E34 M30/M50 Engine Swap

Your engine is hunting. This has nothing to do with the altitude. It is most commonly due to a bad or dirty ICV, but then that behaviour should be the same at all altitudes.

Can you pull codes from the car ? Does the stomp test work or do you have a code reader ?

If you don't and can't get to one easily....try this. Delete all the error codes. Just unclamp your Ecu for a few minutes and reclamp it. Then start the car and see if the engine hunts.

If it still does, it will be time for you to start disconnecting components to identify the gremlin. Disconnect the following, in this order :

1. AFM. Pull its plug.
2. O2 sensor (by removing the O2 sensor relay).
3. ECT (engine coolant temperature sensor, the blue sensor on the cylinder head, closest to the front of the engine and there are two over there). Disconnect it.

Disconnect them all in order until all three are disconnected. Once you've zeroed in on the suspect, reconnect everything else except the suspect and reconfirm, then you've identified your gremlin. Delete the error codes for everything else, replace the defective component, then delete the error codes again.

I once had this same stupid hunting problem. Paid a bmw specialist workshop to fix it. They took 30 minutes and charged me $80. The fault? The switches for the two coolant temperature sensors in the cylinder head (on leads to the ecu, the other leads to the dashboard gauge) had been switched around ! Still can't believe that it was that simple and it cost so much.

Since you've been doing some engine swapping (?) there is a possibility that you mixed something up in the wiring.

Wait.....hmmm........i remember something now. Yes we've seen this problem before. Look at your throttle position switch (tps). Make sure you've connected the correct socket from the wiring harness to this. There might be a switch for the intake temperature sensor right near the brake booster fitting that you could have mixed up with this.

For good measure, please clean your tps and its socket with contact cleaner, apply dielectric grease on both, and fix it back. The tps could well be the problem.

Hmm......you know what ? Add the tps to the disconnect list stated earlier. Put it at the top of the list. See if that solves the hunting.



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I already cleaned the MAF previously and checked for carbon residues but that did not solve it. I cleaned the error codes and started disconnecting as follows:

- TPS: Did not solve the problem
- MAF: I disconnected the MAF, cleaned the error codes and kept the car connected to the fault reader. I started the engine and the moment I put the MAF plug in, the engine almost turned off and generated what seemed like a breathing sound and started hunting again. I am now searching for a MAF with the same number to replace it and make sure that this the cause of the problem. Thanks for your help guys!!

One more thing I noticed, was that one of the exhaust joints gaskets, right after the catalytic converter is leaking some gas. Would that cause additional symptoms?

So you are saying that the engine performs normally when the maf is disconnected ? Then that is clearly the problem.

Hello Again....
The engine is back to the same hunting problem and apparently this time it is not the Air mass as I repeated the exercise and I unplugged the Air mass plug but the engine kept hunting even after the air mass is disconnected. The hunting this time is severe as the engine almost goes dead then the RPM jumps to around 1.2~1.3. I decided to continue in the list that you posted in the previous post, but I was surprised to find that the Oxygen sensor relay DOES NOT EXIST in the E-Box next to the DME. There are only two relays (Fuel pump and DME). How come is this possible? And what do you suggest?

1- Shall I unplug the oxygen sensor plug from below the engine?
2- If I am to continue with the above check list, how do i disconnect the ECT?
3- How can I check if the ICV is working or not?

P.S: The hunting is only happening when the engine starts approaching normal operating temperature. The fault reader registered two readings:

1- Air Mass
2- Oxygen Sensor Heater

Thanks for your help as usual...

Find the o2 sensors relay. Better in the long run. Keep pulling relays and checking for the error code. The ects is under the intake manofold, you must check the bentley manual. The icv is under the throttle body, remove the alternator ducting, key2 the ignition, the reach to the icv and touch it, if it buzzes to the touch, then it is most probably ok.

i just did the maf disconnect test on my car. For some reason, after a few minutes at idle, the engine seems to hunt. No real difference while driving though.

OP, I think your O2 sensors and maf are screwed. Both of these devices are capable of messing with your engine independantly, but when they go bust together, look out.

It is quite likely that your maf is the main culprit, and if you fix that, your o2 sensor code will go away.

Coming back to the disconnect test, if you cannot find the o2 sensor relay despite your best efforts, then just disconnect the plug manually, and see if there are any differences in your engine's behaviour.

Replacing the Air Mass with a new one and manually disconnecting the O2 sensor has fixed the issue. Thank you for your support.

Hi again
I am facing a new problem in the Carbon monoxide (CO) emissions. In mu country, the vehicle has to pass a yearly mechanical inspection. My car is failing the inspection at the CO emissions measurement step. The CO, measured from the exhaust, is reading 8.1 while it should be between 1 and 4 in order to pass. What reasons might be responsible for that? Given that i have a new air mass and RPM is now stable. I am suspecting the O2 sensor as I said earlier, i could not find its relay in the E-box where it should be. Please help. Thanks!

could be an o2 sensor. More likely the catalytic converter. how old is it?

I am sorry I forgot to mention that before inspection, I was checking the catalytic converters and I found out that they were almost blocked, so I replaced them with empty exhausts and did not put new ones. Therefore, the car does not have any catalytic converter installed.

the reason you are failing is because you do not have a catalyst.

Wow good read, did you change the spark plugs and cleaned the injectors. The engine was sitting around for some time the injectors could have cloged.

Hi... I bought a new catalytic converter and installed it. The CO emissions dropped from 8.1 to around 5.0 but still that was a failure. After long diagnostic events, I discovered that the Air Mass plug wires were not connected in the order they should be. I reconnected the wires of the Air Mass plug in the proper order and Bingo, the CO emissions dropped to 1.0 and the vehicle passed. However, I am experiencing now some slight drop in acceleration compared to how it used to be before changing the Air Mass plug wiring order. In addition, in the inspection report, they wrote a note that there is some raw fuel coming out of the exhausts which needs to be followed up. I was surprised by this note as there are no signs of rich engine or flooded engine. Any suggestions??

Yes I did buy new spark plugs and by default, changed all filters and belts and such stuff. However, I did not touch the injectors yet.