1989 325i - Split Second Loss Of Power When Driving

[gilber33]

I guess I should update this as well. The new (not used) CPS bracket solved my issue. The sensor was sitting at the slightest angle that it must have gotten a misreading every once in a while.

I have even put in a different engine since then and the issue is still gone. So in my case it was the bracket.

[digger]

If the tacho is good then i wouldn't be going down path of crank issue as the CPS seems to be sending a good signal (only a scope would confirm that 100% though)

[Kershaw]

Can you rotate the motor by hand 3 or 4 times and track the harmonic balancer teeth to the cps? Or maybe pull the fuel pump fuse, set up a hard mounted camera (so it shakes with the engine) to the car, and crank it over for 20-30 seconds. Even better if you can record in 120 or 240fps. Then watch the video on a computer and slow it down. You might be able to see the crank wobble.

I should update the thread I linked above because I have an update for it. Even though megasquirt solved my issue on that motor, I have a new solution. I swapped in a new motor in with stock motronic and everything runs great. I never considered crank wobble, but that sounds very plausible as the root cause of my issue. I think things like this are never one issue, but compound issues which make it really hard to track down and fix.

[digger]

Does it ever do it idle ? Is it Under load only? Does it seem like the power "totally" cuts out momentarily or is it more that it only loses "some" power

Sounds like bad wiring

[usualdeviant]

The problem is still there. I even got a rebuilt DME straight from Programa, put it in, had the problem, sent it back to have it checked, then they sent it back and the problem persists. Things I tried include used harmonic balancer (can't get new) new OE bolts and washers for it, and chased the threads in the holes in the crank, torqued to spec., used CPS bracket (came with the balancer), wired in my own shielded wires for the CPS, wired in my own fusible link, new OE TPS adjusted correctly... So I'm leaning towards maybe I have a crank wobble. Running 5W-30 right now. What should I try if I have worn crank bearings? 10W-30? 10W-40?

[usualdeviant]

Another update: At this point, I had my suspicions about both of my DMEs. While asking sellers if and how they tested the ones they had for sale, one of them was classy enough to offer me a piece of wisdom - that the original German fuses with a wire that goes from one side to the other, on rare occasion, have a tendency to pull apart under load. I replaced all the German fuses with Chinese ones. Immediately the car ran better, did not stumble or cut out as much. But the problem was still there.

Yes, my original AND my ebay DMEs were both bad. The original made the engine stumble and cut out at low RPM and the ebay one was after 5k RPM. I got another ebay DME, also rebuilt by Programa, and the problem was solved. What caused them to go bad? Old age? ICVs with high resistance? (I can't remember how far out of spec. my old ICV was, but I did replace it in 2017 maybe, with a cheapo Chinese one, and its specs. are pretty close to spec.) Old German fuses that pull apart under load? Something else? Some combination? Not really sure but if you find yourself having to swap DMEs you may want to check these things first. One more thing I might add is the first Programa I got had places where it said 153 AND 173. So kinda fishy there. Could be a good DME just the wrong one?

I've already put at least 100 city miles and quite a few cold starts on it since the good DME was put in, and no issues so far. The only thing keeping my hair on my head was having another car to commute with. This problem started out as a small stutter when taking off at low speed low RPM from a cold start. It started happening, I don't know, maybe 3 years ago. About 6 months ago is when it started the whole choking thing. So I have been going insane for the past half a year trying to figure this out. Hopefully this will help others in the same situation. Fixed it up just in time to pass emissions, made me feel good about the year 2018.

[usualdeviant]

Just a tiny tiny update here. Ran the new upgraded body grounds and alternator-starter cable + new dealer battery. Nothing. Tried driving with ICV unplugged. No issues other than it wanting to stall with foot off gas - normal I take it. So driftxsequence car and my car may have the same issue... Not sure. My ebay rebuilt programma DME + my old DME both bad from a bad ICV? Not sure.

[usualdeviant]

Sounds like the exact issue I am having, my friend! Did you ever get anywhere after changing the bracket? I've swapped quite a few parts trying to figure it out. So I have some theories. Bad body ground(s), bad SI board (gives power to OBC relay which gives at least some power the the DME), broken sheilding on some wire in the harness (CPS, Cylinder Identification Sensor, Ignition Coil). Just about at my wit's end but I am doing a new dealer battery since the AGM O'Reilly battery I had finally tested bad. New body grounds and alternator-starter cable on the way, also having my instrument cluster restored - even if it's not the culprit, it still acts like crap.

[gilber33]

I had my friend riding with me last night and it happened. And he agreed that it felt ignition related.

I'm going to get a new CPS bracket for my next attempt at a fix. I have noticed that the CPS sits in it flush before I start tightening down the bolt to secure it. As the bolt is tightened, it is pulling the CPS at an angle.

Hoping that every once in a while the signal is broken and it's cutting spark. We'll see. I'm running out of time to diagnose and drive the car though.

[digger]

Bad spot on AFM?

[Mike36]

I noticed something similar with an E30 parts car that I purchased. It drove great and shifted smooth, however I noticed this at least twice in the three weeks or so that I drove it. I never bothered looking into it because I was parting the car anyways. Good luck with this, it freaked me out when it first happened to me even though it was for a split second.

[driftxsequence]

IIRC the issue was when the resistance of the idle valve got too high it basically took out the circuit on the ECU. its supposed to be 20/40/20 and it was something like 2k/10k/2k

[gilber33]

Just out of curiosity, is the CPS supposed to be dead center on the gear? I noticed mine is not. Seems to be it is definitely sticks out towards the front of the car, like the bracket it sits in is an 1/8" too long.

Hard to find pics of the CPS installed to tell what its supposed to look like.

EDIT: Actually the bracket must be tweaked a little bit. As I tighten down the screw the sensor doesn't sit flush but points out a little bit. This causes the sensor to read the other edge of the gear, with a real little bit of the sensor over the edge of the gear.

I dont know how picky these are, but could they cause maybe some random misreadings?

[gilber33]

I had an issue like this and there were 2 things I did. First, I disconnected the idle valve. That remedied the issue for a few weeks. Got a new idle valve, plugged it in and the issue happened again. hm so maybe its the ECU? plugged in a new ECU and the issue went away completely. Then a few weeks later it came back...turns out it was the CPS and the ICV/ECU that were intermingled. My issue was like someone turned the key off for a moment and there was a completely loss of power.

Out of curiosity, how would the ICV failing affect this?


The previous owner experienced this once in a while and when I did all the work on the car, I put in a low mileage CPS and the issue persists. I don't think the sensor is my issue, but I guess the only way to fully rule that out is putting in a new one.


And I have heard a few people say mention the ECU when it comes to weird engine stutters like this. Definitely something to look into as well.

[driftxsequence]

I had an issue like this and there were 2 things I did. First, I disconnected the idle valve. That remedied the issue for a few weeks. Got a new idle valve, plugged it in and the issue happened again. hm so maybe its the ECU? plugged in a new ECU and the issue went away completely. Then a few weeks later it came back...turns out it was the CPS and the ICV/ECU that were intermingled. My issue was like someone turned the key off for a moment and there was a completely loss of power.