Absolutely stumped with the crank-no-start I've been struggling with for 3 weeks

[rogerscgreg]

I have a 1987 BMW 325i (E30) that's been a race car for several years now. It uses Motronic 1.3, M20B25 engine. I'm been suffering from a crank-no-start condition for 3 weeks.

I'm going to do the best I can do summarize all of the symptoms I've experienced, tests I've done, and their results. Note that I'm a novice when it comes to working on cars, intermediate when it comes to electrical work.

Onset
Sudden, one day I went to start it and it just cranked without firing.

Symptoms
Crank, no start. Doesn't even sputter. Sometimes, on the first attempt to start it of the day, it will start, then immediately die.

Tests
I've been trying to follow the diagram on e30zone here.

Fuses and Relays

• Checked all fuses
• Replaced the fuel pump relay and the DME relay

Ground Straps

• Inspected and cleaned all ground straps associated with the engine\

Ignition Coil

• Pin 15 has power. Circuit tester glows.
• Internal resistance looked good.
• Unplugged HT cable and inserted a test spark plug. Most of the time, it didn't spark. It did occasionally at the expected 2hz, but couldn't be reproduced consistently.
• Swapped with a known good ignition coil, same results as the old coil
• Verified with an oscilloscope that no spark signal from the DME was being received

Spark Plugs

• Look fine, recently replaced
• Didn't do much testing here because the ignition coil wasn't producing a spark anyway

ICV

• Usually hums. Sometimes it doesn't.
• When I got it to fire and it died, I assumed I had fixed the underlying issue and the ICV had seized, so I cleaned it with carb cleaner

Fuel Pump

• Definitely works because at one point after trying to start it for too long, the engine became hydrolocked with fuel, so I had to take out a couple of the spark plugs and let it evaporate overnight
• Sounds like it primes when I first turn it to "run"

DME and pins

• Unplugged the DME and inspected all of the pins
• Verified 12V at pins 18 and 27
• Checked ground continuity for all grounds
• Checked continuity for input sensors
• Swapped with a known-good DME

Sensors

• Tested CPS with an oscilloscope at the DME pin, saw a sine wave as expected
• Tested throttle position (wide open and closed)
• Verified internal resistance and continuity of coolant temp sensor
  • Even tried starting with it unplugged to make sure it wasn't shorting. Multimeter corroborated that the connection was fine.
• Verified continuity of AFM sensor

Stomp Test

• Unable to trigger with my DME
• Triggered with the DME I swapped in, gave code 1223 (Coolant temp sensor)

My next test is going to be a compression test on all cylinders to see if I have an air leak/compression issue, but I wouldn't expect that to be a sudden onset event.

Any help would be appreciated!
Greg

[digger]

it seems like no spark on any of the plugs? if so have you verified coil control (by the ecu) both at the coil and at the ECU?

[rogerscgreg]

I’ve verified continuity between pin 1 of the ECU and the ignition coil. There is no pulsing of any sort.

[MrBurgundy]

The fuel pump shouldn't "prime" with ignition on.

Should be commanded on by the DME once a CPS signal is sent out.

I would honestly swap a CPS sensor.

Sometimes it will test "good" on the bench but for whatever reason, it still doesn't work.

If you swapped the DME and coil with known good ones, my money is on the CPS. All hall effect sensors are going to be square waves to btw.

[rogerscgreg]

The fuel pump shouldn't "prime" with ignition on.

Should be commanded on by the DME once a CPS signal is sent out.

I would honestly swap a CPS sensor.

Sometimes it will test "good" on the bench but for whatever reason, it still doesn't work.

If you swapped the DME and coil with known good ones, my money is on the CPS. All hall effect sensors are going to be square waves to btw.

Thanks, maybe I should just replace it. Are they available at auto parts stores nowadays? I couldn't find any with a quick search.

A friend of mine suggested it might be a compression issue as well. Do you think that's likely?

[MrBurgundy]

Thanks, maybe I should just replace it. Are they available at auto parts stores nowadays? I couldn't find any with a quick search.

A friend of mine suggested it might be a compression issue as well. Do you think that's likely?

I mean if you got the ear for it, you should be able to tell if a car has no or little compression by the sound of it turning over.

You'd be surprised, i'd call and ask but the oe is the only one i'd use.


I think it was bosch, but idk if you're gonna be able to find one easily.

Hella/Facet are the ones we're seeing now and Hella is the brand to go with if that's the case

[digger]

I’ve verified continuity between pin 1 of the ECU and the ignition coil. There is no pulsing of any sort.

if there is no pulsing with either dme that you tried then it suggests a crank sensor signal issue (unlikely both dme are bad) but that goes against the test where you saw the signal on the scope (a reluctor should not be a sine wave nor a square wave) and that the injectors seem to be pulsing which also needs the cps. a bad waveform should not cause zero spark on all of the plugs and should atleast "kick" periodically as it tries to start

[digger]

Thanks, maybe I should just replace it. Are they available at auto parts stores nowadays? I couldn't find any with a quick search.

A friend of mine suggested it might be a compression issue as well. Do you think that's likely?

if there is no spark then i wouldnt bother with testing compression yet. as a good rule if the engine was severely flooded and alot of oil washed off cylinder walls its a good idea to squirt some oil the cylinders as it will help it start once you get the issue sorted

[rogerscgreg]

if there is no pulsing with either dme that you tried then it suggests a crank sensor signal issue (unlikely both dme are bad) but that goes against the test where you saw the signal on the scope (a reluctor should not be a sine wave nor a square wave) and that the injectors seem to be pulsing which also needs the cps. a bad waveform should not cause zero spark on all of the plugs and should atleast "kick" periodically as it tries to start

This is what I've been struggling with the most. Resistance is good and I have a waveform, I just wonder if that waveform is too weak. The gap at the sensor looked reasonable, but I didn't measure directly.