Rough idle (not vacuum issue) after valve-job

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 cyliner 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

I am thinking that the valve adjustment may be the problem. I did just what you described in adjusting the valves before the head was bolted and torqued to the block. Once I had it torqued down, checked and found that the valves were all out of adjustment again. Suggest you readjust the valves now and they should not be completely quiet. Some slight clicking should be audible.

Bob in Lake Havasu



Edited 1 time(s). Last edit at 08/18/2015 07:54PM by Bob in Everett.

I agree. Tests have shown that the M20 runs better with slightly more valve gap than specified by BMW.

Thanks for the inputs, gents; and I had planned to let things settle in (let the lapped valves seat a bit more, etc) and then do a re-adjust. I'm getting a bit of feedback from other boards about insidious leaks around the "bitch tube" - which seemed to re-seat fine after the zips were cut - e.g. a poor seal around it.

I'll do additional sleuthing, re-spray test lines/couplers, etc., maybe smoke test...and update everyone on what's found. Much obliged!

Quick update: I struck-out on the smoke test (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.



Edited 1 time(s). Last edit at 08/30/2015 10:13AM by moapys.

Well - the latest...: I cleared the 1222 code - it was a holdover from past vacuum noodling - last several starts/trips resulted in no (eg 1444) code. Alas, so much for that red herring... I did re-adjust the valves, stone cold, they were a bit tight, so put them back to a solid .010 - and checked both ends of the rockers for .01 / .007 consistency...all good. I even checed the 02 sensor for good measure; proper waveform and voltage levels.

Finally, I took it to an independent ASE shop for a real smoke test - which they claimed it passed "with flying colors". Since the lumpy idle persists, I am throwing in the shop-towel and taking it to a specialist. I only hope they read my litany of tests run...versus assuming that they're starting-from scratch :-( ...I'll be sure to post the results (beyond wallet depletion) once received!

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.

Did you ever readjust the valves?

Bob in Lake Havasu