M50 2.5L Idle revving up and down on start-up

[paulo 59]

I have bought a replacement engine for my e36 325i.

The previous owner had it runnng nicely about a year ago, but then started to disconnect wiring etc in preparation for pulling out engine + trans for a v8 transplant, but project got put on hold...

He reconnected everything back up again a couple of days ago, so that I could hear enging prior to purchasing.

On start-up, the engine alternates in speed between about 800rpm and 1200rpm about 6 times (about 1 sec. at each speed), then raises to about 2000rpm, and it will sit at that high speed.

Sounds to me like the ecu is compensating for something (air-leak??) or looking for a signal that isnt there, as it definitely sounds like a controlled modulation of idle speed.

We have had a quick check of obvious hoses and connections, and have disconnected battery for about 10 min. to try and reset ECU (O2 sensor was discovered off, but didn't make a difference when reconnected)

Catalyst light is also on, but from my research on the web, that does not necessarilly mean a catalyst / o2 fault - it may represent another fault.

Any help would be appreciated!!

[bravo 34]

You're on the right track. "Hunting" idle is often a vacuum leak. Check all of the hoses on the intake path after the Air Flow Meter, and on the intake manifold for poor connections, or cracks/leaks.

Is it only when cold or warm? or both? Could be a sticky cold start valve?

[pureboiracer 0]

I would be checking any intake sensors, ie maf or map tps ect ect. these sensors being disconected or connected incorrectly can cause a hunting idle. or sometimes no idle. and can also put up the engine light.

[paulo 59]

You're on the right track. "Hunting" idle is often a vacuum leak. Check all of the hoses on the intake path after the Air Flow Meter, and on the intake manifold for poor connections, or cracks/leaks.

Is it only when cold or warm? or both? Could be a sticky cold start valve?

Have only witnessed this when the engine was relatively cold. After several attempts over 1/2 hour (running for only about 30- 40 secs each time) the engine was mildly warm and still displaying the same symptoms.

Will be making more extensive checks tomorrow...