Very rough idle, but, smooths out when MAF is unplugged (but new MAF did not fix it)

[whofarzed]

Hi,
I have a problem. I was driving and my car started jerking and power cut out (as if someone was cutting off the fuel, or cutting the ignition). Now it is undrivable with a very rough idle. Also running very rich with black smoke out the back. Engine vacuum is also down to 10 when usually it is at 20 at idle. I disconnected the MAF sensor and the idle smooths out and vacuum goes back to 20. So I figured for sure it must be the MAF sensor. However, I switched the MAF sensor to a back up one I had and it does the exact same thing. Once I connect the MAF the idle gets really rough and the car is undrivable. I highly doubt both my MAF sensors are broken. Not sure what else it could be? I am not getting any codes on VAG-COM.

I am going to try switching the ECU as I have a spare. Clearly it is something electrical. Maybe a solder joint on my chipped ECU cracked?

Also, it is a 98 AEB (with a K04 turbo), so there are not that many sensors.

Anybody have any other ideas?
Thanks

[FNK]

Did you inspect your spark plugs?
They can be fouled or the gap is too wide to create the spark!

[ricekikr]

Exact symptoms of a HUGE vacuum/boost leak.

[somebody5788]

Exact symptoms of a HUGE vacuum/boost leak.

This

[FCPEuro]

Hello, I would recommend pressure testing the system to check for a vacuum leak. A large leak will give you a rough idle even if the MAF is good, and by unplugging it the computer does not have a proper air reading so it runs off of the ECU's default settings, thus causing it to smooth out. Hope this helps!

[T0mat3]

Very weird as I havr that since2 week ago. Pretty sure mine was caused by a short from a coilpack wire on the turbos compressor housing but unplugging the maf fixes it except the idle mostly. Quite odd

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[1.eighTEE]

you have vag com.. run test on the maf check the g/p check for misfires check fuel trims get more info, i mean it can be all kinds of things
plugs, vacuum leak, bad ICM (although that would more then likely have an active misfire (flashing check engine light) most of the time.)

[T0mat3]

I converted to push down plugs so no more icm it has to be the maf in case boost leak or not, on idle, it should still push the same amount of air if its plugged in or not. And no codes popped up.

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[whofarzed]

Hello, I would recommend pressure testing the system to check for a vacuum leak. A large leak will give you a rough idle even if the MAF is good, and by unplugging it the computer does not have a proper air reading so it runs off of the ECU's default settings, thus causing it to smooth out. Hope this helps!

Ya I originally thought it was a big leak, just seemed strange that unplugging the MAF caused it to smooth out. I will pressure test the system. Thanks. Also, in the past I used to get this same symptom, although it would happen very infrequently and only for a second. The power would cut, car would jerk, then it would be fine. However, this time something definitely totally broke. In the past I thought it was a check valve getting stuck/blowing open, as I pressure tested the system and it would hold over 20psi no problem. I ended up inspecting all 3 check valves and they seemed fine.

Also, it's not the ICM, I tried swapping it with another one I had.

Thanks

[whofarzed]

Ya, I will get more information from VAG COM, however it seemed strange that it didn't throw a trouble code when it can barely idle. I would assume if it was something electrical a code would show up. I will pressure test for vacuum leaks and get back to you guys. However, I don't understand why unplugging the MAF will cause the vacuum reading on my gauge to go from 10 back down to 20?

[somebody5788]

I read the rest of your post and I guess I was a little pre mature. Check your CTS readings and IAT as well.

[whofarzed]

Ok, so I just hooked up the VAG Com and checked all the sensor readings. My problem still seems to point to the MAF sensor. All the other sensors seem to be showing normal values, however, the MAF is reading at 12-13 g/s at idle. However, as I said, I also tried an older MAF sensor (the original), that was working when I took it off a year ago, and it also reads around 12-13. Seems strange that both MAFs would be giving the same reading. Also, because of these MAF readings it is running extremely rich at idle with black smoke/soot coming out the exhaust.

Also, as I said before, I have no trouble codes. However, the second I unplug any of the sensors they show up and I get a code, including the MAF sensor. The idle also doesn't change when I unplug the AIT or Coolant temp sensor. I also tried unplugging the O2 sensors. Also, as I mentioned, with the MAF unplugged the idle smooths out and vacuum goes back to 20 (so I don't see how I can have a vacuum leak). You can even drive the car like this, although you have to shift before 3000. It doesn't seem to be misfiring or anything like that, as the idle is very smooth when the MAF is unplugged.

Is there any other sensor/ or etc. that could cause the MAF readings to be 12-13g/s at idle? If I remember correctly it should be around 3. It's just weird that both my used MAF's give the same readings and make it run rough. I would hate to buy another 300$ MAF if it isn't needed. I also looked at the MAF sensor wiring, however, I can't see this being the cause, as the second I unplug the MAF I get a code on VAG COM. If the wiring was bad, I am pretty sure I code would pop up and I am not getting any MAF codes (besides the high g/s readings at idle).

Thanks

[T0mat3]

Id say buy one and return it if it doesn't fix it, but can't return electrical stuff to vw or audi :s

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[ricekikr]

Also, as I mentioned, with the MAF unplugged the idle smooths out and vacuum goes back to 20 (so I don't see how I can have a vacuum leak).

This happened to me last week. After a WOT run, a pipe between the throttle got disconnected.

Although I wasn't able to log MAF voltage, I did notice the ff:

1. AFR (AEM UEGO) was way rich, richer than 11 afr @ idle, it only reads up to 11afr.

However, I don't understand why unplugging the MAF will cause the vacuum reading on my gauge to go from 10 back down to 20?

goes back to 20
(so I don't see how I can have a vacuum leak).

2. Vacuum was also low (Autometer boost/vacuum gauge) - Theory is ecu was trying to make it idle, thus opening the throttle and lowering vacuum reading, iirc it can open throttle up to 20+%. Try to step on the throttle a bit, your vacuum wont be at 20inhg.

3. Idle and AFRs got way better with the maf unplugged. Making it drivable even.

4. No codes either (vagcom)

With the MAF unplugged my AFRs got leaner, which probably means the MAF was reading something higher than 5g/s. It went from -11 AFR to around 13s @idle when I unplugged it.

And by vacuum/boost leak, I mean something in-between the throttle and MAF. The vacuum leak your talking about is between the throttle and head.

Just for diagnosis, try doing a WOT run (with MAF connected) and see if you build up any boost.

[Seerlah]

As suggested, pressure test your system. When this happened to me, one of the hoses from my dv popped off. Had to unplug the MAF to get it home and diagnose it properly, to find out that the dv hose was disconnected. If you have an AWM, I have a spare MAF I can sell you for $55 shipped. Only had roughly 5k before I replaced it, thinking it was faulty (ended up being my wastegate actuator shaft being bent causing my boost to act funny above 15psi).

[SmokeTheRear]

I agree with the huge boost leak this happened to my car last week while i was driving, and also when i had my blown Ko4 in my car before i changed my turbo for oem Ko4 and checked all my vacuum lines, what i would do is check your vacuum lines. After that clean your TB let it dry for 10 min in the sun put it back in and leave your key on Auxilary for 5 min to not press the gas pedal and let it re-adjust ur TB. If all that fails check your plugs.

[T0mat3]

When this happened to me, one of the hoses from my dv popped off.

who would of thought. The only way I can see this happening is that the side in the intake popping off, causing a leak. Because ppl running bov don't have this problem.



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[T0mat3]

When this happened to me, one of the hoses from my dv popped off.

who would of thought. The only way I can see this happening is that one of the sides popping off, causing a leak. Because ppl running bov don't have this problem.



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[whofarzed]

Yup, you guys were correct. The clamp on the bottom boost hose (that has the diverter valve attached to it) broke and the hose blew off the metal pipe that runs underneigth the car.

Thanks for the help.