I have a stock 98 A4 2.8 Avant. Automatic transmission. Original owner, 70k miles.

For the past year I have had problems with the car jerking/lurching primarily when running the AC, and especially when it is really hot. Now the car is occasionally doing the same thing even with AC turned off.

I feel the jerk/lurch generally when hitting the accelerator. However, at times even when the car is at idle it will sometimes feel like a large load was placed on the engine and the car feels like someone bumped into it.

Happens mainly on hot days, and mainly when the AC is on...
Some days it runs great.

Any ideas what the problem might be?
BTW, the check engine light does not come on and yes the bulb works!
Thanks!

 

I don't care if the check engine light is on, take the car to autozone and have the codes pulled, just because the light isn't on doesn't mean it doesn't have any sporatic codes.
The problems can range from a bad a/c compressor all the way to an transmission issue.

 

I will try to get to an autozone soon. Was reading in another thread that someone had very similar problem and it was fixt with a new check valve and coils. There was some mention of the ICM in some threads--I don't even know what that is. Total NOOB

 

A faulty ICM (ignition control module) would most likely cause a flashing check engine light with a narly misfire.
A vacuum check valve is possible, but it would most likely also cause a check engine light and the engine would run like poo all the time.
I would say its something mechanical like carbon build up on the intake valves or throttle body, but those won't cause any fault codes at all. So if you don't have any stored codes in the ECM then thats the route we are gonna take.

 

I had jerking while accelerating and it was the air mass flow sensor... you can try to unplug it. This will make the CEL go on but while it is disconnected, you can drive it and see if it improves anything. (beware of the much lower MPGs)
Mine didn't jerk at all after I unplugged it and I've already ordered a new one.

Remember, you will have a CEL that you will need to reset, if you try this. It could be something else, this is just a cheap and simple idea.

 

Unplugging the mass air flow sensor could cover up a symptom from a different problem source related to fuel/air/ignition. If you don't have a lean/rich related maf fault, there is really no reason to consider it. I think even autozone's fancy scanners can give output readings for the maf.

There are a few things that will put load on the engine and can cause what you describe. AC compressor going on/off, secondary air pump going on/off. Some people will experience your symptoms when simply putting their windows up/down once it reaches the stopping point. When you say it does this even with the AC off, are you sure it's off? You need to have the hvac unit either turned off completely or turned to ECON to keep the compressor from coming on.

One time I had my AC belt tensioned too tight and it caused a load/surging feeling when it turned on until I losened the tension a bit.

 

I'm sure the AC was off (all fan's off and the system powered down) and I was surprised it was still bucking. It feels like more than just a load hitting the engine--it is a pretty good jolt. One time I thought someone tapped my bumper at a light!

 

sounds like what was happening to me when i was over boosting

 

truthfully it sounds like you have a slipping transmission. my friends integra would do basically the same thing. when he jumped on the gas the engine would rev, and then slam in a gear and you would get a jolt like someone ran into him. i think it ended up being the torque converter but since he totaled it before he could fix it, we never actually found out what it was.

 

Of course I've wondered about the transmission being an issue, but the original owner and knowing I've done all the required maintenance and have never driven the car hard or abused it I would find it hard to believe the tranny would start to die at 70k miles.

If that were the case I'd have to think about totaling my car also!