BMW '89 with M40 Engine having 100000 miles on it. Recently (last 3-4 months) I noticed a problem regarding engine idle.
1. Engine would not idle properly, even if cold/hot/ winter/summer & the revs would go from ~650 to ~950 up and down. This happens randomly though and I couldn't figure a pattern for it.
Also when the engine is left idle and you opened the hood, you could hear & see the engine running smoothly but for a split fraction of a second it would stop working thus shaking the engine a bit and droping the revs below 650.
My mechanic checked the car for vaccum problems but did not find any. The oil & fuel filter were changed with OEMs, and for intake I placed a K&N air filter. I also replaced the idle valve with a brand new one but still no fix. Afterwards, my mechanic also changed a relay that was not in the fuse box but near the engine compartment (don't know where exactly cause I wasn't there) and he said that that relay is used somehow in managing the engine idle revolutions. Oh and in adition, all the fuel hoses were replaced because I had a fuel leak a while ago. Still no fix!!!
The only visible problem that i *think* it may promote an idle problem, is the oil re-circulation hose, which is used to take evaporated oil and send it back to the engine. There seems to be oil around that hose which leads to an oil leak from the hose but I haven't changed that. Could that create idle problem?
Any other suggestions? What other problems are related with bad idling and are common for this engine?
2. When the engine is cold, and i press the gas petal, a cracking sound can be heard from the engine at exactly 1500 revs. Like the valves are hitting somewhere or something similar. This goes away as soon as engine reaches it's operating temperature. Any ideas?
I don't know if this plays a role in everything but the car was powered with LEADED fuel 98 octane from 1989-2004 and then I migraded to UNLeaded 98 octane fuel due to the fact that regular fuel production stopped in my home country. I also tried 95 Octane Unleaded fuel bad I had same problems as well as 100 Octane.
Engine is being serviced at regular intervals, never taken out of it's limits, opened,abused etc. I use 98 Octane Unleaded fuel. Even clutch is the same from 1989
Thank you for your time
Paris
