Audi A6 revs fluctuating

celticexile

Hi Guys, posted in general section as well by mistake;
I'm new to Audi and my 2001 A6 1.8T whilst driving at a constant speed i notice the revs rise and fall slightly, although i dont notice the speed altering.....
Is this a problem raising its head....i've just purchased this with a good service history.

Thanks

haulagebasher

I assume it is a manual gearbos yeah? In that case do the revs increase when you put down the power but vehicle speed does not increase accordingly?Worst case is that your clutch is slipping and beginning to go. But hopefully it is just that your tachometer on the blink.

celticexile

I should have stated its the auto box. Under hard acceleration it changes sweetly and holds gear with no slipping as i can see.

Tropheus

You need to get a scan done with VAG-COM or VCDS to determine if there are any engine faults. It's really the only water to determine for sure what's wrong.

1jcdub

There is a small pvc breather valve under the inlet manifold that gets blocker with oil and needs to be replaced from my experience. The valve normally retails for less than 50 Euro. But get it checked out with someone with diagnostic equipment eg vcds.
This called only causes idle issues when the car gets up to temperature and is sitting at idle.

celticexile

Thanks very much guys, i will take it to guy today to run checks..ill keep updating.
Thanks once again

celticexile

Well just back from my local diagnostic check......no fault codes and everything ok, they recon the rev gauge is a little wonky.

Thanks again guys

unit 1

I had a 1999 version of what sounds like your car, with what sounds like a similar problem.
I never fixed it as I traded it in. The garage where I traded it passed it on in the trade and coincidentally I was in the garage it was passed onto some time later. I'm not a mechanic but from memory I was speaking to the owner and he remembered my car and said it had a problem that the audi garage could not fix, but they figured it out. I seem to remember him saying that there was a tiny crack in the fuel line to one of the injectors and this was causing the revs to "hunt".
Hope this makes sense.

Tropheus

celticexile wrote: »

Well just back from my local diagnostic check......no fault codes and everything ok, they recon the rev gauge is a little wonky.

Thanks again guys

You should be able to see the revs in VCDS which would help identify if it's the rev counter or not.

haulagebasher

Is it doing this fluctuating as you are driving along or is it just while the engine is ticking over. If it is just at tick over it could be the idle control stepper motor set up or the trottle position potentiometer.

I would have thought all injectors are served by a fuel rail rather than individual pipes. And even if one of these was cracked it would leak fuel everywhere due to the high pressure. I think what that refers to is a leak in the pipe from the tank causing it to draw air, which could result in lumpy running.

Could be a vacuum leak also so check all the vacuum pipes. In addition it could potentially be the manifold gasket letting air leak in

In more general terms, in basic engines at least, hunting is a classic sign of fuel starvation in as the engine dies the governor (ICSM) in your case attempts to compensate by opeing the throttle. This might not neccesarily show up a fault code as the system is doing what it is supposed to do. You should look into fuel supply problems too - check or replace your fuel filter and check and clean the fuel tank gauze. If the diagnostic scantool can give you a readout of the fuel rail pressure at idle and at load you should check this against specification values.

unit 1

haulagebasher wrote: »

Is it doing this fluctuating as you are driving along or is it just while the engine is ticking over. If it is just at tick over it could be the idle control stepper motor set up or the trottle position potentiometer.

I would have thought all injectors are served by a fuel rail rather than individual pipes. And even if one of these was cracked it would leak fuel everywhere due to the high pressure. I think what that refers to is a leak in the pipe from the tank causing it to draw air, which could result in lumpy running.

Could be a vacuum leak also so check all the vacuum pipes. In addition it could potentially be the manifold gasket letting air leak in

In more general terms, in basic engines at least, hunting is a classic sign of fuel starvation in as the engine dies the governor (ICSM) in your case attempts to compensate by opeing the throttle. This might not neccesarily show up a fault code as the system is doing what it is supposed to do. You should look into fuel supply problems too - check or replace your fuel filter and check and clean the fuel tank gauze. If the diagnostic scantool can give you a readout of the fuel rail pressure at idle and at load you should check this against specification values.

On mature reflection I do seem to remember the garage owner mentioning vacuam.

celticexile

Thanks Guys,
I'll go back tomorrow and ask him this...

celticexile

Gone on holls so i will get it checked again when he returns...

Media999

Crankcase breather pipe can crack and need replacing on these.

AzureAuto

Maybe check for small vacuum leaks from perished rubber breather hoses, pCV valve casing. This might be causing the "hunting" sympton you are describing. Spary around the intake tract with Wd40 or siila and note if the engine revs increase, if so there's your man. Throttle body reset may also ned to be done especially if it has been cleaned without being reset by VCDS etc.

Media999

Scan with VCDS first anyway and see if it comes up saying theres an air leak.

Check the corrugated pipes first. They crack all the time on the A6. Same with BMWs of the age.

It could be a little tiny crack that you would barely notice but as its corrugated it might expand as car is revving or engine is slightly moving. Years of hot fumes going through them pipes make them brittle. Some of them pipes are an absolute pain in the arse to get near aswell.

The replacements Audi supply arent corrugated anymore. This is why.

Replace pipe. Clean throttle body. Recalibrate throttle body with VCDS.

celticexile

Thanks to you all for replies, obviously talented guys...i will take it up to Daltons as rec by members, to check it out as i'm only so-so on mechs