Quick Clarification Question - MAF Sensor Testing

First off, I'm sorry I haven't dug as deep as I probably need to. I'm trying to get parts ordered before the shipping cut off so they're shipped today and figured I'd ask for a quick answer vs. spending an hour looking for it.



I'm trying to diagnose limp mode on my 04 Jetta Wagon. I unplugged the MAF sensor and ran it.

From what I gather, if it performs similarly or is significantly improved, it's a bad MAF sensor. If it performs worse than when it's plugged in, it's not the MAF sensor.

My question is this - it performs exactly the same as when my car GOES INTO limp mode. However, it's quite a bit worse than how the car performs before it goes into limp mode, right after starting it. When I unplug the MAF sensor, it's acting like it's already in limp mode - no boost.

Usually I have to get it up to about 70 on the interstate or really get into it in town to get it to go into limp mode. When I unplugged the MAF sensor, it appears to have no boost right off the bat.

Can I assume that the problem is not the MAF sensor based on this?

Thanks!

Your findings suggest a dying MAF.
However the best diagnosis would be a VCDS graph.
T

Yes, Carlos is right.
There is a lot of misleading info about the MAF on this forum(s), so be careful what you read.
MAF sensor FAQ, troubleshooting, and replacement for VW and ...

Thanks for the update.

I’m taking it in today to hook it up to a VCDS and will report back. I went ahead and bought a MAF sensor yesterday that will be here on Tuesday.

The prices were all over the place for those things. I found one on carparts.com marked down to $27 so I figured it’d be a cheap test, if anything. If I change it out and it works fine, I’ll chalk it up to the MAF. If it dies again soon, I’ll spring for a “better” MAF sensor and hopefully be out of the woods.

You skipped Step 1.

What DTC is being set? Start there. OBD works. Shot gunning a MAF (or anything else) at it is not likely to get you anywhere but lighter in the wallet.

I won't even suggest anything until you at least have a valid DTC.

All BEWs have the improved late style MAF, and they are not high failure items. And most non-Bosch aftermarket ones are garbage, and may create a problem that didn't even exist before. I've removed so many non-Bosch hot film type MAFs someone put on VAG products over the years, it's crazy.

oilhammer said:

You skipped Step 1.

What DTC is being set? Start there. OBD works. Shot gunning a MAF (or anything else) at it is not likely to get you anywhere but lighter in the wallet.

I won't even suggest anything until you at least have a valid DTC.

All BEWs have the improved late style MAF, and they are not high failure items. And most non-Bosch aftermarket ones are garbage, and may create a problem that didn't even exist before. I've removed so many non-Bosch hot film type MAFs someone put on VAG products over the years, it's crazy.

Click to expand...

I didn't skip it, I just didn't mention it.

There are some cam position codes being thrown as well as one other that I can't remember right now. I'm at work and I have them written down out in the car.

I'm not "shotgunning" a MAF at it. I followed the instructions in the MAF troubleshooting thread that was listed here and the results when the MAF was unplugged made me think it was a MAF sensor.

However, I didn't realize I could create problems by using one that wasn't made by Bosch. Is there any harm in throwing it in there to see if it resolves the problem and then immediately purchasing a Bosch to put in there as soon as it arrives? I don't mind spending $27 to troubleshoot try a new part so long as it won't damage the car further.

It's also worth pointing out - I'm taking the car to a shop later this afternoon who I've talked with extensively about the problem and they claim to be familiar with it. I've asked for a few things and hope I'm not too far off the mark - specifically, VCDS information on both the MAF sensor and the timing. I read another thread that said if the timing wasn't right, you could experience hard starting - which I am.

I've got a laundry list of items that I've gotten off of this site from folks with very similar issues and hope to get to the bottom of it.

Car has 145,000 miles on it and I have service records indicating it has used the correct oil since day 1. I'm hoping this is nothing major; but, I think it's time to take it somewhere to get some VCDS analysis done since I don't have the ability to do that at home.

I'd love to hear what all you have to say about this as I'm already over a grand into this and have no measurable improvements.

I'll be back shortly with the codes.

Thanks.

P0341 Cam. Pos. Sensor
G40 Implausible Signal

P3008 Cam. Pos. Sensor
G40 Signal out of range

P0299 Boose Pressure Regulation
Control range not reached

And then there is a note that also says "Thermostat performance fault"



It's worth pointing out that I'm having some pretty serious starting problems, as well, so I'm asking the shop to look at the timing as well as trying the longer glow plug time setting to see if that will help.

The first shop I took it to was saying to test compression and probably rebuild the engine so I figured I'd ask around, a bit, before going straight to that.

Definitely open to any advice/suggestions/recommendations you guys have to offer. I'm trying to do as much of this as I can at home; but, all signs are pointing towards Vag-Com/VCDS and since I can't do that at home, she's going in today.

For what it's worth, I think I could drive around in town all day without it going into limp mode. It seems to happen when I'm getting into it trying to merge onto the interstate. Cycle the ignition and it's back to normal - until I get into it again.

How about this:
When the new MAF arrives request a return authorization and send it back.
Why join a forum if you don't intend to take very reasonable advice?
No one here who 'knows' TDIs would advise using a cheap MAF that you may not even need.
The crappy one may not work right out of the box.
If it turns out you do need one, only get the real Bosch one using the refund from above.

flee said:

How about this:
When the new MAF arrives request a return authorization and send it back.
Why join a forum if you don't intend to take very reasonable advice?
No one here who 'knows' TDIs would advise using a cheap MAF that you may not even need.
The crappy one may not work right out of the box.
If it turns out you do need one, only get the real Bosch one using the refund from above.

Click to expand...

Seemed like a reasonable way to troubleshoot the problem. I’m asking if it’d work, or not, for a short time - long enough to validate the fault.

I’m not sure if you read, or not, but I said I didn’t realize it could harm the car and was asking if it’d be reasonable to try it out, after reading the reasonable advice provided by another user. Certainly not discounting the advice, trying to salvage the purchase because, again, I don’t mind wasting the $27 if it’ll prove the fault without damaging the car. If it’s too risky to even install it to see if it fixes the problem, I most definitely won’t.

Not really sure what I did to ruffle your feathers, just looking for some help.

OK, so you do NOT need a MAF.

P0299 is low boost. Could be a charge air leak, or a control side (vacuum) leak, or a bad VNT actuator. Something, somewhere, is causing the boost not to stay where the ECU wants it to be. THAT is your limp mode cause.

CMP sensor could be just the sensor, could be the torsion is way the heck off, could be a bad ECU ground (there is a TSB about this), or perhaps a rubbed wire somewhere around the CMP's connector at the front of the engine block.

A DTC related to the thermostat could be a bad thermostat, or a flaky CTS. Since the CTS is so common, and so cheap, I always do them together with the thermostat. Chances are, given the age and mileage, the thermostat is not working as good as it should anyway. I only use OEM Behr or Calorstat thermostats, with a new OEM neck.

FWIW, I have no idea who suggested "MAF troubleshooting" involving unplugging it, but that is a very lame way (albeit easy and harmless) to diagnose a MAF sensor. OBD works, and the MAF is easy to check... but as I said, that is not your problem anyway.

Here is a tidbit: on the BEW engine, it is common for the vacuum line to the VNT actuator, as it clips to the inlet tube (the big plastic one that sorta looks like a saxophone) to rub a thin spot in it and get sucked shut, and evenually rub a hole clean through it. It is difficult to spot, but the best thing is to just remove the hose from the VNT actuator underneath, unclip it and fish it up and out and examine it (it connects to the rest of the vacuum harness near the firewall... there are a couple inline connections).

Often times you can just section in a new piece, and retain the lower portion that has a molded bend to it. I've probably fixed 50+ of those.

Next tidbit is a vacuum leak at the brake booster tube, but usually if that gets bad enough you'll also lose brake assist, but sometimes not. Still something to check. The other two vacuum operated items on the BEW, the twin port intake runner control and the EGR cooler bypass control, are not monitored for position... so if the vacuum system is suspect, the ONLY manifestation you may have as far as engine control is concerned is the VNT, which will cause lack of boost.

Wow.

Thank you for all of this information.

I actually read the disconnect the MAF sensor troubleshooting but on here on the “limp mode” thread.

I’m gonna print that out and take it to the shop with me this afternoon, just in case. I’m picking the car up tonight when they’re done, so if they can’t get it solved while I’m there, I’m gonna be greasy this weekend.

I really appreciate the advice.

Yea, Oilhammer is next level.

BobnOH said:

Yea, Oilhammer is next level.

Click to expand...

Totally. Really glad he weighed in.

Once I tackle this limp mode issue, I’m going straight into the hard start issue. Hopefully it doesn’t end up being a compression/engine issue that results in a rebuild, like the original shop suggested.

Wagon said:

Totally. Really glad he weighed in.

Once I tackle this limp mode issue, I’m going straight into the hard start issue. Hopefully it doesn’t end up being a compression/engine issue that results in a rebuild, like the original shop suggested.

Click to expand...

That sounds pretty bogus, you'd have to really abuse one of these things to need a rebuild before 300k miles.
I'm guessing it's pretty hard to find a good mechanic who knows the PD. You'll want to get a short list of good mechanics and interview. Most places will be clueless, but they won't fess up.

BobnOH said:

That sounds pretty bogus, you'd have to really abuse one of these things to need a rebuild before 300k miles.
I'm guessing it's pretty hard to find a good mechanic who knows the PD. You'll want to get a short list of good mechanics and interview. Most places will be clueless, but they won't fess up.

Click to expand...

I think you nailed it.

Wagon said:

Seemed like a reasonable way to troubleshoot the problem. I’m asking if it’d work, or not, for a short time - long enough to validate the fault.
I’m not sure if you read, or not, but I said I didn’t realize it could harm the car and was asking if it’d be reasonable to try it out, after reading the reasonable advice provided by another user. Certainly not discounting the advice, trying to salvage the purchase because, again, I don’t mind wasting the $27 if it’ll prove the fault without damaging the car. If it’s too risky to even install it to see if it fixes the problem, I most definitely won’t.
Not really sure what I did to ruffle your feathers, just looking for some help.

Click to expand...

No mussed plumage here but if a stranger told me how to save $27 I might thank them.

This shop is also saying I need to rebuild the engine, essentially. I brought them all of this information and they had it hooked up and running diagnostics for about three hours, among other things, and their results are much more detailed and point the same direction.

I will post the information they gave me once I drive to the house (at 45mph).

The only information I got is some canned testing procedure type stuff. It’s not what they did while I had it in. The shop said they’d put together an email with everything they checked, what they found, and a quote for the proposed repairs. I’ll share all of that once I have it.

What ended up being the issue Wagon

It's nice to get a post back on solutions, Wagon hasn't been active since June.
Buhler?/
Jess- do you have codes?

Hi everyone,

This is all fantastic information, and I'm bummed that Wagon never got back. I'm dealing with a similar issue, and thought I'd pick up where he left off.

Mine is a 2004 BEW Golf. Boost levels were usually 12-16psi, then for about a month I noticed that they were more like 12-14, then I'm in a situation where if I go above about 6, limp mode kicks in, sometimes with CEL, sometimes not. The only code it gives is P0299.

I've been over and over the vacuum lines, including that great tidbit from oilhammer about the spot where it rubs thin -- mine does have some wear, but no hole.

Actuator starts at about 3psi, finishes moving at 19. Not sure it goes as far as it should, though, so I may well have coked vanes; turbo may be OEM (though actuator has been replaced) to the engine, so may have 200,000+ miles on it. Unfortunately the downpipe nuts are so rusted that as soon as I put a socket on them they rounded off, so I've been unable to inspect/clean. I almost have the money for a new VNT-17, so if that's the issue then fine.

I checked the hard line to the brake booster. I'm getting about 24psi at the hard line nipple and 23psi at the vac in line to the N75.

Also I've cleaned the EGR valve twice in the last year, and what I could see of the intake manifold from there was lightly oily but no soot.

However, it would suck to buy a new turbo and be right back in limp mode.

So. The specific point I'm at is regarding the solenoids. Most information I've read has to do with the ALH on this issue, and I have some spare ALH solenoids: however, they appear to be different from the BEW solenoids, and moreover, the N18 does not look anything like the N75 on the BEW, so just swapping it doesn't make much sense to me.

Is an ALH N75 swappable with a BEW? I tried it, but it seemed to run worse. I checked Bentley and searched here, and got nothing that cleared this up for me.

I'd be grateful for input and advice. I'm pretty stumped.

Sent from my BND-L34 using Tapatalk

I too had the P0299 DTC, but my actuator did not hold vacuum due to internal corrosion.
I did replace the actuator, and that cured the limp mode, over boost I was experiencing. A constant CEL with the new Garrett actuator forced me to have it tuned out, VNT delete.

Quick update--I bought a new BEW N75, same behavior: boost ok up to 7psi, then limp mode.

Replaced the vacuum line down to the actuator, we'll see if that has any effect, but I doubt it.

I'm preparing to try and get the downpipe nuts off, clean the hot side if possible with engine cleaner, and buying the new VNT-17.

Sent from my BND-L34 using Tapatalk

Going back to post 7 in May:
'P0299 Boose Pressure Regulation Control range not reached'

7 psi may not enough.
You need to log the boost requested and actual.
I think the requested will be higher then the actual by more then 4 psi which trips limp mode.

Would be a good time to log MAF to see if it's returning valid data.

Before I bought a new turbo I would definitely remove & clean it.
It would also give you the opportunity to check the actuator for rust, proper travel and adjustment.

Was the intercooler checked for leaks?

I found a leak in one of my pipes using soapy water and a spray bottle.

Last, I thought people use oven cleaner to clean the inside of the turbo.

wonneber,

Thanks for the input. First of all, I totally meant 'oven cleaner', I just had a brain fart.

Update: Got the exhaust flange nuts off, sprayed oven cleaner all up in there, every 20 minutes for about an hour and a half I got in there, sprayed more in, worked the actuator rod with a mityvac.

Interestingly, it tends to stick at about 7psi, and doesn't always keep moving after 7psi unless you physically push it. But I did what I could, reattached, ran it hard, and a marginal improvement; it reliably drives up to about 45mph with decent boost levels (not above 7-8psi as per the OBDII), but as soon as I do a hard run in 3rd or 4th, it cuts out and triggers the CEL.

That makes sense, what you're saying about requested boost probably being higher. I don't have a "real" VAGCOM cable, I have one of the $20 ones and haven't gotten it to work with my laptops. Lame. So unfortunately I can't log that stuff at the moment.

There is one potential leak, and that is at the compressor outlet pipe, the one between the turbo and the pancake pipe. I pulled it off, inspected the tabs, and the tabs are still intact, it just seems there's a little... Play. Like, 2-3mm. Just for kicks, I taped it all up to see if it would make any difference in the boost, and it didn't. But duct tape probably doesn't stand up to boost pressure anyway.

How big of a leak does it have to be before it has this kind of effect? I'll try spraying down the system, I've been under there and done a visual inspection with the car off and didn't see anything obvious, but not with it running.

So, I hesitate to pull the turbo off and clean it/redo it myself. I did that previously and ended up having a catastrophic runaway condition afterwards--probably due to a crap Chinese CHRA, but I don't want to repeat the experience. I'm a novice mechanic, and there's plenty of stuff I feel comfortable with, but the internals of turbos make me nervous. My current plan is to go ahead with the upgrade I've been saving up for. I bought a brand new Garrett GT1749V (778445-001, supports up to 175hp) and all the bits, and they're on their way. During the install, the intake is coming off, so I'll be able to confirm whether a coked intake/EGR is the issue or not.

Any ideas as to how I can determine whether my loose compressor pipe is the issue, besides buying a new one? Can I stick an extra rubber o-ring on there or something?

Thanks everybody. I will update one way or another once this is resolved.

I think you are mixing up psi with inHg in your vacuum system. No big deal but it is a
bit confusing to read especially when the limp mode occurs around 7 psi also.
You should find out what is causing the actuator to stick at (I'm guessing) 7 inHg and
need a 'push' to get past it. That could be at the root of the limp mode.

Flee -- you're right. When referring to the readings on the mityvac it's inHg. It is confusing going between boost and vacuum; I actually thought there was a correlation there but probably not...

Sent from my BND-L34 using Tapatalk

It's funny the way the actuator works.
The actuator pulls the arm when you start the car and releases it a bit as you go. At least that's what I see.

I have read you can take the actuator off and work the arm going into the turbo back & forth by hand for 5 minutes or so and it may free some of the carbon build up that could cause sticking.
Might be a good time to give it a bit more oven cleaner.

It's only 2 nuts, a plug, and a e-clip.
There's a VW bulletin about the actuator rusting internally.
Tie a piece of thread or dental floss around the clip when you put it back on to make it easier to find if it pops off when your trying to get it on.

Hey everybody, I wanted to update this. No solution yet, but this is what I've done:

-Replace OEM turbo with Garrett VNT-17, braided oil line, lower EGR pipe, all new gaskets and nuts

Upon reassembly, even with the Garrett lower EGR VNT-17 pipe, the EGR pipes don't fit "like stock", and I had to spend a lot of time slowly tightening all the EGR pipe fasteners so that they fit. There are slight leaks, on the underside and the top of the EGR cooler where the pipes join (evidenced by soot marks). I have tightened these to the best of my ability, and probably need new bolts to properly torque them (old ones are rusty and could get stripped with any meaningful torque).

However, with the new turbo, I get *exactly the same behavior as before*: Idle is smooth and normal, the car boosts up to 4-6psi on level surfaces, then limp mode with P0299 as soon as I'm going up a hill or really get on it (above 45mph always).

So I checked my vacuum tubes again, and I sprayed down the IC piping with soapy water while my wife held it at 2200rpm, no evidence of leaks. I call a local garage with lots of VW experience (not a TDI specialist), and we run through the scenario; he offers a smoke test, which I was interested in.

$112 later, he's smoke tested and found no leaks, found a sticking rear brake caliper (corrected), a loose tie rod end, and replaced one vacuum line, to the butterfly flap valve/lower runner flap vacuum actuator on the air intake. Brake caliper didn't fix the issue. His diagnosis: your butterfly flaps are probably so coked up that it's blocking the intake. Hm. Sounds possible, as my intake did have a layer of soot on it.

Last night, out comes the intake, pull it apart, and the butterfly flaps, while they are somewhat stuck in place, are in full vertical position and in no way impeding airflow. Nonetheless, I soak it in biodiesel, scrub it out with a brush/scrape a lot of crap out of both halves of the intake, reassemble and start 'er back up. (If I'd had more time I would've deleted the butterfly flaps, but right now I just want boost...)

No change. Same behavior.

On a whim, unplug MAF to see if, after all that, I had a bad MAF sensor. Car dies within a second of starting. Plug the MAF sensor back in, smooth idle, max 4PSI boost.

An oddity: At times, I get good boost (very responsive, 9+PSI) when I have just started the car and it is still cold (under 100F coolant temp).

So. To recap:

Work done:
-Vacuum lines checked for integrity by myself and qualified mechanics
-N75 replaced
-MAF sensor appears to be working (though I suppose it could be giving faulty readings at higher airflow conditions?)
-Snowscreen delete
-Replaced fuel filter
-Apparently no boost leaks in IC piping
-Doggie collar on upper IC tube to EGR valve
-Brand new factory Garrett turbo, brand new gaskets, so coked vanes are not the issue.
-Spring clamp plus extra hose clamp on rubber connector between plastic air intake tube and compressor intake, because it felt too loose

Known potential issues:
-Slight leak in EGR piping (gaskets are present).
-Potentially damaged wires to smart actuator (wires intact, some damaged insulation on purple, I believe, wire)
-The in-tank fuel pump may need to be cleaned.

This weekend I may be able to register the car in a county that does not require emissions checks. If that's the case, I may, for diagnostic purposes only of course, be able to temporarily of course, blank off the EGR piping to see if what I'm dealing with is simply a leaky EGR.

...Beyond that, I am at a total loss here. I'm going to try to find an experienced TDI mechanic in my area (Cleveland). I think there's a guru in Columbus, but I'd rather not make that (sloooow) drive.

Any recommendations or suggestions are welcome.