clancy

My kid's 2001 A4 2.8 stick shift just starting bucking while driving in town. I am getting his poor description over the phone. He also said it would die at traffic lights with the clutch in, but restart easily. No CEL. Car was driving fine the day before, only change was a fueling.

This sounds like a fuel supply issue to me??? But the fuel pump was recently replaced by my trusted shop. Fuel filter is nearly new as well. I know it is not much to go on. I will take my VCDS to see what I can learn as well as a few tools. Any suggestions on what to look for? And what tools to bring? He is an hour away so I will be working in his apartment parking lot. I am not really a great DIY. I do basics and take it to my trusted shop for anything more. But I would hate to pay to tow this the 40 miles for something I should have seen and been able to do myself. Also replaced the PCV hose assembly about 6 months ago. It is about due for a timing belt service, been about 60,000 miles. But I assume this would be an engine wrecker if it broke. Could it jump a tooth +/- and cause this?

Any advice??? thanks

G_claudz

Without Vag-Com you are going to fighting a war of a hit or a miss, you need to get that car scanned, that will point you to the right direction. However, your symptoms suggest an air/fuel mixture issue, so try and see if you are getting any MAF sensor errors like signal too low or o2 sensor errors like internal resistance too large or too low. I am not sure if the 2.8's run on MAP sensors too, but if they do try and see if it does not throw a code for that too.

G_claudz

If the timing belt broken, he would know it and the car would not fire up again, if it jumped a tooth that would just mess with the timing.

clancy

Yep,agree. But I doubt oxy sensor issue would be this drastic. I will scan with my vag com when I get there. Just trying to anticipate what to bring with me. Would MAF issue cause bucking? How about coils? Is jumping the timing on a worn belt common?

Volhv

Bad spark plugs can cause bucking. Check the gaps, and look for fouling.

An old belt could loose some its teeth, but they don't generally stretch enough to jump teeth.

clancy

Sent him to Auto Zone to get a read on any codes. None???? Assuming their person did it correctly. I was hoping there might be something to give me a hint on what to expect when I get there tomorrow. If it had been spark plugs or coils or ignition module then I would have expected some codes to show. Maybe it is just some air hose or vacuum hose issue.

I will report back.

Articus

I don't think the autozone scanners are able to pick up ALL the stored codes from every module. Only a VCDS will.

clancy

Checked the car yesterday. Only codes were for a random miss in a couple of cylinders. Cleared them and watched for misfires in real time. It only had a couple. Checked everything else and saw nothing of concern on VCDS. Drove the car and it ran fine. Got it hot and high rev'd in second gear several times. Ran fine. Checked for misses again at idle, nothing. During a visual check of hoses etc. found one loose connection on one of the crankcase vent hoses. Fixed it. Pronounced the car fine. Not sure that was the issue, but it ran well, so what can I do? Will see if issue returns. Otherwise I am calling it a random event.
Thanks for all the advice.

carkarl

As mentioned above, fouled plugs can cause misfires. They can also clean themselves if the cylinder fires enough times and heats up enough. Might be worth asking the kid whether he ran the engine very briefly to move the car when it was cold, before all this happened?

I had a similar experience in my b5 over a decade ago, but going so far as a blinking CEL when I started it. Eventually, my mechanic and I decided that I'd fouled the plugs the day before by starting the engine and stopping it too quickly to swap parking spots with my wife. Ever since, I always let the engine run for at least a couple minutes before turning it off, and have never had this particular problem again.

Because of the blinking CEL, I called a tow truck but he said we'd have to pull the car out of the underground garage before he could get it on his flatbed. I started it and tried to drive gently and quickly out of the garage to minimize the danger to the engine. By the time I got outside, the engine was running smoothly and the CEL stopped blinking. Since it wasn't blinking anymore, I figured I could drive it to my mechanic instead of towing, but then the CEL cleared before I even got to the main road.

Volhv

I'd change the plugs... who knows how long they have been in there. Gaps are probably big enough to stick your foot through! It may run fine when babied, but if he sticks his foot in it up hill............. I'd check for misfires under load.