Why is actual air mass decreasing at full load at high rpms?

runningallday

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Jul 22, 2017
Location
Iowa City, IA
TDI
1999.5 Jetta TDI manual
Bad maf, worn tuebo, exhaust restriction? Air filter dirty?
Exhaust restriction sounds possible. Air filter is new. Can maf be diagnosed from these graphs? Other signs of a worn turbo?

I'm going to replace the vacuum tubing because it's all 20 years old
 

Nero Morg

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2003 Jetta wagon, 2001 Jetta TDI, 2014 Passat TDI
Usually the maf readings will read above 800mg/str during a hard 3rd gear pull. A good maf will read well above 1300mg/str. If it doesn't, a bad maf is suspect. I'd refresh the vacuum lines and test again, cause if the turbo isn't actuating correctly it won't have enough airflow to raise maf readings.
 

runningallday

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1999.5 Jetta TDI manual
Usually the maf readings will read above 800mg/str during a hard 3rd gear pull. A good maf will read well above 1300mg/str. If it doesn't, a bad maf is suspect. I'd refresh the vacuum lines and test again, cause if the turbo isn't actuating correctly it won't have enough airflow to raise maf readings.
alright sweet. VCDS is awesome, i can actually slowly track down the problem this way. although, all in all the car is running pretty good... just wondering if it had a little more power at the high end
 

runningallday

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1999.5 Jetta TDI manual
snow screen was maybe 50% clogged up? had a lot of dried bugs in it. also lots of dust/powder caked onto the screen. so i cut it out. (i also have the extra foam layer on my air filter, that's supposed to replace the snow screen).

now i just need to decide what i need to go do, that requires me to drive at 70 mph in 3rd gear :D
 

runningallday

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1999.5 Jetta TDI manual
snow screen removal made no significant difference. ill swap out the vacuum hoses when i have time, then try it again. after that ill try a new MAF sensor.
 

jimbote

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spiral arm, milky way (aka central NC)
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If boost actual is tracking with requested and known good maf then most likely a restriction on the intake or exhaust. With the intake plugged a drop or flatlining in maf readings usually tracks instantly with a drop in power. If it's exhaust there's usually a slight delay between the flatlining in maf readings and the noticeable drop in power.
 
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runningallday

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Iowa City, IA
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1999.5 Jetta TDI manual
If boost actual is tracking with requested and known good maf then most likely a restriction on the intake or exhaust. With the intake plugged a drop or flatlining in maf readings usually tracks instantly with a drop in power. If it's exhaust there's usually a slight delay between the flatlining in maf readings and the noticeable drop in power.
well here's the plan. i'm going to swap out the vacuum tubing soon, that should be done anyway. and i have a new MAF on the way, that's only $60 for a bosch. those are the easy repairs. the intake was last cleaned around 100k miles ago, it's due for a cleaning, honestly i think this is the most likely culprit... if it's not solved after that, then i could replace the catalytic converter, looks like you can get one off ebay for $130? I'd prefer not to make my car louder in any way, and as tempting as it is to punch a hole in it, i don't think i'd be happy with the smell/noise. do you know if the muffler/resonator might also need to be replaced?

I'll keep running the same test (specified and actual mass air in 3rd from 2000 rpm up to 4500 rpm) after each repair, to see if anything changes.
 

PakProtector

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Jan 5, 2014
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AnnArbor, MI
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Mk.4 and the Cummins-es
Thanks for putting this up. Going to have to check my BEW...I am going to suspect the MAF should any similar issue present itself.
cheers,
Douglas
 

wonneber

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2014 Jetta Sportwagen, 2014 Tiguan,2003 Jetta 261K Sold but not forgotten
Have you looked inside the intake manifold through the EGR valve (or removing the valve) to see if the intake is clogged?
Only costs an EGR o-ring & some time.
I would hold on the exhaust side for now.
 

runningallday

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Iowa City, IA
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1999.5 Jetta TDI manual
Have you looked inside the intake manifold through the EGR valve (or removing the valve) to see if the intake is clogged?
Only costs an EGR o-ring & some time.
I would hold on the exhaust side for now.
At first glance I didn't see anything significant. But I only peeked in the easily removed part on the left of the EGR. The MAF sensor should be arriving Friday, that's an easy swap. I also picked up a mityvac so I'll test my vacuum tubing before and after swapping it, haven't done the vacuum tubing yet either.

The car's still running fine, no CEL, just the discrepancy in specified vs actual mass air at full load >4k rpm.
 

runningallday

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Quick update: replaced the MAF sensor, no significant change besides reading slightly higher than it used to. Replaced all vacuum hoses except the one to the turbo, no change. Mityvac definitely showed a huge difference however - before, it would go to 15 mmHg pretty much instantly, and then fall 1 per second. After replacement, it took many many pumps to get it to 15 mmHg, and then it just held there without moving at all. Crazy.

youtube video: vacuum tubing before and after

I did take a picture of the inside of the EGR. It's more gummed up than I thought. Cleaning the EGR + intake will be the next step. I was doing some reading and someone mentioned an "EGR Cooler", is that important during the intake cleaning?
 

runningallday

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1999.5 Jetta TDI manual
some more MAF sensor info at idle:
EGR at 4.8% (closed?) - specified air mass is 255, actual is 535
EGR at 84.5% (open?) - specified air mass is 255, actual is 365

What do these numbers even mean?

Also I'm going to change the fuel filter tomorrow and test again after that.
 

CableJockey

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Apr 20, 2019
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South Dakota
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2002 Golf GLS
With an intake looking like that, I'm not surprised the engine is choking at high rpm.
Egr cooler is mounted to the rear of the intake...

Remove and clean your intake!
 

Curious Chris

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Pineview GA
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I had a MAF on my 2002 that would hit 850 under full throttle then wander down to 500. No Check Engine light as VW did not anticipate this failure mode
 

runningallday

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1999.5 Jetta TDI manual
I had a MAF on my 2002 that would hit 850 under full throttle then wander down to 500. No Check Engine light as VW did not anticipate this failure mode
I replaced my MAF sensor, no change. The previous MAF sensor seemed to underestimate the air flow by maybe 10-15% though? Yours changed after you swapped it out?
 

runningallday

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1999.5 Jetta TDI manual
Uh, that's nasty -- take it off and clean it.
I don't have a torch with me, and I think I need to heat up some nuts to remove it? I think when I visit some friends for Thanksgiving I can work on it then. I'm planning on boiling the intake manifold with some baking soda and dish detergent, that seems pretty safe and I anticipate it would work pretty well and be lower risk than burning it out.
 

runningallday

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One thing I just figured out - the air mass is measured in mg/str. Which means mg per stroke. Which means, the engine definitely uses up a lot more air when the rpm is higher. So this clogged intake is making more and more sense. Can't wait to get it done...
 

Nero Morg

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Some auto parts stores offer a parts cleaning service. You can ask around. Just be careful if you do the burn it out way. Don't need to melt your intake.
 

Genesis

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I don't have a torch with me, and I think I need to heat up some nuts to remove it? I think when I visit some friends for Thanksgiving I can work on it then. I'm planning on boiling the intake manifold with some baking soda and dish detergent, that seems pretty safe and I anticipate it would work pretty well and be lower risk than burning it out.
You wouldn't happen to have some diesel fuel laying around would you? :)

Take it off, soak in 5 gallon bucket full overnight, run brush down it, repeat, then use a few cans of brake cleaner on what's left. Not as fast as the "fire fire fire" method but it certainly works.

No real drama in getting it off most of the time other than reaching around things. You do need a new gasket.
 

Nero Morg

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Just don't soak it in a strong mixture of degreaser. It actually will start breaking down the aluminum. Had a buddy do that.
 

runningallday

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1999.5 Jetta TDI manual
You wouldn't happen to have some diesel fuel laying around would you? :)

Take it off, soak in 5 gallon bucket full overnight, run brush down it, repeat, then use a few cans of brake cleaner on what's left. Not as fast as the "fire fire fire" method but it certainly works.

No real drama in getting it off most of the time other than reaching around things. You do need a new gasket.
i got the 'intake manifold kit' from urotuning: https://www.urotuning.com/products/...-all-tdi-1993-2007-aaz?variant=32196527063095

yeah still sorting through all these different iterations, trying to figure out the best way. honestly i don't see why boiling it with Dawn detergent wouldn't be the safest option? another one that nobody mentions is just putting it in the oven at 500 degrees. that'll burn any carbon off it, and shouldn't melt any aluminum.

I read quite a few posts about people using a basic solution, and now im rethinking the baking soda idea, because it really does oxidize the aluminum. some people mentioned they had to use bigger bolts when reinstalling... i'd prefer not to do that lol. the easiest way is to just buy a used intake manifold off ebay, they're only $60, in comparison to all this time and effort im spending trying to figure out how to clean it, and then actually go through the process of cleaning it, that'd be a bargain! however i do enjoy working on my car :)
 

ToxicDoc

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You don't want to put this thing in the oven unless you want a significant amount of smoke and bad smelling fumes in your house. I've done 2 intakes and strongly recommend putting on a BBQ grill and using the hand torch method. Afterward I used a little oven cleaner for just a few minutes sprayed inside to help loosen remaining residue then rinsing it in a sink. A stick or screwdriver then finishes scraping the little bit that is left.
 

Genesis

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Sevier County TN
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'03 Jetta Wagon
If you put it in the oven what is in there will burn off all right -- and the smoke will likely set off your fire alarm in the house. Plus the smell, well.... I suspect you won't like that very much. What's in there is not just carbon; it's a sludge made out of oil vapor (from the PVC) and carbon (from the EGR) and it's NASTY.

I would not use alkaline or acid-based cleaners on it either (which means no Simple Green, etc) as both will etch aluminum and you want those nice sealing surfaces to still be nice. I've used the diesel soak method before with good success; it's just not fast.
 

STDOUBT

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I used the 5 gallon bucket method with a very mild degreaser after scraping out as much as I could with a sacrificial butter knife and a few nylon bottle brushes. I can guarantee this degreaser full strength will not harm aluminum:
 
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