1987 engine cut out issue --All Bentley DME 1.1 tests performed

[mrgesq]

I have a significant engine cut out problem. I have searched and read virtually all of the posts describing this problem but have not found a fix. I have a 1987 E 30 convertible 5 speed with a later engine, I believe from 1989. (PO didn’t change the timing belt) Car starts and idles fine, maybe a little high at 1000 rpm. When driving, the engine cuts out momentarily and randomly. It’s only for a second or less. If driving, the momentum of the vehicle keeps it going because it cuts out for a second or less, then keeps going. When it happens, the check engine light comes on, the MPG gauge pegs all the way to the left at over 45 mpg and the tachometer drops, but jumps right back and keeps going. It’s like someone is sitting next to me with a master engine switch flicking it back and forth randomly. It’s definitely electrical, but I cannot find the source. Very annoying.

Here is what I have done: new main and fuel pump relays; new cap and rotor; new coil; new crank sensor; used known good Motronic 1.1 DME, cleaned all pins in diagnostic port with dialectic cleaner.

I performed ALL of the Motronic 1.1 Control Unit electrical tests on page 49 of the Bentley Manual. Three tests were out of range:

1. No continuity in throttle switch at wide open (there is continuity at closed position and loses continuity when rotated open slightly, with audible click);

2. New Crank sensor (Beck Arnley) reads 874 ohms (at sensor cable terminals and DME). Bentley says should be 540 ohms +/- 54.

3. Idle air stabilizer valve reads 45 Ohms. Bentley says should be 40.

Would the wrong ohm output in the crank sensor cause the problem? What about a short in the throttle switch? It almost feels like it happens more when depressing the throttle. I am at a loss.

Any suggestions? Thanks in advance.

Mike

[ForcedFirebird]

Check the main relay socket. It's the three relays under the cover on the driver strut tower. If that relay loses power, the DME completely shuts off. We have to zip tie them in place in the endurance cars at work because they can jump out of contact easily (specially when a driver tries mowing the lawn in the race car).

If you have an 89 engine, the PO may very well have used the 89 harness. Check the injector plug under the throttle body (c191). The pins are often turning green/chalky.

Is the CPS old and the wire crusty? I have had many ~900ohm sensors that work fine. There was a batch several years ago that were reading 900, and would be dead out of the box - but that seems to have changed lately as many aftermarket sensors we install are good, even though they read at 900. We typically talk everyone into using OEM BMW sensors as they last the longest and the most well constructed. The plastic-cased sensors seem to go bad after just a couple of years, where many circa 1988 sensors are still working fine (with the exception of the plastic on the wires breaking from age). If it's an older sensor, have a friend wiggle the wire where it goes into the sensor while you ohm the wire. They like the break right where the wire enters the sensor.

With the throttle switch being open at WOT, you will just be lacking upper end power, has nothing to do with drive ability. If the idle switch is broken, you will have idle troubles since the DME "thinks" your foot is resting on the throttle.

[mrgesq]

Thanks for the response. The main relay and CPS are both brand new. The Relay seems tight in the socket and passed the Bentley test with an audible click. Plus, the cut out happens on smooth flat roads. No correlation to vibration.
The old CPS was frayed at the entry to the sensor so I thought for sure I found the problem. The new CPS is Beck Arnley, which I thought had a solid reputation.

[EbzyyE30]

Not sure if your describing what I was feeling in my 89 coupe 325i, I would be driving and as soon as I hit 3000-3500 rpm it was like my engine shut off and misfired horribly the check engine light would come on for a split second and then gone and then went back to driving normal. I was stressed on trying to find out what the problem was, I took apart my MAF, the idle control unit and and looked inside the throttle body and all components to find absolutely everything covered in blow-by. I cleaned everything and went for a drive and it was gone no more horrible shut off feeling. as stupid as it sounds it helped a lot so i'm just sharing my experience. (This also happened two more times as blow by was persistent because I think the car had been sitting for a while by the previous owner, Hasn't happened to me since)

[mrgesq]

Thanks for the response. I took the throttle body apart and it's pretty clean. So it the Intake. The Idle control unit was new last year. The throttle plate was out of adjustment and the throttle switch was not working at WOT so I searched for new Bosch Throttle Position switch. I could not believe the prices. The cheapest I found was at the dealer with the BMWCCA membership at $282. It's two micro-switches in a plastic case! He also said it's the last one in the US., which may be accurate. I could not find them anywhere new on the Internet.

I also replaced the Crank Position Sensor with a genuine BMW sensor with the correct 545 Ohms resistance, but it still cuts out once it warms up. It also seems like I can make it happen by rapidly pressing the accelerator. That's why I thought it might be the throttle position switch issue. That's on order now so we'll see. I have my doubts that's it, but we'll see.

[mrgesq]

Looks like I got it solved. The last thing I did was to replace the Throttle position sensor, adjust the throttle butterfly to spec and remove and clean the MAF sensor with dialectic spray cleaner. I also cleaned the contacts on the MAF sensor connector. Took it for a 20 minute drive and ran great with no cut outs. Idles a little low at 600 rpm but otherwise good. My guess is that the Throttle position sensor was sending an intermittent false idle signal because the little protrusion that sticks out on the idle side microswitch (inside the sensor) was depressed and would not pop back out when the throttle was moved off idle. Only other thing I can guess is a bad connection to the MAF combined with dirty MAF sensor. Seems to have solved the problem after a month of searching, testing and replacing parts.

[ForcedFirebird]

Nice! So it was a backwards-situ from normal. Typically the WOT doesn't work, and the engine lacks power. In your case, it was stuck in "idle mode". Makes perfect sense.

Thanks so much for closure. So may have issues and never close the thread with resolve.