Sudden and repeated Engine cut out -All Bentley tests done. calling thejlevie

[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. 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. It’s like someone is sitting next to me with a master 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 opens when throttle rotated);
2. New Crank sensor (Beck Arnley) reads 874 ohms (at sensor plug terminals and DME). Bentley manual 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

[superj]

Our 88 used to that too. Ours had loose connections at the afm causing trouble

[richardodn]

Based on your symptoms, it's definitely the ignition cutting out and is an intermittent failure. You can tell that because of the tach dropping immediately to 0.

Suspect things
Main relay - a failure here will kill power to the DME and fuel pump.
Crank pulse sensor - a failure here will cause the DME to stop seeing timing data and shutting everything down.
DME - the ECU itself could have internal problems

[mrgesq]

Thanks for the response. I have a new oem Relay and crank position sensor and I replaced the DME with a know good used unit. Both test correct at the pins on the DME.

Update: New BMW OEM crank Sensor with 545 Ohms resistance. Still cuts out, but only after it warmed up. Also seems like I can almost make it happen by pushing the accelerator pedal rapidly, like a fast acceleration. I pulled the throttle body and the Throttle position sensor does not work at WOT and after I cleaned it with dialectic cleaner, neither does the idle position. New OEM throttle position switch on order for $282 bucks. Ouch.
The hunt continues.

[richardodn]

It's not likely the tps. It's basically 2 simple switches. 1 switch closes at idle and is open off idle. The other switch closes at WOT and is open the rest of the time.

[msservices]

You may want to check for corrosion in your wiring harness between the fuse box and the ignition switch/steering column/dash cluster. I know that there is a point there where the harness sits in/up against the sheet metal in an area that’s prone to moisture/rust. I’ve heard of this problem which causes symptoms which would act like a faulty ignition switch

[H.J. 1981]

Last time i had a sudden cut of the engine while driving, i checked the fuel pump since it felt as if someone was clamping a fuel hose. Once verified the fuel was not the issue, i started the car and it idled funny, i disconnected the ICV connector and replaced it and worked just fine, i am guessing mine was getting sticky despite that i had it recently cleaned.

Tested the ICV connectors, all within spec. Do you have a used ICV somewhere you can quickly swap out and test?

Richard is most likely correct with an ignition issue, it is the only symptom that would cause the tachometer to drop to 0, are you absolutely sure all the connectors are firmly in place? any corrosion? do the relay pins look rusty? Ignition and coil wires/connections in order? no loose wires anywhere? even the simplest bolt/nut is snug nicely?

[smittyatl]

Bad AFM caused this on my e30. Full throttle acceleration followed by the car dying then restarting under its own momentum.

[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 and replace the fuel filter. 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.

[msservices]

Thank you for posting the feedback!

[mrgesq]

Update: Since I replaced so many parts with new, I put the "new" aftermarket idle air stabilizer back in the car so everything would be new. I don't recall where I got it (on line) , but in was not OEM Bosch. It immediately went back to idling at 1100 rpm and the Stumbling and cutting out. Gotcha! I replaced the "new" with the original Bosch unit and it went back to idling at 600 rpm and runs like new. 30 MPG over a mixed 600 mile drive.

So, the moral of the story is to replace anything that sends a signal to the DME with OEM components. I learned the hard way.

[msservices]

Good to know, Thank you