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A quick MAF/sluggish mode query (UPDATE)

Hi all.

I know this problem has been discussed to death, but I have only seen a brief reference to my query in one FAQ document.

If I unplug my MAF and it makes no difference to performance, does it definately mean the MAF is faulty, or could the sluggishness still be caused by something else that overrides the symptoms of a faulty MAF?

I have run some diagnostics and the MAF expected/actual plots of block 003 are sometimes close, and sometimes far apart.

I also have an intermittent error code for the fuel temp sensor (G81), this jumps between around -29 degrees and the default -5.6 degrees. I would assume it is a bad connection, and the fact that the value is usually negative is down to my cheap software (eBay cable + VWtool)? My reasoning for this is that I understand the fuel temperature at usual loads should be around the 30 mark.

Please note that the sluggishness cannot be solved by stopping and restarting the engine. It seems okay at low revs, then on motorways (highways?) it takes ages to accelerate. The EGR valve and VNT actuator seem to move plenty, and I cleaned my intake manifold the other day.

Any help would be greatly received!

Jon
 

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No, something else that is restricting airflow like a clogged catalytic converter can cause the same symptoms. But unplugging the MAF is a quick and free test and most of the time the cat is fine and the MAF is bad. Some other things like bad turbo or bad ECU could also cause this problem.

Faulty fuel temp sensor won't make any noticeable difference in performance because I had one too that was traced to a chafing wire in the injection pump wire harness.
 

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Discussion Starter #4
Thanks for the quick replies.

One thing I didn't mention was that the car seems to chuck out a lot of burnt diesel fumes when idling (and possibly when driving!) but no smoke.

My requested/actual turbo pressures seem okay, with the actual values lagging about a second behind the requested, at all revs.
 

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Discussion Starter #5
Thought I'd continue my old thread rather than make a new one. I have replaced the MAF and it has made no difference.

My '98 Passat TDi suffers the following symptoms:

-Loss of power at medium-high revs. The car feels generally sluggish in all gears.
-Strong exhaust fumes on idle (also possibly while driving?)
-Presence of oil in the piping between the inlet side of the turbo, and the inlet manifold (through intercooler and both lengths of pipe)
-Some blue smoking when pulling hard in a low gear (cold, not sure if it occurs when the engine is hot)

However:

-Only fault code is the fuel temp sensor (G81)
-MAF sensor has been replaced with no improvement
-VNT actuator moves smoothly ~10mm
-Diagnostics show boost pressures (actual and requested) are close, max ~2000mb


Can a bust PCV cause poor performance? I read somewhere it will essentially cause a boost leak but it's connected to the inlet side of the turbo, which doesn't make sense to me. I'd guess that's why there is oil everywhere.

I should also mention that the throttle position in one of the data blocks (I forget which) only ever hit a maximum of 67%... that doesn't sound right to me?

Any help would be much appreciated. I can try and get some diag graphs up but I don't have access to a 32bit PC at the moment (I used VWtool).
 

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The CCV (very similar to PVC) will not cause poor performance. The engine is always pumping out positive crankcase pressure because the vacuum pump exhaust goes into the crankcase and beause diesels have high blowby. As you've found, it does put a bit of oil mist back into the intake.

The throttle position is suspicious but I'm not sure. Try this: with the car ON but engine not running, measure it through VCDS measuring blocks. I know it's there but I've never noticed how high it goes. It should sense all the range. In addition, the accelerator pedal has a few sensors on it, if it's bad it should throw error codes and throw you into a permanent high idle. The VW engineers were careful to avoid stuck accelerator pedal problems.
 

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Discussion Starter #7
unusual fix?

Wow, I have been playing with my diagnostic cable and have possibly found a fix.

I noticed the throttle position only went from 0 to 67% on data block 11, when moving the pedal through it's entire range. I removed the lower part of the dash to see if the pedal mount was bent or something, and there was a rubber bump-stop preventing the pedal from moving further. I removed this, the pedal now moves to give up to 90% throttle before it hits the bump-stop mount, and my car drives like I would expect a 110bhp car to drive (I can actually feel the turbo now!).

While this is good news, I wonder why the rubber bump stop was there? It is an audi/vw genuine part. The mount for the pedal assembly seems fine - no obvious damage.

Any thoughts would be appreciated.
 

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To prevent revving it to hard maybe, mine has the same thing but mine goes like a rocket even with it in place, it also ran good without the new injectors lol. I think removing it maybe working but it might be hiding the initial problem whatever it may be.:)
 

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Discussion Starter #9
I adjusted the pedal so it travels from 0-100%... what a difference! The car pulls like I expected it to now.

Under high load, full turbo and in 2nd gear, I get quite a lot of grey smoke from the exhaust. It seems this is generally normal judging by some forum posts I've seen, I guess it's pretty coked up having been driven up to 67% for a while.

Still can't get rid of the smell though. I can smell it whenever I'm stopped.

Thanks for the help chaps.
 

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Welcome to the forum. welcometomyturbodies

-Only fault code is the fuel temp sensor (G81)
That isn't the error code thats the component with the component number. What error code did your scanner show you?

Are you from Bath, UK?

If so add UK at the end as there can be Bath in loads of countries just the same as Birmingham and London are in the States?
 

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Discussion Starter #11
That isn't the error code thats the component with the component number. What error code did your scanner show you?
Yeah I was just referring to the component number, didn't write down the code at the time.

Good point, I'll correct my location!
 
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