Jerky transition from idle to part-throttle

[Munk]

Hi
I'm new to this forum but thought I'd give it a try.

In my '89 M20b20 (Motronic 1.3) I have an annoying problem with the transition from idle, to part throttle.
Engine idles just fine, but the moment my TPS clicks off the idle-switch, the engine revs up to 1300-1500rpm. In other words - it's either 7-800rpm or 1300-1500rpm - Nothing in between.
This makes for an annoyingly jerky ride at low speeds, where you only just touch the throttle ever so slightly.

Things I've tried so far:

Cleaned up the ICV with brake cleaner. (3-pin plug T-type) It rotates freely, and opens/closes all the way, when tested with a 9V battery.

Replaced both coolant temperature sensors (blue and brown)

Replaced oxygen sensor in the exhaust

Replaced vacuum hoses (no air-leaks when pressurized)

Cleaned out the throttle body.

Replaced Air flow meter.

Replaced the ECU

Deleted the carbon canister, and plugged the hose (did this as I replaced the fuel tank recently)

Cleaned relevant electrical plugs with contact cleaner (none had any rust/corrosion anyway)

Tried different adjustment of the TPS.

NONE of the above have had ANY effect on my problem.
I suspect that my ICV isn't getting the right signal/voltage, but don't really know what to do at this point.

I've had the car since 2008, and daily driven it until 2 years ago. The problem have been there for as long as I can recall.

Any inputs?

Kind regards:
Niels Munk, Denmark

[earthwormjim]

Have you tried adjusting the throttle butterfly valve itself? You can adjust how much air it allows to leak by when in the idle position. Perhaps it allows too much air to bypass through, thus when you barely touch the throttle, the butterfly allows too much air through for the engine to operate at RPMs between 800-1300 rpm.

[Munk]

Have you tried adjusting the throttle butterfly valve itself? You can adjust how much air it allows to leak by when in the idle position. Perhaps it allows too much air to bypass through, thus when you barely touch the throttle, the butterfly allows too much air through for the engine to operate at RPMs between 800-1300 rpm.

The butterfly valve closes shut when in idle position as it is. The adjustment option you are refering to, is the little M5 screw secured with loctite, on top of the throttle body, right?

[earthwormjim]

The butterfly valve closes shut when in idle position as it is. The adjustment option you are refering to, is the little M5 screw secured with loctite, on top of the throttle body, right?

Yes, it's secured by thread locker from the factory. The butterfly valve is adjusted from the factory to allow a small amount of air through when at idle.


After replacing my AFM, cleaning the throttle body, and fixing my various vacuum leaks, I found I had to adjust the screw slightly to maintain a smooth idle. Since you seem to have replaced everything in the engine when it comes to idle and low RPM, that screw is about all I can think of.

Have you also changed the crank position sensor?

[ak-]

Test the TPS with a multimeter

[Munk]

Have you also changed the crank position sensor?

I replaced that sensor about 6 years ago, as it suddenly failed, and left me stranded :-(
I'm not really sure if the problem was there before or not.

[Munk]

Test the TPS with a multimeter

I've tried to attach a picture of my TPS before and after cleaning.
I also resoldered the internal connections on the full-throttle switch as the were flakey.

My TPS should be good, at least the idle-switch wich is the only one of the switches in play here?

[325e '87]

Is this a automatic transmission car? Two things that can affect that rpm range are air conditioning and an auto trans. I don’t know how the ecu handles the loads of these two components but my idle speed can be a little funky when my air conditioning is on.

[ak-]

I've tried to attach a picture of my TPS before and after cleaning.
I also resoldered the internal connections on the full-throttle switch as the were flakey.

My TPS should be good, at least the idle-switch wich is the only one of the switches in play here?

Nice! Test it anyways. I bought a new one from BMW and it failed in 2 years causing weird problems.

[Munk]

Is this a automatic transmission car? Two things that can affect that rpm range are air conditioning and an auto trans. I don’t know how the ecu handles the loads of these two components but my idle speed can be a little funky when my air conditioning is on.

It's a manual transmission & no Air conditioning :-/

[Munk]

Nice! Test it anyways. I bought a new one from BMW and it failed in 2 years causing weird problems.

Both switches tests fine for continuity when tested with multimeter, though when I cleaned it, it took some elbow grease to get the full-throttle switch working again.

I might try and replace it with a new version, just for the sakes of it.