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View Full Version : M50 swapped E30 running rich



okohigg4
05-14-2024, 10:25 PM
Hello all,

This is my first post ever so please bear with me for all the newbie mistakes.

I have a '91 BMW 318 is, with an M50B25TU engine out of a '95 525i auto, courtesy of a friend. My current issue is as follows:

The car is running rich and idles badly after warming up. When I first start up the car, the idle is smooth but after warming up and driving it for some time, it starts to have a very rough idle. While driving it on the street or the highway, I see no issues but as soon as I come to a stop, the car lobes very badly and I can smell unburnt gas out of the exhaust. I pulled the dipstick the other day to check the oil level and it had risen. I initially thought that the head gasket was done for but the oil did not look like brown milkshake when I did the oil change. Also, the dipstick smelled of gas.

Secondly, after getting stuck in stop and go traffic, or even idling for more than 15 minutes, the car starts hesitating and idle becomes worse, almost to the point of stalling out (but it does not). The engine shakes like a mf, pushing the accelerator makes car jerk forward and I have to get the revs higher than 4000 for it to not do so.

I am not able to do a stomp test as the CEL is not wired. I bought a code scanner but I am not able to get any read as it looks like the ECU is not communicating with the scanner (although the scanner shows power). Also, this scanner was OBD2 which was connected using an OBD1 adapter.

I have cleaned the MAF and the ICV, but to no avail. I am going to check the fuel injectors and see if any is leaking.

I looked around in the wiring harness and did not find a pigtail for the O2 sensor. Jacked up the car and looked at the exhaust but could not locate the O2 sensor either. The car has aftermarket headers, which has the bungs welded in, but have been bolted off. Could this missing sensor be the issue? I am working on getting a AFR kit.

Any input on this will be greatly appreciated. Thanks!

fo3
05-16-2024, 03:45 AM
Sounds like MAF, but could be MAF wiring.

It's always going to run rich or poorly with no O2, but a bad O2, TPS etc should not make it stall or rough really rough (because it will just run default mapping). But a bad MAF really screws up the running.
Try unplugging it, then it will run default rich and not stall. If unplugging it makes the engine run smooth (but rich), then the MAF is bad.

The culprits you're looking at for very poor running is MAF, maybe ICV, maybe TPS, maybe cam position sensor.

okohigg4
05-17-2024, 12:48 AM
Thank you for your advice. I will test the maf. Currently waiting out for the afr kit, will install the O2 sensor and test it out.

Also, how does cam position sensor play into this?

fo3
05-17-2024, 09:18 AM
CPM just a curve ball last option guess as it helps control injector timing. Unlikely, so it's the last thing to check though.

okohigg4
05-18-2024, 12:07 AM
So, I was driving back from work today and got stuck in traffic. This time, the idle was constant at 500 rpms but the engine started shaking as it was dying and this could be felt through the car. I immediately pulled over and disconnected the maf sensor connector. The engine started running strong again immediately but the idle bounce started at the same time, bouncing between 500 and 1500 rpms. This confirms your theory that MAF is bad, but could this idle bounce be caused because of the maf being disconnected?

Shall I do smoke test to rule out vacuum leaks? Thank you again for your input!!

- - - Updated - - -

Also a side note, the engine bay was very hot when I opened up the hood. Do you think the maf sensor wiring is affected by this?

fo3
05-18-2024, 06:50 AM
Smoke test for sure, I replaced the ICV hoping it was that because it had a small leak on the plug area. Smoke test should be the first thing you do.

When I had a bad MAF it would run smooth, but very rich with it unplugged, but stalled and would not run at all with the bad MAF plugged in.

I replaced TPS, MAF and ICV trying to diagnose it - I did MAF last because a M50B25NV MAF is over $1500! A TU like yours is only $300 though

When everything was put together it hunted really bad for about a day and I was worried about what I have done and it's still stuffed? But after a day it sorted itself out.

But mine did not hunt/bounce in RPM with the bad MAF unplugged, it ran smooth but very rich. E: When I tried a cheap chinese resistor MAF it did run rich and hunt like crazy though. Check if you have a genuine MAF or a chinese resistor garbage one.

okohigg4
05-18-2024, 01:11 PM
I have a Bosch maf. Looks like a genuine oem part.

Anyways, I will try to get my hands on new sensor. Pelican is selling one. Is Bremi a reputable brand ($200) or should I bite the bullet and buy genuine BMW part ($600)?