Hi, i have an Audi 80 2.0E petrol, from 1994 in good condition with 230,000kM however it has since a few years had an intermittent fault that recently has become quite frequent.

The engine RPM are oscillating +-200rpm when the engine idles or is at low (under 3000rpm) mostly. So, one has a very unsmooth sensation as the engine and car jolts due to the sudden increase/decrease in RPM.

I thought this might be due to an old/blocked fuel-pump because the car didnt start recently and only restarted when the breakdown person tapped the fuel tank/pump with a hammer to "unblock" it.. We replaced the fuel-pump but the oscillating RPM issue still persists.

Can anyone advise? The really strange point is that the issue only happens about 30% of the time... and suddenly appears/dissappears with no real pattern/reason. You can start the engine, it then runs unsmoothly and after few minutes it works fine. The reverse also applies.

 

Though I don't think there's anything majorly wrong here, I recommend that you do a compression test first. Compression issues are always felt the most at low RPM's, and also are stronger when the engine is cool.

Next, I would thoroughly clean the area around the fuel distribution module (or whatever the name of it is...) under the hood. The part I'm talking about is basically a box that functions as a splitter to supply each of your injectors with fuel.

Follow your fuel lines up from your injectors. They will all four come together at this box that I'm talking about. What you're looking for is the fuel pressure check valve. I think it's mounted on the side of that box, with a fuel line into it from the gas tank (fuel pump/filter) and a fuel line running out of it (to the box with all the metal hoses).

It sounds like you have a chunk of something that is holding the valve slightly open. If it's unable to close fully, there will be low fuel pressure at start-up causing a long delay until the engine fires (which is how I understand the explanation of your issue) It will also cause the engine to run poorly at low RPM, because it can't close enough to regulate the high fuel pressure coming from the fuel pump.

If I'm right, you're going to have dark spark-plugs, evidence of running rich. You'l be able to check the plugs easily, since you have to remove them in order to do the compression test, which is what I recommended as your first step.

Good luck. And please post your progress to help your fellow members.
LG

 

Hi LG,
many many thanks for the quick advice... I'm fairly hands off when it comes to car maintenance (depend on a garage most of the time!) However this problem has eluded both an Audi and independent garage over the years (probably because it was very intermittent and only showed up 1 in 10 or 20 drives... now however its v.regular).

Anyway, I will attempt to follow your advice and identify/check the parts you mention... Will repost if i get stuck.
cheers
Peter

 

The problem is with your air idle control valve. If you follow the duct from the fuel distributor to where it connects to the intake manifold you will see a cylindrical module attached to the duct by the manifold itself. It will connect to the duct on one side and have a hose coming off the bottom and a plug in wire connector on the other side. You can try spraying it out thoroughly with carb cleaner with the part removed. If this doesn't help I've found replacements at the wrecking yard cheap. This is the part that smooths your idle though, focus on this.