M40b16 engine cutting on aceleration

[blood69]

Hi everyone
I'm having an pain in the ass issue with a 316i m40b16 engine
If I apply any engine load on the car, it cuts power on what it looks like it's an ignition issue. The rev counter drops to zero and then goes back to normal.
I changed all the ignition system last year. Plugs, ht leads, dizzy rotor, dizzy cover and ignition coil.
Also last year I changed the CPS and the ECT sensors.
All new stuff.
This behavior of the rpms suddenly dropping on the tach below on what should be reading when the cut happens, gives way the CPS sensor, harness or ECU.
Last year I put a brand new CPS from Valeo and fixed another issue that I was having at the time.
Assuming that the E30 are known to Eat CPS sensors, I bought a new one from Vemo but when I tried on the car the issue remains.
I check both resistence from both CPS and they are at 560ohms. I also checked conected to the harness and pin 48 to 47 from the ecu plug and got the same results. So this eliminates the harness I guess.
I open the ecu looking for shorts, first test was pin 47 to pin 2 (ground) got resistence. But pin 48 to ground got 3.3ohms. I don't know if this is by design, I've tested all the caps and resistors from the ecu and didn't find any short.
The AFM I did a test driving the car and the voltage goes smooth, when the cut happens the AFM is still giving voltage, no suddent drops.
I also tested the one year old ignition coil and got primary of 2.5 ohms and secondary 8k Ohms.
The last thing that comes to my mind is the harmonic balancer, I clean it up and it doesn't seem very rusty.
What do you guys think? It's time for a Standalone Ecu or I am missing something here

[Speedtouch]

I would start with checking the basics, such as engine-to-body earth strap(s), and that you have a decent DME (ECU) relay.

Also verify that the ignition switch is giving a reliable supply to the engine electrics.

[blood69]

I would start with checking the basics, such as engine-to-body earth strap(s), and that you have a decent DME (ECU) relay.

Also verify that the ignition switch is giving a reliable supply to the engine electrics.

Thanks for your reply.
I had a friend with his e30 here at home and i swapped his ECU, DME and Fuel relays to my car. And the issue remained.
I also tried my relays on his car and they worked fine.

About the ground, I do get good ground measured with the multimeter both in the block and the head, about 1.8 ohms.

You mentioned the ignition switch, you mean here the key operates to start the car?

[Speedtouch]

Yes. If the ignition switch has worn contacts, you won't be getting a reliable power feed to the DME/ECU.

You could try bypassing it altogether, and running a direct +12V feed to the ECU, obviously ensuring you are feeding the power to the correct terminal(s).

You mentioned it only dies under load, so does it run reliably while just sat idling?

[blood69]

Yes. If the ignition switch has worn contacts, you won't be getting a reliable power feed to the DME/ECU.

You could try bypassing it altogether, and running a direct +12V feed to the ECU, obviously ensuring you are feeding the power to the correct terminal(s).

You mentioned it only dies under load, so does it run reliably while just sat idling?

It doesn't die. It cuts like it's a Lunch Control on a old Honda. The rev counter drops to almost zero then goes up again. Like for a half second I loose power, like it's a sooner rev limiter.
My first thought was the CPS. Then the ECU.
If the ignition switch is cutting power under load its possible for the ecu to disconnect momentarily and cause this issue. Also I have to test the alternator to see if is going over or under voltage when this happens.

[Speedtouch]

So it's 'bogging' down when you try to accelerate? In that case, there's probably an inlet manifold vacuum air leak somewhere. I would carefully check any hoses, couplings, etc, for cracks/splits, and try spraying starting spray or brake cleaner around the inlet manifold with the engine warmed up and idling: if it temporarily rises in revs, you have a leak there.

There could also be fuel starvation going on, so perhaps change the fuel filter to ensure there's correct fuel pressure.

[blood69]

So it's 'bogging' down when you try to accelerate? In that case, there's probably an inlet manifold vacuum air leak somewhere. I would carefully check any hoses, couplings, etc, for cracks/splits, and try spraying starting spray or brake cleaner around the inlet manifold with the engine warmed up and idling: if it temporarily rises in revs, you have a leak there.

There could also be fuel starvation going on, so perhaps change the fuel filter to ensure there's correct fuel pressure.

Thanks for your help.
I managed to read voltage from the AFM when the cut happens. It's steady it drops after the cut but not when the cut happens.
I checked for vacuum leaks, don't got any, good 200 to 250 milibar after the throttle body.
Also my fuel gauge at the FPR stays steady when it happens. So I don't think it's fuel related problem.
I also tested out with another AFM, the cut pressisted.
I still wanna test the tps but I don't think that's the problem.
From the engine head to the chassis I get good zero in diode mode on the multimeter. So in theory I have good ground.
My money is still on some bad conection provoking this issue. Some voltage spike or drain going to where it's not supose to.
My relays and fuses are all good.
I still have to try routing the ignition switch. Providing direct 12v to see if that's the issue.

[Speedtouch]

I would also try adding an extra engine-to-body earth strap, if only temporarily, to see if it makes any difference...

[blood69]

I would also try adding an extra engine-to-body earth strap, if only temporarily, to see if it makes any difference...

Im defennatly going to try to earth to chassis today.
Yesterday I tried different used CPS sensors and got the issue even worse.
I also tested the tps sensor that it's working great.
I mesured the ignition switch and I have steady 12v on the green wire even when the cut happens.
I tried to pry the trigger wheel and check for play and axial movement.

[Speedtouch]

Have you tried cleaning out/replacing the idle control valve?

[blood69]

UPDATE.
I used a timing light pointing to the trigger wheel marking compared with the arrow.
It advances and retards timing depending on the rpm and engine load.
When the cut happens the light goes off. So I can't see what happens.
I guess it retards timing or advances to mutch.

I also put a ground wire from the head to the chassis. The ohms reading from the head to the chassis improved a bit, but the cut problem pressists.

I'm guessing the issue is the CPS since everytime I mess with it, the problem gets worse.

Bmw where I live asks for 150 euros for one. I bet it's not even the original one.

I'm baffled how the Veleo CPS that I put last year is already busted. The Vemo one that I tried brand new didn't make any difference.

Now I'm questioning if paying 150 euros for a BMW cps sensor and waiting 3 or 4 weeks to arrive would make any difference.
I guess it's a risk I have to take.

With the engine running and applying throttle I don't see any axial movement on the trigger wheel. This is driving me nuts 🥲

[blood69]

Have you tried cleaning out/replacing the idle control valve?

I see what you mean, I had that problem before. When acelerating the engine cuts because of the ICV.
I disconnected to see if it made any difference. But the rev counter drops below the actual revs the engine is producing and some pops and bangs comes out of the exhaust if I keep acelerating, regardless of having the ICV disconnected or not.
I I'm pretty confident that's not the issue. The car idles fine.

[Speedtouch]

I'm wondering if it's a possibility your exhaust system is blocked somewhere - have you tried revving the car right up to the red line with it stationary? Alternatively, it could be a blocked fuel filler cap breather causing a restriction...

[blood69]

I'm wondering if it's a possibility your exhaust system is blocked somewhere - have you tried revving the car right up to the red line with it stationary? Alternatively, it could be a blocked fuel filler cap breather causing a restriction...

If I apply light throttle I can rev the car. If I apply more than 50% throttle it cuts and the rev counter drops to almost 0 rpm instantly then rises again showing rpm.
Most certainly the ECU is not reading the CPS correctly and it shuts down for safety. Now why is not reading that signal correctly that's where it gets tricky.

[Speedtouch]

Have you checked the notorious green wire that runs behind the dashboard to the ECU? The Wiki guide provides more info:

https://www.e30zone.net/e30wiki/index.p ... oubleshoot

[blood69]

Have you checked the notorious green wire that runs behind the dashboard to the ECU? The Wiki guide provides more info:

https://www.e30zone.net/e30wiki/index.p ... oubleshoot

This is awesome stuff. I tried the light bulb on several locations.
I have here a video with the bulb conected to the negative of the coil, and to the chassis. I invite anyone to see what whappens.
[media]https://photos.app.goo.gl/mMaR7PsmNULuJYyy5[/media]

[Speedtouch]

For sure, it is not cool rock steady! Perhaps you have a faulty rectifier in the alternator that is causing the voltage glitches? You could try temporarily disconnecting the alternator and try the test again...

[blood69]

For sure, it is not cool rock steady! Perhaps you have a faulty rectifier in the alternator that is causing the voltage glitches? You could try temporarily disconnecting the alternator and try the test again...

I was going to disconnect the alternator several days ago. But the big wire was starting to break apart while I was taking out the nut. So I decided not to because I don't have any spare wire.
In the meantime I ordered an original CPS sensor from my local BMW dealership. It will take 3 or 4 weeks to arrive.

[Speedtouch]

I would try disconnecting the alternator anyway, particularly if the wire is showing signs of fatigue! You may find it runs OK with it disconnected...

[minesapint]

When you say the engine is cutting out on load, is this when you are driving the car on the road and loading the engine, or do you mean when you are increasing engine speed whilst stationary in the garage?
I’m going to assume it’s the latter, stationary car.
I can’t see how the problem can be related to the CPS, as the engine runs and idles, This would not happen with a broken CPS. The CPS is the only means the ECU knows the engine is running. The CPS either works or not, Yours is working or the engine would not run at all.
You said when you were using the timing light, and the engine cuts out, the timing light turned off, therefore the supply to the spark plugs has been lost, this spark is coming from the coil which is initiated by the ECU.
You’ve swapped out the ECU with an appropriate known good one and the problem persists you say, so it may not be the ECU that’s at fault, but that’s not a guarantee.
I’m minded to suggest the TPS (throttle position switch) could be the fault, They can become clogged with oil. Read the section on TPS in the Wiki which explains how to check-out its operation. They have three switching positions, throttle closed ….ICV controls air supply, throttle slightly open…..AFM signals ECU, and above 50-60% open (where you say engine cuts out) this is WOT (wide open throttle) ECU is fed directly. It may be this final switching stage is not making contact.
Worth a try. Good luck.
Cheers