vbittnv

Hi guys, I appreciate the help. This is my second post regarding a rough/vibration at idle for my 2002 a4 1.8t tip quattro. Needless to say, I have yet to solve it. I now have some gas mileage information and on my last tank of 90% city driving non agressively, I achieved 12.2 miles per gallon. It has 70,000 miles and I have done the following. I changed the timing belt, associated belt, water pump etc. For the vibration, I have:

1. Cleaned the MAF with cleaner. No change.
2. changed fuel filter. No change.
3. Changed plugs and gapped to .028". Old plugs looked clean and good, no fouling or wetness.
4. New air filter.
5. changed all fluids, 93 octane gas.
6. hooked up a vacuum gauge to the car. When I started ut up, the gauge registered 15 inches or so untill a pump kicked on about 5 seconds later, it went up to about 19 inches or so. Im not sure what this means. I might have a leak somewhere. I tried to listen and didn't reallly hear much.
7. I hear "crackling" from underneath the car and excessive "heat waves" from the exhaust near the downpipe. I know the cat and turbo are located under there, but the crackling seems very odd to me. My old a4 didnt make this noise. Even after light driving, this happen, not just after agressive driving. Cat issues perhaps?
8. coils seem ok, I pulled them out one by one when the car was running, and they all seemed good.

The rought idle happens when the car is in any gear, even park, but worst in D. The car otherwise is pretty goo, some slight vibration in the lower revs (3000 rpm and lower) but after 3000, the turbo kicks in and its really smooth. There are no check engine lights, no codes. It is 40 degrees or so in chicago and Im willing to do anything to fix this. I now have a vagcom so I can measure or test anything. Is there anything I can do with the vag to test the o2 sensors?, MAF, etc. Thanks for the long read, and any help would be great. Jeff

daveak05

Check/test oxygen sensor? Is the crackling sound with engine running? Because if so...well, your cat may be fried for one thing...turbo maybe too.

D-

vbittnv

Yup, the crackling is when the car is running.I hear it near the mufflers primilarily while running, but also under the front of the car. The noise sounds like the expansion/contraction noise on other cars sometimes but on this car, it seems much more frequent and louder. It crackles under the front also while its running. As far as the turbo, it seems fine, no weird noises, boosts perfectly fine, but perhaps you are right about the cat. What was your point about the o2 sensor? Thanks

daveak05

If o2 sensor bad, will get a rich mixture which will overheat exhaust system and cat, also result in very poor mileage, rough idle, etc. Maybe replace, it's simple procedure. Check for black exhaust too to verify a posible bad o2 sensor...or engine codes.

D-

bananas

If you replace your O2 sensor, don't buy it from the dealer as they will rip you off. I got mine from http://www.autopartswarehouse.com (OEM connector, etc).

It turned out that my O2 sensor wasn't going bad as I thought so I have my old one (still good, methinks) sitting in my garage.

vbittnv

their vacuum values especially at startup with what I'm seeing. Is there anything I could check with a vagcom? o2 readings perhaps. If I replace the o2's, should I replace both or one? I also seem to have smoke during running, like a whitish smoke, not oil or bluish, not black but still smells a little like gas. Any more insight would be great.

Thanks

daveak05

I don't think the 1.8T has 2 oxygen sensors, but I could be wrong. I don't know that a vacuum issue would cause what you're describing, at least not the crackling exhaust, whitish smoke(head gasket?), etc.

You could either research this more, or take it to a pro. If not dealer, then try some respected German auto repair shops.

D-

bananas

I watched my guage this morning: started at 15" and went to 20" once warm

vbittnv

Im sure my headgasket is fine as I'm not having any cooling issues, etc. Im leaning toward the o2s, cat, and Im going to replace the maf for good measure. Thanks guys.