M20B25 Cutting Out on Idle

[Eddie1990]

I have an M20B25 in my BMW E21, it has always been running a little rich with a slightly high idle, never got to the bottom of it.

However, yesterday I went for a good drive and everything was fine until it started cutting out on idle all of a sudden.

Through the throttle range it's fine, if a little lumpy, but when it drops to idle, it dies.

I've posted a video below of it starting, and then dying.

It's also running very very rich, with some blue and black smoke coming from the exhaust, it also sounds like it might possibly be misfiring, but that clears once the revs are brought up.

Video of idle issue:



Any help would be great, thanks.

[TPS]

ICV?

[Eddie1990]

ICV still buzzes?

Does that not necessarily indicate it's working?

[Eddie1990]

Cleaned out the ICV and took it for a drive. At first it seemed fine, but after 10 mins or so, it was cutting out again.

Below 2500rpm its lumpy too, like it's misfiring maybe?

As said, it cuts out only when the car is allowed to idle, so when I dip the clutch, or put it in neutral.

[Eddie1990]

As a test of the blue plug, I was going to take the car out and when it starts cutting out, unplug the blue plug and try the car again with it disconnected.

Under normal conditions, will the car run without the blue plug?

[Brianmoooore]

Car should start from cold without the blue plug connected, but should then deteriorate as the car warms up. At working temperature,it should not be happy at all.

[Eddie1990]

So that might explain my issues when the car is hot?

My other guess was that the coil has deteriorated and is suffering when hot?

[Eddie1990]

I've just checked with BMW and the blue plug CTS is NLA from BMW and no UK dealer has them.

What's best?

[jslot]

Have you checked the operation of your throttle position switch?

[Eddie1990]

I checked it a few months ago and it was fine.

It clicks when the throttle is moved, but I haven't checked the readings since this issue has presented itself.

[Eddie1990]

Okay just tested the TPS.

The guide on E30Wiki states:

"remove its plug and connect a resistance meter to pins 1 and 2. You should get a short circuit ( 0 Ohms) with the throttle closed, going open circuit with the throttle just off its stop. Pins 1 and 3 should be open circuit until the throttle is 2/3 open, when it should go short circuit."

I'm getting:

- Short circuit (0 Ohms) between pin 1 & 2 until the throttle comes off the stop.
- Open circuit between pins 1 & 3 all the time, no matter where the throttle is.
- Open circuit between pins 2 & 3 until the throttle is open 2/3 and then it goes short circuit.

So surely this is wrong? Can anyone confirm that the guide is correct?

I'm basing this on the following pin configuration:



Any speedy response would be super awesome

[jslot]

That sounds like its working fine.

[Brianmoooore]

That quote from the wiki sounds suspiciously like it was lifted from a post of mine a few years ago.
Your readings agree with the wiki in full - 1 and 3 are open circuit all the time because 1 and 2 become open circuit before 2 and 3 become short circuit.
There isn't likely to be anything wrong with the actual plug to the coolant temp. sensor. It's the sensor itself that can go faulty, and these are readily available, BUT it should be tested first AT THE ECU PINS. Details in the wiki.

[Eddie1990]

I'm on Motronic 1.3 I believe (sorry )

ECU code ends 073.

Did some tests today:

CTS

Cold: 2390 Ohms (ambient temperature about 18 degrees)
Warm: 250 Ohms

Both readings matched at the ECU.

CPS

520 Ohms

Coil

Primary Terminals: 0.5 Ohms
Terminal 15 to King: 5000 Ohms
Main HT lead to dizzy cap: 1900 Ohms

Also did a compression test:

Dry

Cyl 1: 150
Cyl 2: 150
Cyl 3: 150
Cyl 4: 155
Cyl 5: 150
Cyl 6: 150

Wet

Cyl 1: 190 (+40)
Cyl 2: 170 (+20)
Cyl 3: 185 (+35)
Cyl 4: 143 (-12)
Cyl 5: 135 (-15)
Cyl 6: 165 (+15)

Compression test done on cold engine at WOT.

Anyone got any suggestions based on all of this?

As a side note, I took it out and gave it a good drive and the fault appeared to have vanished, car runs okay, if a little rich still, however I'm dubious as to if it's actually sorted, as all I did was test everything and cleaned the TPS.

[Mitchen]

As a side note, I took it out and gave it a good drive and the fault appeared to have vanished, car runs okay, if a little rich still, however I'm dubious as to if it's actually sorted, as all I did was test everything and cleaned the TPS.[/i]

Did you let the engine warm up when doing this drive? Sounds very reminiscent of when my CPS went.

[Eddie1990]

Yep, gave it a good drive including getting it up to full temperature.