M42 won't rev

[$C300]

Hello everyone, having a issue wondering if anyone has run into this before.

So the car is a 87 325is, I swaped I'm a 91 M42 I had bought for another car.

Got the car running this weekend and when it idles it's about 1-1.3k but when I try to rev it, it revs up to about 2.5-3k then falls on its face and if I keep the throttle open it will bounce back up then back down like a really lazy two-step. The car isn't triggering a check engine light yet.

I have checked the tps seonsor and it check with in spec as well as the plug showing 5v. No difference when unplugged

If the AFM is unplugged it will throw a cel light and stumbles. The AFM door swings freely and apears to be operating correctly.

IAC again will throw a check engine light when unplugged however not a noticeable difference when unplugged.

I am still utilizing a external filter the 87 came with, i can hear it prime and apears to be working. I do have a internal from a 318is that I have not swaped in yet.

I swaped o2 sensor just to eliminate it and it did not change anything.

I'm starting to think it's ignition related, I'm currently running m42 coils and some spare m20 wires. I was planning on going COP so I'll probably order that in the next few days.

Plugs were relatively new, and the fuel filter came with the car but looked relatively new (I'll be replacing that soon).

any help will be greatly apriciated.

[Gumby]

I'm having a similar issue. Sometimes it's 3.5k sometimes it 5k and sometimes never does it. Need to get out a voltmeter

[unixnerd]

I had an M30 Alpina do this once, low fuel pressure. Can you stick a fuel pressure gauge inline with the rail?

Also check the whole mess of pipes under the intake. A vacuum leak there can be a bigger deal than you'd imagine.

[Best_name_available]

this is definitely an issue, cars have to r3v to be on r3v after all, i hope this gets resolved soon. you will be missed long boy

[$C300]

I had an M30 Alpina do this once, low fuel pressure. Can you stick a fuel pressure gauge inline with the rail?

Also check the whole mess of pipes under the intake. A vacuum leak there can be a bigger deal than you'd imagine.

i havnt check fuel pressure yet, however the way it cuts out and bounces back makes it seem more like a ignition or cam/crank sensor. Fuel pressure test is something i will be doing soon.

iv already done a smoke test the car has absolutely no vac. leaks

[$C300]

Update

Its been 4 months and still baffled....

Converted the fuel system to single intake pump for M42 (fuel pressure is good), did a COP conversion, new TPS sensor, swapped out the CAM and Crank sensors to one from a E36. and still having the same issue.

The Idle air control valve seems to be getting stuck, will hold the car at 1200-1500 until i tap on it with something.

Still having the same reving issue, Anything over 2200-2500 rpm it will hit that range then drop to idle then back up.

Hopefully ill have some time this week to pull the valve cover off and see if the timing marks line up in case the chain jumped a tooth.

Any help would be appreciated.

[varg]

Revving then dropping and revving up again means it's highly unlikely it's the cam timing. Judging by the fact that I've seen a a thread where at least one other person had this problem and threw tons of parts at it to no avail, I'm going to guess this is caused by a bad ECU or a wiring fault. ECUs are fairly complex electronic devices, they can fail in bizarre ways. I had a Volvo 240 LH-Jetronic 2.2 ECU which would consistently rev to ~2,700rpm, cut out, fall to ~1,500, then rev to redline just fine. The problem would then not repeat unless you allowed the revs to drop below 2,000rpm again, and went away entirely when the TPS was unplugged. I also had another LH2.2 ECU that wouldn't trigger the fuel pump relay, and the relay trigger had to be jumpered to a "hot when key on" wire to make the car run.

[$C300]

Already got a new ecu from a working m42, I'm also starting to think it's something in the harness.

The only changes I have done was splice in the c104.

I'm going to start looking for a harness...

[varg]

Do a continuity test of the wiring for the TPS, Cam and Crank position sensors

[$C300]

ill do some more poking around tonight, hopefully its as simple as a bad wire in the loom i can just bypass.

[$C300]

Update

Did continuity test, no signal from tps. Unplug tps car revs fine, cam sensor and crank sensor continuity tested fine.

Now I have a cel light even with the sensor plugged in but she will rev up. The tps sensor is a 3 wire however there is only one input on the ecu, need to trace wire and bypass now.

[varg]

So no continuity between pin 12 of the ECU connector and pin 2 on the TPS? From my reference, which is a little easier to read than that, the 3 pins are 1: +5V, 2: Signal, 3: Ground. Sensor ground on the ECU is pin 43, +5V is pin 59.

[$C300]

Yes issue was between pin 12 on the issue and pin 2 on tps.

Fixed it last night, now I just have to finish the rest of the car.