hesitation from idle and low idle speed

[Morgenster]

Hi,

I tried hijacking an old thread to get some answers, but that didn't prove useful. So I'm trying a new one.

I recently bought a 1991 520i with M50 engine which has 260k km on it.
It's the best car I've ever driven (not that I've driven many, but still).
It's fitted with an LPG aspiration system. Where I live that's a serious bonus considering that premium fuel will cost you about 1,4 euro/liter and LPG is at 0,4 euro/liter.

I noticed something a little bothersome though:
When starting the car cold, idle will go down from 1,2k RPM to about 800.
After warm-up idle drops to 600 or less and at that point I sometimes get slight hesitation when I rev up. Tapping the accelarator doesn't give a quick rev, but a slight slowdown. holding throttle for more than a second gives me a steady rev.

In warm idle there's some misfires, which disappear at higher revs. Spark plugs have just been replaced.
For what it's worth, my mechanic said that the AFM is relatively new and shouldn't give problems.

I started to suspect the Throttle Position Sensor. Bentley says the range in resistance should be from 1K to 4K ohms. I get 2K to 5,6K, but without apparent flat spots.

Now here's my theory:
With 2K resistance at idle could the TPS be fooling the ECM into thinking that throttle is slightly open therefore not opening the ICV wide enough, subsequently causing slight hesitation when opening her up? Could it also be the reason why I don't feel a difference in pull between half-open and wide open throttle?

The reason I ask is because I don't really know the tolerances on the TPS measurements and I don't feel like replacing a 60Euro potentiometer (that's just wrong for a simple pot sensor) if it's not malfunctioning.

ANY input would be wildly appreciated since none of the other forums I visited had anything to say on this.

PS: I've been lurking a little to see how things work around here and I'm excited to see so many e34 enthousiasts in one board!

[DaveVoorhis]

I started to suspect the Throttle Position Sensor. Bentley says the range in resistance should be from 1K to 4K ohms. I get 2K to 5,6K, but without apparent flat spots.

That could be the problem. I'd definitely try a new TPS.

[BillionPa]

also clean the ICV with carb cleaner, and make sure the ignition coil is at the right spec too.

air filter looking good?

[DaveVoorhis]

make sure the ignition coil is at the right spec too.

He said he's got an M50. There are six coils!

[Morgenster]

Ignition doesn't seem to be the problem. Coils are OK.
Car runs rich in idle, you can smell petrol.
ICV got a dose of WD40 and idle doesn't fluctuate.
Maybe carbon buildup in the ICV vacuum lines?

[DaveVoorhis]

Ignition doesn't seem to be the problem. Coils are OK.
Car runs rich in idle, you can smell petrol.
ICV got a dose of WD40 and idle doesn't fluctuate.
Maybe carbon buildup in the ICV vacuum lines?

Tried a new TPS?

[632 Regal]

air leak in the intake system, find it, fix it, reboot the computer.

[Dazzanx]

Ignition doesn't seem to be the problem. Coils are OK.
Car runs rich in idle, you can smell petrol.
ICV got a dose of WD40 and idle doesn't fluctuate.
Maybe carbon buildup in the ICV vacuum lines?

I too have an E34 520I with the M50 block and have experienced the same problem. As this is only my second BMW, the first being a 320I touring, I was quick to spend a lot of money on the verious induction sensors, only to find that the problem was still there.

If only I had found this forum earlier I could have saved £££'s simply by checking for air intake leaks and cleaning the ICV. Also, cleaning the throttle body and butterfly plate made a better improvment.

Check this page out for more info if you hav'nt already.

http://www.bmwe34.net/e34main/mainte...Adjustment.htm

[ILoveMPower]

I have a similar problem on my M30, I believe it's due to a tiny crack in my intake boot.

[Morgenster]

My idle has improved in the past weeks simply by driving the car hard and long... The coils haven't been changed yet and now that the ride has improved. I doubt that that was the problem. So most likely filthy components were the culprit.

Cold start will sometimes give me some slight misfire, but that's not too frequent.