'89 525 missing a cylinder or two...

[khav]

So...recently purchased an '89 525i for a song. Valve train was a bit noisy, but solid and clean. After some suspension bits I was on the road and life was good...until last week. I was heading up a long incline at 65mph and felt a serious drop in power accompanied by a rumble that felt like a dead cylinder.

I nursed it home...same miss at idle and under load...zero power up any kind of hill, but moves pretty well on flat highway...or at least enough to get home. Pulled the plugs and noticed a chunk of the center electrode insulator was missing from the #1 cylinder...yikes! so, replaced plugs, cap and rotor hoping for an easy fix..they were toast anyway...but no go. Coil and wires check out...pulling the wires fron #2 and #4 make little difference at idle, so I think those are potential suspects.

No external vaccuum leaks but, interestingly enough, removing the oil filler cap causes a drop/recovery in idle similar to a vaccuum hose removal...hmmmm...never seen that.

Anyway, compression test tomorrow and hoping the broken bit of plug insulator didn't bugger the valves, but I wanted to get a general impression of potential issues from the seasoned members here who may have been in a similar situation.

Please and thank you-

Kurt

[LouieD]

Welcome and good luck. The crankcase is under constant vacuum hence the stumble at idle with the cap off. Normal operations there. Compression test and/or leak down test should yield answers or at least point you in the right direction.

[Binjammin]

Yeah, if you've got a repeatable skip on only two cylinders, have replaced all tune-up parts... it's time for some testing. I wouldn't be surprised if it's time for a head gasket. Be prepared for the worst, but remember the m20 is a cheap, plentiful motor.

[ross1]

The plug might have come apart from detonation which could also have damaged a piston(s) or rings. I doubt HG as the cylinders aren't adjacent.
Just saw an M20 advertised here or on b-nut with alleged 20k for $500 if you need one.

[Touhou]

my 4th cylinder rings are toast, but i still drive on anyways.
once catastrophic failure occurs, motors will be swapped :P

[5mall5nail5]

An M20 short 2 pistons is called an M10, and if thats the case, throw it in the trash

[khav]

A few days late, but checked compression tonight...#6 is the culprit with 80 lbs. The first clue was when the plug came out...nearly spotless. So, hoping to only find a bad rocker tomorrow and not a toasted valve. I'm thinking that the phantom issues with #2 and #4 were due to their place in order. Onward.

Oh, 5mall5nail5...I heard that red anodized calipers reflected gamma rays and made the rotors last longer

Kurt

[5mall5nail5]

Originally Posted by khav

A few days late, but checked compression tonight...#6 is the culprit with 80 lbs. The first clue was when the plug came out...nearly spotless. So, hoping to only find a bad rocker tomorrow and not a toasted valve. I'm thinking that the phantom issues with #2 and #4 were due to their place in order. Onward.

Oh, 5mall5nail5...I heard that red anodized calipers reflected gamma rays and made the rotors last longer

Kurt

Dudes - sounding like a headgasket if the plug is super clean! Then again if the intake valve isn't opening that'd do it too... but then you should have very very very very little compression.

[khav]

Originally Posted by 5mall5nail5

Dudes - sounding like a headgasket if the plug is super clean! Then again if the intake valve isn't opening that'd do it too... but then you should have very very very very little compression.

Compression hit max quickly so wasn't thinking gasket or rings...but I'm getting a little beyond my depth here. I'll report back with the findings.

Kurt