The Dreaded Mid to High RPM Cutout

[It's Soda Not Pop]

https://youtu.be/ESLp3COHiqI?si=mhXr5FAPAD76H30n

So I've perused this forum and a few others for days looking for an answer to my problem. Now there have been resolutions on here and the forums, but nothing has worked for me so far. When I get on the throttle and reach 4500 to 5000 rpms (it has done it around 3500) the engine cel flashes and the tach bounces. Now to go into detail about what I've changed.
- CPS (oem bmw) twice
- Cap and rotor (oem or bosch)
- Coil (oem)
- Plug wires (irelandengineering 8mm wires)
- Plugs (NGK)
- All Relays
- Fuse #9 and Fuse #11
- DME twice (with a stock chip and back to my SSSquid tuned one)
- AFM twice
- Stomp test works so I assume the TPS is fine (it's not old)
- New DME coolant sensor
- Fuel pump is a Walbro 255
- New FPR
- New Fuel filter

I've tried jumper wiring the FP Relay the cutout/hiccup still happens. I've tried wiggling all the connections while the car is running to make it do it. Nothing happens. Now this car is hard to start when it's cold out. It will initially fire up, but it will die if I don't blip the throttle in time. But after that it idles fine. It will drive fine for the most part. It's only during hard driving or free reving high that it does this. I'm going to add a video (hopefully it's clear for you guys to see it).
I'm about ready to burn this thing down. Just a few months ago it was fine. Now I will say this I did have to rewire the fuel pump to get it's power from the battery and the signal from the FP Relay(the one in the engine bay). You don't think the car believes it doesn't have a fuel pump now because I've done that. Because there is still 12 volts but almost no load now. All the load is from the battery to another Relay to my fuel pump.

[It's Soda Not Pop]

Guys I'm really at a loss here. I feel like for last 5 days or so I've done nothing but read and try different stuff. Nothing has worked. What would cause a car to idle fine and drive fine( quarter to half throttle shifting around 4000 or less) but act up at WOT. I've gone through 3 different CPS it's not the sensor.

[Panici]

Have to try and narrow down if it's a fuel or spark issue.

Does it happen at high RPMs but only part throttle? Trying to see if it's related to engine speed or load.


Ignition:
What about your ignition system grounds?
Try gapping down the plugs.
Check your timing with a light.
What is the system voltage at the battery? Alternator putting out 14v? Same voltage at the coil under load?

Fuel:
Put a fuel pressure gauge in there, see if it's holding steady during WOT.
Install a wideband O2 sensor and gauge.


EDIT:
Thinking a bit more about it, Flashing CEL is typically a misfire or knock on OBDII cars.
Does it indicate the same on the 173 DME E30?
No knock sensors on an M20, so how would the ECU determine there is a misfire happening.
I'd bet you're losing crank signal at the ECU, what else would the ECU have visibility into?

Following that line of reasoning, what is between the ECU and the CPS?
Harness and C101 connector.
Clean out the C101 and make sure the pins are tight.

To verify if the CPS is missing some pulses, you'd need an oscilloscope to check. Can the sensor distance to the crank (gap) be adjusted?

[It's Soda Not Pop]

Have to try and narrow down if it's a fuel or spark issue.

Does it happen at high RPMs but only part throttle? Trying to see if it's related to engine speed or load.


Ignition:
What about your ignition system grounds?
Try gapping down the plugs.
Check your timing with a light.
What is the system voltage at the battery? Alternator putting out 14v? Same voltage at the coil under load?

Fuel:
Put a fuel pressure gauge in there, see if it's holding steady during WOT.
Install a wideband O2 sensor and gauge.


EDIT:
Thinking a bit more about it, Flashing CEL is typically a misfire or knock on OBDII cars.
Does it indicate the same on the 173 DME E30?
No knock sensors on an M20, so how would the ECU determine there is a misfire happening.
I'd bet you're losing crank signal at the ECU, what else would the ECU have visibility into?

Following that line of reasoning, what is between the ECU and the CPS?
Harness and C101 connector.
Clean out the C101 and make sure the pins are tight.

To verify if the CPS is missing some pulses, you'd need an oscilloscope to check. Can the sensor distance to the crank (gap) be adjusted?

So it can happen at part throttle but not as bad as full throttle. I couldn't get it to do it in 1st gear. But 2nd the bucking happens around 5k to 5.5k. 3rd gear is bit lower 4k to 4.3k. And 4th it's happening around 3.5k.
As far as I can tell everything grounded correctly. I gapped the plugs around .028 .027.
The car back fires pretty good during this event. And it does it on the overrun as well.
Like I said before this is 3rd cps put on the car. Oem bmw sensor as well.
This car 6 to 7 months ago wasn't doing this. The only real modification that I've done recently was rewiring the fuel pump. Which I'm considering converting back.
Could the harmonic balancer be moving at high rpms and just can't see it when I rev it from the engine bay?
The cps gap can be adjusted. But it has a little over 1mm gap right now.

[Panici]

So it can happen at part throttle but not as bad as full throttle. I couldn't get it to do it in 1st gear. But 2nd the bucking happens around 5k to 5.5k. 3rd gear is bit lower 4k to 4.3k. And 4th it's happening around 3.5k.

The car back fires pretty good during this event. And it does it on the overrun as well.

This sounds like loss of spark to me. Does the tachometer drop out as well?

Check & clean your C101 connector, inspect your wiring harness between the CPS and the ECU (Easier said then done).


Ultimately, you need to verify that you're getting a consistent crank trigger signal at the ECU and not dropping pulses.
If you don't have the tools (oscilloscope etc) to do this, rent them or bring it to a shop that has them on hand.

[It's Soda Not Pop]

This sounds like loss of spark to me. Does the tachometer drop out as well?

Check & clean your C101 connector, inspect your wiring harness between the CPS and the ECU (Easier said then done).


Ultimately, you need to verify that you're getting a consistent crank trigger signal at the ECU and not dropping pulses.
If you don't have the tools (oscilloscope etc) to do this, rent them or bring it to a shop that has them on hand.

Yeah the tach is jumping backwards. In the video I posted you can see the tach, coolant temp needle, and the fuel economy meter jump.

[It's Soda Not Pop]

So a few months back (around Halloween I believe) I attempted to convert my service interval lights into shift lights. I wasn't able to get it work correctly for me. In fact the lights kind of go crazy all the time. I wonder if this new board is causing some kind of electrically gremlin.

[Panici]

So a few months back (around Halloween I believe) I attempted to convert my service interval lights into shift lights. I wasn't able to get it work correctly for me. In fact the lights kind of go crazy all the time. I wonder if this new board is causing some kind of electrically gremlin.

Anything is possible. Remove the new board and see if it makes a difference.

I wonder if a short to ground on your tach signal going to that SI board would kill spark.
Maybe it's a shared ECU output to the tach and coil?

Still recommend the earlier advice:

Check & clean your C101 connector, inspect your wiring harness between the CPS and the ECU (Easier said then done).

Ultimately, you need to verify that you're getting a consistent crank trigger signal at the ECU and not dropping pulses.

[e30austin]

consider the TPS being full of oil through the throttle shaft bearings. i know you said you replaced it, but it's worth looking, along with the C191.

a friend had a customer car that came in to his shop for the infamous 4k hard cut. he swapped every single engine component (harness, DME, sensors, etc) off a known good, running driving car, including the goddamn chassis harness, to no avail. what eventually "fixed it", was converting it to a PNP Megasquirt controller.

hope your situation ends better than that.

[moatilliatta]

Harmonic balancer tight? Big and small bolts?

[E30SPDFRK]

It sounds dumb, but I had a car doing very similar issues, and the O2 sensor ended up fixing it. The car had a newer sensor installed, but it was the wrong one for the car and it would miss under load with a tach drop and a CEL flash.

[It's Soda Not Pop]

consider the TPS being full of oil through the throttle shaft bearings. i know you said you replaced it, but it's worth looking, along with the C191.

a friend had a customer car that came in to his shop for the infamous 4k hard cut. he swapped every single engine component (harness, DME, sensors, etc) off a known good, running driving car, including the goddamn chassis harness, to no avail. what eventually "fixed it", was converting it to a PNP Megasquirt controller.

hope your situation ends better than that.

I'm going with Jake's Turkey and Cheese PNP. Also I put a hole in the TPS for that reasoning.

[It's Soda Not Pop]

Harmonic balancer tight? Big and small bolts?

I tested to see if they were tight. They are. I did find a NOS one. I order it just in case.

[It's Soda Not Pop]

Anything is possible. Remove the new board and see if it makes a difference.

I wonder if a short to ground on your tach signal going to that SI board would kill spark.
Maybe it's a shared ECU output to the tach and coil?

Still recommend the earlier advice:

I'm going to swap the board back out. Because the coolant temp needle is also jumping. I'm hoping that this is the fix. I've checked all my connectors. They all look really good. My harness is really good considering the age.

[It's Soda Not Pop]

It sounds dumb, but I had a car doing very similar issues, and the O2 sensor ended up fixing it. The car had a newer sensor installed, but it was the wrong one for the car and it would miss under load with a tach drop and a CEL flash.

I thought about it being the sensor. That's the only sensor that hasn't been changed since 2016.