I will paypal $100 to the first person who can tell me how to fix my car.

veget8

Veteran Member
Joined
Aug 9, 2002
Location
Randolph, VT
TDI
Golf, 2002, Silver
Edit:

This issue has been resolved. Please see the end of this thread for the conclusion.

---------------------------------



I have tried a lot of things, and I haven't been able to get my TDI running the way it ought to. I will paypal $100 to the first person who can tell me how to fix my car. This is not a joke or scam, it's an act of desperation :)

The car:
2002 Golf TDI GLS
Manual transmission
95,000 miles
Greasecar for the first 70,000 miles
Stock chip
DG Racepipe
Boost gauge

Symptoms:
Decent low end power, but lack of (almost no) top end power.
Poor mileage (34 mpg).
Top speed is around 75-85 mph depending on whether I am going up or down hill. Power is fairly good until about 2800 rpm, and then it drops off pretty quick. The car won't go past much past 3200 rpm in any gear, if it does, it creeps by it. If I jump hard on the pedal in first, there is a quick puff of black smoke around 2000 rpm.

Doesn't run rough, idle seems stable. starts a little hard when cold, but nothing that alarming to me for a diesel w/ 95,000 miles on it.

Around 3000 rpm, a VAG-COM shows MAF requested 800+ while actual hangs around 450-550 (average, with some oscillation above and below). I can post plots if requested.

Using the 2.0 MAF gives it considerable more pep, but still falls off around the same spot. Makes it smoke like the dickens, too.

Here's what's been done to the car since this problem developed, mostly in an effort to try to resolve it:
-Replaced MAF and did Cage mod
-Replaced air and fuel filters
-Descreened
-Cleaned intake manifold
-Removed EGR
-Checked cylinder compression. All cylinders between 400 and 450 psi with about 10% variation
-Replaced injector pump (and upgraded to 11mm)
-Replaced injectors
-Leak tested vac lines
-Tried replacing N75
-Checked for boost leaks
-Replaced turbo / exhaust manifold / actuator
-Replaced catalytic converter
-Mufflerectomy
-Replaced ECU

Feel free to ask questions if I can clarify anything. Thanks in advance to anyone willing to take a stab at it :) I am game for any helpful suggestions at this point.
 
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mrchill

TDIClub Enthusiast, Super Secret Diesel Ninja Vend
Joined
Sep 16, 2003
Location
MASS! home of THE WORLD SERIES CHAMPION RED SOX! x
TDI
96 B4v red \ 98 Mk3 green\98 Mk3 Jetta black\ 99 Mk4 Jetta green x2\ 99 Mk4 Golf silver x2\ 99 Mk4 Jetta black\ 97 B4 sedan green\04 JSW gold\03 JSW silver
You are likely going into limp due to overboost. The actuator is probably bad. Common in the Northeast. Do you have a cel on? If so, do you have a code to give us?
 

bam_bam_dip

Veteran Member
Joined
Nov 17, 2004
Location
Belton, TX
TDI
99.5 Jetta TDI GL
Still sounds like a MAF. Does the "falling off" after 2800 rpm go away or change significantly with the MAF unplugged?

Try the redline test both ways....MAF plugged and unplugged. What rpm will it hit??
 
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Herm TDI

Vendor
Joined
Nov 21, 2001
Location
Richmond, Maine...The far side of Witsend
TDI
2002 Golf GLS Malone Stage 3, P+520 nozzles, 11MM Inj pump, Sachs VR6 clutch, Stelth Race Pipe, Immo Deleat, EGR Deleat
veget8 said:
I have tried a lot of things, and I haven't been able to get my TDI running the way it ought to. I will paypal $100 to the first person who can tell me how to fix my car. This is not a joke or scam, it's an act of desperation :)

The car:
2002 Golf TDI GLS
Manual transmission
95,000 miles
Greasecar for the first 70,000 miles
Stock chip
DG Racepipe
Boost gauge

Symptoms:
Decent low end power, but lack of (almost no) top end power.
Poor mileage (34 mpg).
Top speed is around 75-85 mph depending on whether I am going up or down hill. Power is fairly good until about 2800 rpm, and then it drops off pretty quick. The car won't go past much past 3200 rpm in any gear, if it does, it creeps by it. If I jump hard on the pedal in first, there is a quick puff of black smoke around 2000 rpm.

Doesn't run rough, idle seems stable. starts a little hard when cold, but nothing that alarming to me for a diesel w/ 95,000 miles on it.

Around 3000 rpm, a VAG-COM shows MAF requested 800+ while actual hangs around 450-550 (average, with some oscillation above and below). I can post plots if requested.

Using the 2.0 MAF gives it considerable more pep, but still falls off around the same spot. Makes it smoke like the dickens, too.

Here's what's been done to the car since this problem developed, mostly in an effort to try to resolve it:

-Cleaned intake manifold

Feel free to ask questions if I can clarify anything. Thanks in advance to anyone willing to take a stab at it :) I am game for any helpful suggestions at this point.
You mentioned that you cleaned the intake manifold assembly.

Did you also clean the air inlet passages in the cylinder head? I had an A-3 come to me after it had been in the hands of a "stealership" for a persistent low power problem.

I removed the intake manifold and found #3 air inlet passage about 90% blocked/ clogged and #4 air inlet passage was more then 50% blocked.
 

BleachedBora

Vendor , w/Business number
Joined
Oct 16, 2003
Location
Gresham, Oregon
TDI
'81 DMC-12, '12 Audi TT 418 hp/383 tq, '08 GL320 CDI 275 hp/448 wtq - '81 Caddy ALH, '04 Golf BEW 2" Lift, '85 HMMWV
All those things might work, but he wanted to know how to fix his car, he didn't ask the cheapest way!
Replace the engine, and the entire drivetrain. That will most assuredly fix your car. My paypal address is the email address found in my profile! ;)
Yay--more college money! :p
Good luck!
-BB
 

veget8

Veteran Member
Joined
Aug 9, 2002
Location
Randolph, VT
TDI
Golf, 2002, Silver
Mr. chill - I will answer your request for a boost plot shortly. Unfortunately, the actuator has been ruled out as it was replaced with the turbo only a month or so ago. The only CEL code I have is:
16786 - EGR System: Excessive Flow
Because I have replaced the EGR valve with a racepipe.

bam_bam_dip - If I recall the car performed somewhat more sluggishly with the MAF unplugged, and still rev'd to about the same amount before falling off. I will do this test again with real numbers for you. Incidentally this MAF was brand new out of the box on Monday, so I don't think it's bad, but it's still a good point.

Herm- When I was at your place in late August to get the turbo replaced, (i was the guy with the greasecar decals) we had the intake off. Do you recall if we checked the intake ports? Is it something we could have overlooked, or would you have thought to check while you had the intake off (I ask because you are very meticulous and I wouldn't be surprised if you looked). If not I will take the intake off and peek at it with a mirror after some of the easier suggestions have been addressed.

Bleachbora- Nice try. I would actually consider awarding the money to you if you actually could conclusively prove why changing the engine would solve the problem. The very nature of my attempts to resolve this are evidence that it's not necessarily something involved directly with the engine or drivetrain. For example, an engine change will not resolve a problem with a clogged catalytic converter - the entire exhaust is generally left installed on the vehicle when an engine is replaced. Another example would be the ECU- this component is mounted to the body of the car above the engine compartment and would most likely not be included with or replaced with a new engine. If any components such as these (and there may some I haven't replaced, such as the drivetrain control module) are the problem, your suggestion would not work. All things considered, I may indeed end up trying something along the lines of what you suggest, but I can promise you that it will be pretty far down the list of things to try.

That all being said, let's say going forward that all suggestions must be backed by some theory :rolleyes:
 

Galactic_Warrior

Veteran Member
Joined
Feb 14, 2002
Location
Milwaukee, WI
TDI
02 Jetta
Was the power drop off (at say 2800-3000 rpm) a sudden problem or was it a problem that slowly developed?

If it was a gradually worsening problen I see three possible concerns for investigation.

1. Clogged intake ports. As previously mentioned, this can act like a clogged intake and eventually limit air flow, thus power and RPM's.

2. Your car is a 2002. Have you pulled the cam cover to inspect the camshaft? 2002's are known for soft cams. A gradually degrading cam lobe (or lobes) can act just like a clogged intake and reduce the airflow. If this is the problem, it is not too bad of a fix.

Another thought....
3. Have you checked your timing? Severely retarded timing can cause hard starting, low power at higher speeds and reduced RPM's (but still decent low end power).

Please advise; we all want to see your car up and running!
 

Bugdope

Active member
Joined
Sep 19, 2006
Location
Alaska
TDI
2000 Beetle TDI
I'm new to the TDI game and don't deserve any money. However, I do have a VAGCOM and have been playing a bit just checking things out. Doesn't the MAF graph above definitely indicate that the actual MAF is not following the requested. I believe that the actual MAF should stay at 800 or above. The question of course is why? You have replaced the MAF and ECU !! Could there be a bad connector or cable from the MAF ??
Best of luck.
 

Galactic_Warrior

Veteran Member
Joined
Feb 14, 2002
Location
Milwaukee, WI
TDI
02 Jetta
Veg,

Please re-plot your MAF readings.

You have what appears to be good data, but you are plotting it along the X-axis based on the recorded data sequence.

If you can re-plot with the X-Axis only being RPM in a pure linear fashion, it will be easier to interpret (and then validate the theory of the MAF or maf circuit) being the culprit.

If this is not clear, PM me and we can exchange e-mails and I'll re-format it for you (I was a test engineer in a former life and was used to this type of data reduction).

BTW, the data does suggest some type of air restriction or a consistant error in MAF values!

Best of Luck!

Edit:

Does the following accurately represent the events of your test drive? Please advise.
 
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johnsonej

Member
Joined
Feb 13, 2005
Location
Phoenix
TDI
Jetta 2001 TDI GLS manual ALH
buy another MAF sensor from performance-cafe.com for $59.I drove around for a year with a bad one. I bought one a several months ago
and still running strong. I have an ALH with no modifications. Replaced the timing belt, rollers, tensioner and serpetine belt at about 82K.
it is always a possibility that a new sensor could be defective.
good luck.
 

bam_bam_dip

Veteran Member
Joined
Nov 17, 2004
Location
Belton, TX
TDI
99.5 Jetta TDI GL
Greasecar, correct?? Are these plots on WVO or D2?

It's really odd that you say your car runs "slightly" better on a 2.0 gasser MAF the a 1.9 tdi MAF. It should be the other way around.
I'm still leaning towards the MAF, but not 100% confident. If possible, I'd like to see plots of:
1. Maf plugged in (got that)
2. Maf unplugged
3. 2.0 Maf
All for comparison, and
4. a working tdi Maf, if you can borrow 1 from someone else would be fine.
 

veget8

Veteran Member
Joined
Aug 9, 2002
Location
Randolph, VT
TDI
Golf, 2002, Silver
GW- Thanks for your words of encouragement and help. It was a long, long time ago, but IIRC the car gradually developed the upper end problem. I have checked the timing with a VAG-COM, and it is on _upper_ line on the VAG-SCOPE, if you know what I mean. The timing is on the advanced side of the recommended range, as is typical. As for the valve timing (i.e. crank vs. cam), the cylinder compression would show a problem. As for the cam lobes, I have had the valve cover off many times and haven't noticed anything strange, but I will pop it off again and look. Anything in particular I should look for? Clogged intake ports are not out of the question, but I seem to remember checking them when I cleaned the intake manifold as part of the standard procedure. Also, Herm had my manifold off last month and I think we would have noticed something (but let's see what his recollection is).

You are correct about the plot. It is a log of a driving run and not really a good 1500-3500 accel run. However, the pedal was to the floor the entire time for the last part of the graph. You can see what happens to the "actual"- starts out bad, gets worse (ridiculous even). The little upward tweak on the requested must have been a bad data point of some sort. I'll do a better plot later on (tommorrow or sunday at the latest).

As for the bad MAF, here is why I am seriously doubtful it's a problem with the MAF sensor itself:

I have 4 MAFs:
-original, bosch tdi MAF
-1 pierburg MAF
-1 Bosch 2.0 MAF
-and as of monday, 1 new bosch tdi MAF

They all plot pretty much the same and cause the car to behave relatively the same. The only major difference being that the 2.0 is consistently higher across the board, but the shape looks the same, and the specified drops like a rock. This means that either:

1. they are ALL BAD (which the first 3 i might buy, but the brand new one too?!) or

2. they are indicating the air flow as measured properly, which would indicate an obstruction or problem with the induction

3. there could be something wrong with the harness or circuit that gets the measurement to the ecu (which you and bugdope acknowledged). I will do the MAF plugged/unplugged test and tell you about how high it revs/power difference. That should indicate something in this area, right?

Ok, got a list of stuff to try. I'm gonna get some sleep and hopefully by the end of the weekend I will get back to you with some answers. Thanks to all for your help thus far - I know you're not just doing it for the money :)
 

veget8

Veteran Member
Joined
Aug 9, 2002
Location
Randolph, VT
TDI
Golf, 2002, Silver
All plots and test are done using diesel, and it's safe to assume they will until this is all solved. The car hasn't run veg since about 20,000 miles- just to keep variables out of the equation (although I highly doubt it has anything to do with this problem, since every possibly related component has been replaced).
 

oldpoopie

Vendor
Joined
May 14, 2001
Location
Portland Oregon
TDI
2001 golf gl, 2006 jetta, 1981 ALH swapped rabbit pickup, 1998 beetle
I hope I'm not stating the obvious, but have you checked or cleaned out the snowscreen????? I noticed you said descreened, Do you mean the snowscreen or the screens at the MAF?
I'd also check the operation of both the check valves in the Vac lines. Particularly the black/white one.

You mentioned the greasecar kit. Is it possible that repeated backflushing of the grease kit could have greased up the tank, blocking or partially blocking the sender at the tank, limiting fuel flow?
 

BleachedBora

Vendor , w/Business number
Joined
Oct 16, 2003
Location
Gresham, Oregon
TDI
'81 DMC-12, '12 Audi TT 418 hp/383 tq, '08 GL320 CDI 275 hp/448 wtq - '81 Caddy ALH, '04 Golf BEW 2" Lift, '85 HMMWV
weedeater said:
I would love to see this plot overlaid with actual boost....
That would be interesting!
So you've checked the timing...and one would assume that one of the MAF's would be fine.

Justin, he says in the original post that he removed the snow screen, so I don't think it's that...
Fuel sender though, that's an interesting idea! :)

I still like the idea of a possible port not being cleaned when the intake was cleaned.

Anyway, glad you enjoyed my joke--If I were to transplant the entire guts of my car into your car (since it's an A3 it'd be a lot of work, and all wiring, clusters etc... would not fit very well...) But IF we could transplant it all I guarantee you would have a lot more top end power! Your car would be a beast! :)
Sorry for the problems though, with my black '97 we replaced almost everything on it trying to fix problems. Turned out one of the problems was a $5 coolant temp sensor; that's after we spent almost $9k on parts and labor for various things.
Good luck!
-BB
 

chromeBuddha

Veteran Member
Joined
Jun 30, 2006
Location
Arlington, TX - DFW metroplex
TDI
2002 Golf TDI manual
The OEM vacuum line to the N75 valve was replaced with a thin wall vacuum line. At high vacuum, the line actually collapses, pinching off and returning the turbo veins to the idle, low boost position.

Arg...still can't fall asleep...as many BS answers as I make up...should be in a coma by now.

With regard to the fuel restriction idea put out above/below, how would that effect the MAF actual? What drives the MAF requested, the accelerator pedal position? Or some combo of the RPM vector and other inputs?

Are you able to redline with no load? If not then running the vehicle from a jar of diesel could eliminate the fuel tank veggie grease theory.

You mentioned the cat earlier. What have you done to investigate that? Get a used exhaust from a salvage yard, knock out the cat element and hook that up.

I am pretty sure I couldn't be any more help awake, but man you have sent a lot of $ at this problem.

Were all the listed mods made after the problem started or some before. A sequential timeline would be helpful.
 
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quartersaw

Veteran Member
Joined
Sep 20, 2005
Location
Albany, NY
TDI
2002 Jetta Wagon, '96 B4V,'99 2 door Golf
Remove the top of the injection pump to visually check fo crud,and corrosion.If there is crap in there,then the pump is your problem. What is your fuel temp. number in Vag-Com with the engine stone cold with the ignition key turned to 'on'?? If it seems like a strange number vs. ambient air temp,I would suspect the fuel temp sensor.
 

Dave_D

Veteran Member
Joined
Jun 9, 2003
Location
Gaithersburg, MD, USA
TDI
2015 Passat Titanium Beige six speed manual & Jetta, 1999.5, Tornado Red
The plot shows the actual MAF as falling far short of the requested MAF. Either the MAF is bad and the reading is thus wrong or the MAF is good and something is preventing airflow from meeting the request. The fact that you see the same behavior with several different MAFs argues that the reading is accurate and something is restricting your airflow. As noted earlier it would be interesting to see requested boost and actual boost ploted as well to see if the turbo is doing its job and how this tracks with the MAF. I am at a loss at this point as to what hasn't been checked in the intake path, but that is where I would concentrate my efforts.
 

Galactic_Warrior

Veteran Member
Joined
Feb 14, 2002
Location
Milwaukee, WI
TDI
02 Jetta
A quick revisit....

A diesel engine basical needs three things to run. Air, fuel and compression.

You have verified the compression. so let's rule this out. The next is either air or fuel. Considering that the car runs reasonably well until about 2800 RPM, this leads me to belive that there is a restriction in either the air or fuel stream that is allowing "basic" operation, but not full power. The nature of the problem getting gradually worse also supports the restriction theory.

You have basically replaced the entire fuel system (pump, injectors and filter). I would take Old Poopie's advice and pull the fuel strainer in the tank and verify that you are not getting fuel starved. You can do this from inside the car (remove a panel under the rear seat for access). Clean/ replace the strainer to make sure the fuel can be drawn freely to the filter. Likewise, I would also consider another fuel filter replacment, if you see evidence of the strainer being gunked up with grease. It may also be a good idea, since everything is apart, to blow compressed air through your disconnected fuel lines to verify that the lines are not restricted.

I'm still betting that you have an air issue though. My basis for this is the MAF readings. MAF's do go bad, but I find it improbable that all 4 were bad. I would run a MAF and boost log to see what is up. If you have good boost, but poor MAF readings, this would make a stronger case for an intake restriction. You have no snowscreen and a new air filter. Please verify that the turbo outlet to the intake manifold tract is intact and secure..I doubt an issue, but let's be sure. Also, is the intercooler relatively clean?

If these steps don't produce results, I would examine both the intake ports and the cam. There were a few posts by JasonTDI about bad cams. If the lobes are wearing (either intake or exhaust), the high RPM flow will be limited. I don't have cam lift specs, but maybe a little reseach is warranted.

Also very important...don't get too frustrated and remember to take an occasion beer break!
 

veget8

Veteran Member
Joined
Aug 9, 2002
Location
Randolph, VT
TDI
Golf, 2002, Silver
Time Line

Quarter, chrome, and OP- all of the mods stated were made after the problem had developed, and in most cases because of the problem. The pump has been replaced with a new one, and I haven't run any veg through it yet. Cat was replaced with a brand new one yesterday.

Chromebuddha- yes, I've had the problem for over a year and a half, and I by my estimate I have well over $3000 into it. Most of that was spent in the last 2-4 months. While it's unfortunate that I've had to do that, it's been worth it - The car is quite special for a number of reasons. It has achieved some historical firsts and became fairly well known, but I won't get into that here.

Basically the summary timeline looks like this:
------------------------------------------
Sometime in summer 2004 - * * Problem developed. Laggy turbo, crappy low end power, crappy high end power. With the chip I had at the time, it not only had no high end power, but smoked like a locomotive if I tried to put my foot into it. Probably not a fueling problem.

2004-2005 did all the easy investigations (replace maf, removed snowscreen, air filter, etc.). Most of these resulted in various improvements on the low end power. None of them ever resolved the problem of high end power.

06/2005 Cleaned intake - it was real bad. made a big difference on the low end power. Removed EGR and installed a DG racepipe so it wouldn't happen again. Pretty sure I checked the intake ports with a mirror, but not 100%

10/2005 Last time I ran on veg.

Winter 2005-2006 suspect it was in this time frame I damaged my ECU with a battery booster that puts out 40 amps. The low end power was around the same as it was before I cleaned the intake manifold (coincidentally). This was resolved later (see 09/06)

06/2006 Replaced pump with remanufactured 11mm pump. No improvement, to the dismay of SVO+TDI skeptics everywhere ;). Still crappy low end power, and slow turbo spooling. Makes MORE smoke (holy cow). More indication that this is not a fueling problem. Hammer modded to set IQ around 10-11 to control smoking (it's still set there as far as I know).

06/2006 Checked compression. Replaced injectors with new ones, stock size. No improvement at all.

08/2006 Brought to the VW stealership to have it "fixed". Mostly for entertainment value, which was lucky because that's exactly what I got out of it. They claimed they tried a new MAF, ECU, and a bunch of other tests but they couldn't figure out what was wrong. They told me they weren't interested in pursuing it any further because the car had been modified from stock. Specifically they mentioned the missing EGR valve (didn't touch the grease kit though, to my surprise :p). Later, not to my surprise, I found out that there was no way they tried a new ECU. The connections were stuck on there as though they hadn't been touched in 2 years (which they hadn't).

08/2006 Brought it to Herm's. Replaced turbo/exh. manifold/VNT actuator, changed air filter, fuel filter, tried brand new MAF. Tested vac lines, N75 switch. No improvement in high end power, and turbo is still laggy. Air filter helped with the low end power, and it was evident that the VNT actuator was binding, but wasn't the big problem. Replaced socketed wetteraurer chip with stock chip- less power, no more smoking! Probably should have reset IQ to something normal, but I don't think it is making that much of a difference (am I wrong?). Herm points out ECU is not throwing codes and is suspicious of it. He enlightened me about the damage a high output battery charge/booster can cause to TDI's.

09/2006 Got a brand new ECU. Installed it and convinced the stealer activate it (IMO 3 with GEKO or whatever- what a pain). Car begins to perform wonderfully better in the low range (like right after I cleaned the IM)... another hint the dealership had not tried this. But STILL NO IMPROVEMENT on the high end!! Not what I expected as an outcome. Oh yeah, and car reports codes normally now so I have a CEL for the EGR (yay.)

09/2006 Put 2.0 MAF in. *Relatively* speaking, the car is now a torque monster (although I notice the turbo still lags more than it should, but it's been a while since I have driven w/ stock chip) below 2500 rpm, but this MAF makes it smoke like a train again. Still no improvement on the high end- about 2800 rpm and the car goes limp.

10/04/2006 - bought a brand new MAF in case my old burnt out ECU was toasting MAFs. Car is back to normal sluggishness in the low range, and still no improvement in the high end. Only a slight puff of smoke if I really get into it at 2000 rpm.

10/13/2006 - Cat replaced with new, high flow type. Mufflerectomy. Car is loud, (seems to have) slightly improved low end power, but as usual still no improvement in the high end.

-----------
The above timeline is based on a log book, but there is some recollection that could be off a little. Please don't let the information above bias you - It's been a long and complicated past. I think the best way to go about finding this problem is based on factual data on the car the way it is today.

I am putting together a worksheet to collect all of the data people have requested thus far, including boost plot, maf plot, redline loaded and unloaded, etc. Thanks for your patience - as I have a lot of other crap to do today I don't know if I'll get any testing done. Tommorrow should be better.

Thanks everyone for your continued support! You guys are all awesome for helping me.

Talk to you soon
 

davetdi2004

Active member
Joined
Mar 8, 2005
Location
Charlotte, NC
TDI
2004 Jetta TDI PD 1.9L, Gray
Sounds to me like the intercooler could possibly be partially clogged, maybe with oil residue. This would cause flow problems at the high RPM range on a sustained basis.Probably not too noticeable in short bursts.
 

Burl4561

Veteran Member
Joined
Mar 25, 2005
Location
JAX, FL
TDI
-'02 Golf 4dr
I agree with the general direction of the conversation. Logging requested and actual boost will be quite telling. While you are at it, I have heard a suggestion of logging Actual and requested Fuel.

I haven't tried it yet, but will be doing it on mine tomorrow.

Your initial post said "leak checked lines and N75" Have you tried a differnt N75?
 

bam_bam_dip

Veteran Member
Joined
Nov 17, 2004
Location
Belton, TX
TDI
99.5 Jetta TDI GL
I'm inclined to agree with Galactic_Warrior, I have a hard time believing all 4 MAF's are bad. And since your smoking like crazy, it sounds as if your motor is starving for air. The MAF is requesting more air.....you see my point.
 

BugBug

Veteran Member
Joined
Aug 19, 2006
Location
Minnesota
TDI
2001 Beetle TDI, 2005 New Beetle
Have you checked the applied vacuum from the vacuum pump while driving i.e. under load at full throtle?
I read somewhere that if the engine is cold, the ECU limits boost. If the coolant temp sensor is defective (I believe this is a dual element sensor (one for the ECU the other for the guage)), then the ECU is cutting your boost.
Let us know what you find.
 

Fortuna Wolf

Veteran Member
Joined
Apr 28, 2006
Location
Wilmington, NC
TDI
2002 Jetta TDI Auto Sedan
Man, its time to go get some gauges! :)
Get a mechanical vacuum gauge and a mechanical boost gauge. I'd drill a hole in the back of the intake manifold for the boost gauge if you really want to measure the boost at the engine. Otherwise to measure boost produced by the turbo, tap right after the air compressor and before the intercooler. I think that for diagnostics the intake manifold is better. The manifold gets warm, but won't get over 200F, so most plastic fittings will be safe to use with it.

The vacuum gauge is for the vacuum system so that you can see if the vacuum pump is working properly. Once you have found that its working properly use it to measure vacuum in the fuel lines right before the IP, this will give you an idea of clogged fuel systems (excellent if you're running a WVO system!). Or sell it to me...

The OEM vacuum line to the N75 valve was replaced with a thin wall vacuum line. At high vacuum, the line actually collapses, pinching off and returning the turbo veins to the idle, low boost position.
At higher RPMs it requires less vacuum on the turbo to produce the same amount of boost... I still think he should get a mechanical boost gauge.

He says that his injectors are working fine, and I am guessing that the problem is not strictly an underfueling problem, since he says that its smoking. Right? I'm leaning more towards some sort of air problem.

However, there was a suggest to check for buildup in the fuel lines.
Try this, remove the hoses from the IP, and as you would mainline a can of diesel purge, mainline a bottle of diesel (wedge it in there tight) and try driving on it.

Lastly, what if there was a huge amount of backpressure in the exhast, preventing the VNT from producing much boost, or the engine itself from working? Can you remove the exhast hose from the turbo and test?

I'd replace all of the vacuum hoses with polysiloxane hoses, just to be sure.

Edit: saw another thread where you replaced the cat and exhast. I guess that's not it. :(
 
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paramedick

TDIClub Enthusiast, Vendor
Joined
Jul 29, 2001
Location
Versailles, Kentucky
TDI
2015 Audi Q5 TDI, 2000 New Beetle
My take.

MAF is fine. Definitely clean the checks valves in the vacuum lines and confirm they are all good.

Replace the N75. Testing this is iffy.

Confirm that the fuel and coolant temps are tracking each other consistently.

I like Justin's idea of checking the fuel pickup. I also thought that before I read his reply. Drill that puppy out if it still has check valves. Before you put everything back together, use compressed air to blow out the fuel supply and return lines and see if you get crud. Obviously, not into your tank!
 
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