320i '95 M52: Uneven acceleration and fuel consumption gauge rocking

[dr.e36]

Hello. I'll try my luck on this forum also

I posted a similar post under the O2 sensor replacement DIY 15 mins ago. Didn't see this technical forum until just now. Sorry for that.

I need your help. I have a problem. My e36 320i M52B20 (oct.'95) (275000 kilometers) accelerates unevenly most of the time (about 95%) and during this the fuel consumption gauge sways - rocks up and down evenly about once every second or less for a few liters (metric unit - about two pints) in each direction.
(I am not from an English speaking country so I hope "fuel consumption gauge" is the correct phrase - the thing that shows fuel consumption under the RPM meter).

Of course, the gas pedal is kept perfectly still during this, so the problem isn't due to my leg shaking or anything like that

The previous owner of the car and I studied the problem quite extensively. The phenomenon is not due to any external factor, e.g. wind, road. It happens in all situations. It does not matter whether the car is cold or at the working temperature (although when I have been driving it for 1 hour or so, the acceleration is higher and the fuel consumption much lower, but that is probably normal and due to engine materials being evenly hot and properly dilated)

1. It happens when the gas pedal is depressed from about 1/4 to 3/4 of the way. At low acceleration and at full throttle the acceleration is even, however I think it is not high enough for this engine.
2. The biggest differences in acceleration are seen between 2000 and 4000 RPMs. Above that it is much less pronounced.

Follow this link to see the video, made by the previous owner of the car around 2 years ago:

During this experiment, he held his foot perfectly still on the accelerator. The rocking of the fuel consumption gauge can bee seen above 60 kilometers per hour (above 2000 RPMs, driving in the fourth gear):
bmw Pictures, bmw Images, bmw Photos, bmw Videos - Video - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting


3. While driving at a steady speed and same gas pedal position (again on a straight road and not on a hill or anything), the car very finely but distinctively accelerates and decelerates, again the consumption gauge mirroring the effect.

This may be important, however it is a recent issue and was not present for almost two years since the problem was first seen: A couple of times when I started the car, one cylinder, I think, didn't work.

This also might be important: Sometimes when I press the pedal, the car doesn't immediately accelerate nearly as fast as it does other times, but waits and then slowly starts to accelerate.

I don't know if it is related or if it is normal for this engine, but below 3000 RPMs the acceleration is in my opinion very low. Above this barrier, the car is a rocket compared to below it.

I have to ask the previous owner, if he replaced something and after this he noticed the problem or if it couldn't have been related to anything.

- I replaced the rubber hose that connects the airflow meter with whatever is behind it.
- The car didn't show any problems when my mechanic (who is famous for his technical expertise regarding these cars) hooked it up on his tester. The spark plugs looked normal and the cylinders didn't show any unusual deviation from normal data.
- The airflow meter was replaced by the previous owner. I don't know if he bought a new one or a used one. He said, that when he replaced it, the problem was almost fixed. There was a significant improvement with the acceleration. However, slowly the problem started to present again.

My mechanic thinks that the problem could be that somewhere there is air going in through some crack. That would cause the vacuum to drop and would cause the problem in his opinion.

Any comments and ideas will be greatly appreciated! Thank you in advance!

Bye

[JhwShark]

Not that familiar with your MY...but surging/hunting cyclically...
Since you indicate one cylinder may not be working on starting I would consider checking the FI seals but first pull each plug and tell us what you see is there consistent/uniform appearance? Any wet with gas, oily, or sooty?? Take a pic and post...
I would check all vacuum hoses and replace most due to hi KM's.

You could also check your ignition related sensors (temp, tps/cps, O2...) for their values, if degraded may be a factor at mid-range.

Take a compression reading as well, wet and dry, while the plugs are out and post the result.

[dr.e36]

OK, thanks! I will tell my mechanic. I don't have any of the necessary tools. If the problem is not solved by anything you suggested, I'll come back.

Thanks! Bye