M42 Starting & Misfire Problem

[rlich8]

1991 318iS
41xxx miles
Clean, unmodified car.

Car has trouble starting, but eventually does. Misfires/does not run on all 4 cylinders. Some kind of metallic knocking/pinging (!!! weird !!!). Problem first occurred yesterday, then went away and the car ran just fine. Reappeared today. I have garaged the car for now. No CEL light.

Plugs look good. Pulled the cam cover, the timing chain is tight and there is no wear at all on the sprockets; looks exactly as a 41k mile car should. Oil is super clean.

Recent maintenance performed: cleaned and rebuilt fuel injectors, new fuel filter, new plugs. Oil change approx 1k miles ago.

Any ideas?

[5Toes]

Im curious what this problem is gonna turn out to be, free bump!!

[varg]

So did it start doing this gradually, all of the sudden? Does it ever run smoothly or is it a dead miss?

[rlich8]

I got a little curious yesterday, so I began with some basic diagnostics. After checking the obvious things (spark, fuel, crank and cam sensor), I decided to do a compression test.

The first cylinder checked out just fine. The second cylinder unfortunately has 0 compression. The spark plug came out looking like as if it had been dropped and was pretty mangled.

I haven't had the time to pull the head yet, but it looks like I'll be doing an engine rebuild.

Worse things have happened, I guess. THE WEIRD THING is that it happened, then went away, and now of course the damage has been done. I'm curious to see what (probably) put a hole in the piston.

[varg]

Compression test. That's what I was going to suggest if you said it was a dead miss. No point in attempting to diagnose it with the head on. It's time to pull that baby off and assess the damage.

[5Toes]

hey man its not too hard to replace the head on the m42. The set of cam locks you need I swooped up off eBay used once for $40, and the actual gasket set you need with bolts is $200 (VR upper gasket set, goetze head gasket, VR head bolts). Id recommend ordering two new block dowel pins, 12 new exhaust header copper nuts, as well as a new thermostat.

Its not really that hard, just takes two people to gently set the head into place. It can be done in 10 hours or less.