Post-interference rebuild; poor idle (non-vac') - 'thoughts?

Wed Aug 19, 2015 1:22 am

Folks, I'm at the end of my wrenching-skills, following a post-belt-break
valve-job. The vehicle is an E30'vert with the M20B25 (non-eta) engine.
I had a few valves break, and couple bent - I replaced bad ones
from a donor head, all were 'roll-tested' for straightness, dip cleaned
and polished - and finally lapped in-port upon re-installing. Yet;
I have a "lumpy" idle (made worse by removing the oil-fill cap
on the valve cover), so it certainly seems to be top end blow-by; BUT:

1) After doing compression test - all good/spec - I did a leak down...
A) All cylinders similar, under 5% loss on any one at TDC
B) No audible hiss from exhaust, throttle or in rad/coolant
C) However, EVERY cylinder had slight air hiss through vc's oil cap
- It was audible, but not enough to even wiggle tissue paper
2) When rebuilding the head/lapping valves, I cold-adjusted them to spec
A) Running, the valvetrain is quiet; no audible tapping or clicking
B) No other anomalies in running; pulls strong, no lagging or smoke
3) To try cylinder isolation, once plugs were reinstalled, did a plug-yank
A) Sequentially removing plug leads, tried to find "weak" cylinder
B) Spark removal equally degraded idle, irrespective of cylinder

It's official; I am out of ideas. Any next-step tests welcomed. I am
loathe to yank the cover with nothing apparent to look-for <?!>.
....'Thoughts? THANKS, EVERYONE

Wed Aug 19, 2015 8:29 am

My money is on an air leak also. In addtion to willnz points i would work through all of the rubber around the intake and the TPS.

As i have personally found even when a hose looked fine removing it and inspecting it revealted cracking and small splits in the corners. My rought running was not as a result of a single air leak but seveal small leaks i found.

hope this helps.

Wed Aug 19, 2015 10:01 am

aimlessrock wrote:
As i have personally found even when a hose looked fine removing it and inspecting it revealted cracking and small splits in the corners. My rought running was not as a result of a single air leak but seveal small leaks i found.
.

This probably applies to 50%+ of all E30s out there!

Thu Aug 20, 2015 12:33 am

Fair enough, gents; I will refocus on unmetered air / vacuum leaks. I had gone through the rubber pretty scrupulously - but did NOT double check the bitch tube's re-seating: It seemed to re-engage nicely upon snipping the zip ties...but that potential oversight is getting a bunch of votes...

I'll be sure to post how the sleuthing goes, and thanks for the inputs!

Paul

Sun Aug 30, 2015 6:28 pm

Quick update: I struck-out on the smoke re-test results (having made a jury-rigged bidge for past E46 troubleshooting), as it indicated no evident errant wisps.

I finally did a stomp test - was late in considering that, having no CEL illuminated - to see if it reinforced the vacuum thesis; TADA = it yielded 1-2-2-2 (Lambda 1) code: This still can be sensors, vacuum, O2, etc; so I guess I am still back to checking final connections made upon finishing the head rebuild ; ...never any easy ones, I guess...but all ideas still welcome.

Sat Oct 03, 2015 5:03 pm

Still seeking "indie" in INDY: ...All - per the above, I began looking for e30 expertise in Indianapolis: I've now tried Merkle (dissatisfied/no-help), Mikar (great guys; too busy) - and it looks like I'm relegated to Reggies...who is well respected by the racers, but I suspect will be VERY costly as a "boutique shop"...

Does anyone have any favorite BMW service experts in the greater Indianapolis area? It's quite driveable - but wouldn't relish a trip to Chicago unless my only recourse. 'Continued thanks, guys.