Okay, here goes. I've done multiple searches but haven't quite found the exact same symptoms.

Background: 2002 A4, 3:15s, B&B Bullets, CAI, 77K on the odometer, never been tuned.

I've noticed I'm running a little rich as there is a lot of black residue on the exhaust tips every week when I wash it. The issue is, that once the car is warmed up (about 5 minutes after driving) the car will "buffet" at idle, intermittently. When it is warm outside and the car is running warmer, the buffeting is worse. When it gets real bad, the car will sometimes backfire when I take off from a stop, usually between shifts. If going uphill in "D" at low rpms it will also buffet. Under normal driving conditions there are no issues at all. It will get worse if the air/heat is being used, and it gets better if I put it in neutral at a stop. I AM NOT GETTING ANY CODES AT ALL. When it is really cold here it runs better longer...until the car warms up again!

I ran some Techron, thinking I got some bad gas at first. I've sprayed carb cleaner and wd-40 to clean out the check valve. I've cleaned the throttle body. Cleaned and re-oiled the intake.

Any ideas? I'm at my wits end, and I really don't want to start replacing components to troubleshoot that way! If it was O2 sensors I should get a code, same if I had a bad plug, right? I'm at your mercy oh great ones of C5 forum!


Thanks,
Glen

 

TTT....really? No one?

 

Here for you man!

I hate just replacing stuff to! Here is where I would start:

1. The car has TWO modes of controling the engine When it cold it runs in open loop and the Air/Fuel ratio (AFR) is controlled by a pre-programmed table.

2- Once it warms up, it goes into CLOSED LOOP and the O2 sensors control AFR. When you add a cold air intake (especially a front grill forced air intake like VARARAM), you change the way your MAF see the air flow and its output is different than the way the factory intended it to work.

3- Find some one who has some sort of data logging equipment. Something like EFI Live, HP Tuners or even someone who has an advanced code scanner like one that I have for my C6 (ACTRON 9180)

With data log software you can see what the LONG TERM FUEL TRIMS are doing and see how your O2 sensors are working. If the O2 sensors are LAZY and not switching, it will cause the engine to run poorly.
The LTFT should be slightly NEGATIVE (taking fuel away). They fine tune/ control the AFR +- 25% If they are way positive, the engine usually runs rich and poorly. I bet your fuel mileage sucks!

Data logging with any of the top tuning tools is FREE> You just need to find someone with tuning software tools to help you. If you lived near me, it would be done already!

Some things to check:

- Make sure that your air filter is clean and dry
- Pull a plug or two on each bank and inspect it. Make sure that there not fowled, worn and gapped correctly.
- Make sure that you do NOT have a vacuum leak. Check the PVC vacuum lines and the air ducting between the maf and TB.
- Are you using the C5 DIC DTC code reader to read your DTCs?
- CATs If your cats are clogged, it can cause problems simular to yours. If you have access to an infrared temp probe, read the CATs and see what temp they are operating at. How is your WOT power?

Hope this helps

Bill

 

Bill, as usual you are the man! Thanks for the tips. I have a friend with EFI live and we are going to take a drive tomorrow and check some things out. I will post my discoveries! Thank you again, sir!

 

Okay, ran EFI Live today at lunch and this is what I found:

At idle while in Drive, 1, 2, or 3 I get cylinder #2 misfires. The count goes up everytime I feel the car stumble as well. When in neutral or park I don't get misfire counts and when I'm driving I don't get them either. O2 readings look normal in that they do "the wave" and pretty much stay between .15 and .85 with the occasional drop below .15 but generally consistently between the normal .15 and .85. My initial thoughts, then, are to check plugs, wires and coil packs. Maybe move coil pack from 2 to 3 and see if misfires follow?

 

Sensors look text book! Swap out that coil, check the plug wire 250-750 OHMS (350 is normal) and just get some new AC Delco Irridum plugs.

IF,,,,,you need new wires, get the GM Performance Red wires. Best bang for the buck PERIOD. Call Gene Culley. He has them for right at 60 bucks

Keep us informed!!

 

I might just replace the wires anyway. Got some cheap Autozone ones last time and learned my lesson about getting what you pay for! Radio noise is horrible! Thanks, Bill. I'll update soon!

 

I had bot new coil packs from MSD and two of them went bad within a year. Put the old coils back on and haven't had a problem in over 8 months.

 

Would also guess plugs/wires...$65-70 and 1 1/2 labor

 

I just did plugs/wires 20K ago, and even then I wasn't going to do wires except when I got to plug #8 and tore the boot! Plugs/wires are easy, so if that's the fix, I'll be very happy. Even if I chase it down to coil pack, no problem. But today....today is all about Turkey!!

 

Sounds pretty consistent with a vacuum leak. If it's the top of the intake/throttle body gasket, etc. you can sometimes verify it by spraying around with carb cleaner with the engine idling. Idle will speed up if it sucks the solvent through a leak. If it is a leak, it's probably around the cylinders with the misfire code. A backfire usually is caused by a lean condition or unburned fuel igniting in the exhaust. Maybe slight possibility you have some bent pushrods. Swapping the coil packs to see if the misfire moves is a good idea too. Good luck, Michael.

 

Okay Bill, was finally able to get to it today..whew what a week!

Here's what we did in chronological order:
1) Removed plug wire off #2 and tested it. Ohms came in at just over 700 which is high, but they were cheap wires so not surprising.
2) Removed plug #2, looked good, rechecked gap and all was fine.
3) Swapped the #2 and #4 coil packs and just reconnected the plugs and wires to original locations.
4) Hooked up EFI Live, went for a test ride and as soon as it was warmed up a bit and I idled at a stop sign the cylinder #2 misfire PIDS started kicking in as usual.
5) Pulled the plug off the coil pack (#2) and fired it up - it showed a healthy spark and misfired as expected.
6) This time we pulled the #2 and #4 plugs AND wires, swapped them and went for a test drive again.......here's the kicker...you ready?

NOTHING HAPPENED!! Couldn't get a misfire no matter how hard I TRIED! I turned the heat on which usually makes it unbearable, I drove around a lot to get her nice and warm. Put it in 1st, 2d, and 3d at idle and NOTHING! I've taken three trips now and there is no indication of the problem whatsoever. The confusing thing is I replaced no components at all, just moved the coilpack, plug and wire from #2 to #4. ????

Oh, it between one of those steps I sprayed some carb cleaner in various places looking for a vacuum leak. Nope.

So now everything is fine, but I don't know why it is. Healing hands?

I think I'm going to get the new red wires and some new plugs and just do a tune anyway. Thanks for all the help, Bill.

very respectfully,
Glen

 

Glen

S T O P!!!!!!!!!!!!!! DO NOT TOUCH ANYTHING! It works!


I agree,,wires seem to be the most plasuable suspect! RED WIRES ARE YOUR FRIEND>

BC

Bill