Just got my 99 A4 back from shop and they can't figure it out. They are leaning towards a coil pack though. It's hesitating a bit when accelerating but no check engine light is on. Every now and then it will flash, less than a second but that's rare. Seems to hesitate more when it's warmed up. If I don't drive it for a week I can go a day or two with none at all. Any ideas on how to diag without a code?

 

I don't know what they did, but if the battery was disconnected while it was there, the car may need a throttle body alignment. Disconnect the battery again for a half hour or so, then hook it back up and turn the key on without starting the engine. Listen under the hood for a humming noise. That's the throttle resetting. Once it stops, it's done.

This kind of thing can be caused if the TBA isn't done, and it's worth doing it. It may not fix it but it's worth a shot since it's easy and quick.

 

wow thank you for teaching me somthing imthedevil!

 

Definitely try the TBA, that can make a big difference in a hesitation. It could still be a coil though. If the TBA doesn't seem to fix the issue, pull into a garage, take off the plastic engine cover, start it, and turn out the light and just let it idle. Watch the coil pack area and listen carefully and see if you can visually see a spark arcing anywhere at any of the coils, before it makes contact with the ignition wire. If there is an arc, when you see it you'll hear a distinct tick that isn't synchronous with the normal lifter tick and the normal firing of the spark plug. Surely you've heard the sound made when you shock someone with a static electricity discharge after rubbing your socks on carpet and proceeding to make contact with another person or metal. Thats the sound of the spark arcing, it will likely be closely followed by the stutter sound of the idle dropping for that split second.

If this is the case, you're likely looking at a common failure of the rubber insulation of the coil packs. It gets a hairline fracture in it right at the point where the wire snaps into the coil and this weak spot causes the several thousand volt jolt of energy to arc through the insulation and thus not shoot down the wire to the spark plug as designed. You can temporarily remedy this by locating the spot it is arcing through and globbing some electrical RTV on the area, but it is more advised to just replace the coil pack.

Also, I just realized that you didn't specify what motor you have. What I just told you is based on the V6 setup that has a coil pack harness similar to a distributor cap on older cars. If you're running a 1.8t, the coil packs sit right on top of each individual cylinder and will likely not follow the same diagnosis as I just described.

 

well if it's fine when you leave the car sitting for a few days, i'm beginning to think it may be the ignition control module starting to go out. it likes to fail when it gets hot.

 

Mmm, good thought hiwords, I forgot about that hiccup.

 

what years does the TBA trick work? all years or only face lift? i have a 96 12v.. mine like to act cooky at times.. rather annoying too

 

Sorry for not clarifying the engine. It is a 1.8t. It has been doing this for a bit and that's why I took it into the shop. They couldn't get a definitive read on the failure. I think I might just replace the ICM and coil packs myself and see if it fixes it rather than paying diag fees again.

 

Phil: I can't guarantee it works for all pre facelift, but I know it works for mine, but I have the 30v engine so that could be the difference. Best way to find out would be to try it, just do what Devil said and see if you can hear the TB humming away as it realigns.

OP: I'd say start with the coils. To the best of my knowledge, 4 new coils is cheaper than one ICM, but I could be wrong, seeing as I don't have the 1.8t motor. If you have the money for both, it won't hurt to do both at the same time, but I find it useful to start with the cheapest replacement first when on a budget.

 

2000+ do not have the ICM. That's AEB only. Could be a vacuum leak. Could be bad spark plugs, a bad coil, a clogged catalytic converter, or any number of things, really.