Erratic RPMs E30 325

[OP6 27]

Just put in a replacement P/FL M20B25 engine into a F/L E30. It runs, but, 2 issues which I am sure are related.

1. The engine will idle but probably on 5 cylinders, the missing cylinder may come in at about 2000rpm, anyway at 2000rpm and higher, on constant throttle, the revs drop to idle and then back up just like a dirty ICV. The drop to idle and back up occurs in about a second and occurs all the time unless the throttle is slowly increased, in which case it performs normally.

2. Taking the oil filler cap off makes no difference to idle or oscillations, suggesting an air/vacuum leak somewhere. There is a reasonably strong smell of fuel coming from the oil filler when open.

This engine sat in the garage for a couple of years and was supposedly "good" when pulled.

Leaking inlet or exhaust valve seal? If so, how to fix.

Your thoughts please gentlemen.

Cheers Don

[Jono51 58]

check all the basics

so is it an unknown engine, did you here it running before??

firstly start with a compression test

Edited April 14, 2013 by JKSE

[OP6 27]

with the engine warmed up, it will now idle on all 6 cylinders happily with a good idle.

however although the engine is very reponsive to the throtle is very revy. when the throtle is held at one fixed position (say 2500 rpm) it will drop to idle and then rev right back up. it doesn't misfire just does this as if it is hunting.

[///MPROVD 131]

with the engine warmed up, it will now idle on all 6 cylinders happily with a good idle.

however although the engine is very reponsive to the throtle is very revy. when the throtle is held at one fixed position (say 2500 rpm) it will drop to idle and then rev right back up. it doesn't misfire just does this as if it is hunting.

sounds like a vacuum leak taking the filler cap off wont change revs much if a leak is already present maybe try a smoke test and see. just a thought