Post interference valve-job poor idle not vacuum; 'thoughts?

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Post interference valve-job poor idle not vacuum; 'thoughts?

Postby moapys » Wed Aug 19, 2015 12:08 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 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
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby Yoshi » Wed Aug 19, 2015 12:18 am

M20s have a pretty strong PCV system. There's this huge line that goes from the valve cover to the throttle body POST AFM. That means that when you open the oil cap, you're creating a vacuum leak and that's why it idles worse.

With the oil cap in, how lumpy IS the idle? Sometimes that's just how your engine is. But also, I'd suggest checking for other vacuum leaks. Mainly the charcoal canister line under hte throttle body. Sometimes the bitch tube under the intake manifold can leak. Basically, get your intake system 100%. I think your engine and head job went great.
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby moapys » Wed Aug 19, 2015 2:39 am

Thanks, Yoshi; it ran smooth-as-silk before the T/B-idler sheered, causing the valve tap dance... That's what tells me that something is amiss after the rebuild.

Admittedly, I may have been hasty to claim "not vacuum" - as I did perfunctory spray tests on hoses / joints...but unmetered air could be at fault I guess. I replaced the 'big bootie' (rubber intake plenum) just to be sure, but it had no affect. I'll do some additional rooting around on the intake side. Thx again!
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby Yoshi » Wed Aug 19, 2015 12:05 pm

T/B-idler? what's that?

It does seem like something head related would be to blame but you also had to take apart so much else. Any of the things you took off could now be an issue. Gaskets don't reseal correctly, you forget stuff, 30 year old rubber cracks.
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby 87e30 » Wed Aug 19, 2015 12:51 pm

Yoshi wrote:30 year old rubber cracks.


So I should probably replace that condom in my wallet? :-D

Yoshi wrote:T/B-idler? what's that?


I believe he meant timing belt tensioner, if I'm not mistaken.
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby M20_fever » Wed Aug 19, 2015 1:52 pm

So I should probably replace that condom in my wallet? :-D

Doesn't matter, you won't need it.
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby M20_fever » Wed Aug 19, 2015 1:54 pm

So I'm assuming the car idled fine before the timing belt broke? 9 times out of 10 when you have a vacuum leak on an M20 after pulling the intake it's the bitch tube not sealing. It nearly impossible to diagnose since you can't really check up under the intake. Maybe with a smoke test, even then it's hard. Did you replace the seals on the tube with new ones or reuse the old stuff?
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby 87e30 » Wed Aug 19, 2015 10:19 pm

M20_fever wrote:
So I should probably replace that condom in my wallet? :-D

Doesn't matter, you won't need it.

BURN!

back on topic.. Might I suggest taking a video of engine start, idle, revving, and idle while removing oil cap? That may be of use to someone trying to give advice. Post it up on youtube and link it.
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby moapys » Wed Aug 19, 2015 11:23 pm

Great inputs, guys - and I just cleaned and silicon-greased the existing bitch tube seal (had been done a couple of years before)... And yep; it was the belt tensioner/idler whose shaft sheared putting in this situation. The b'tube seemed to snap nicely back in place after the zip ties were cut - but I admit not to scrupulously checking that. While it would seem that if there was that much unmetered air / vacuum loss through the b'tube, that the filler cap removal would have less impact...but I'll dismiss NOTHING.

Lemme do a b'tube check, and do some "filmography" in the next night or so (before the nice weather is gone!).
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby Ryki » Thu Aug 20, 2015 7:55 am

Hmmm, b'tube filmography huh?
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby 87e30 » Thu Aug 20, 2015 12:31 pm

Where's this nice weather you speak of? It's been hotter than ass here, humid, and rain an the worst times. The highway on my way home last night was 3 feet under water!
Schwarz 5-speed 87 325is (newest project)
Arktisblau 5-speed 86 325e (current DD)
Schwarz 5-speed 87 325e (parts car/LS1 swap?)
Gazellenbeige automatic 85 318i (70,000 miles)
Alpine white 5-speed 87 325is (my first e30, possibly for sale)
99 Suburban
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby moapys » Tue Aug 25, 2015 10:22 pm

Well, I did a re-spray (used a couple of solvents), plus used the boroscope to look at the b*tchtube - and it looked seated and tight. I'm having a chap by who has better diagnostics gear; maybe he can pull an early code that will give us a hint.
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby M20_fever » Thu Aug 27, 2015 2:38 pm

Have you done the stomp test? That will give you any codes stored in the ECU.
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Re: Post interference valve-job poor idle (maybe not vacuum)

Postby moapys » Thu Aug 27, 2015 10:52 pm

AH; the stomp-test... I had forgotten about that - and was awaiting an aquaintance to bring his code reading gear by (mine is just OBDII). Excellent! I will remind myself of the "dance", and perform it tomorrow - 'looking forward to any insights thereafter. Thx for the reminder, Yoshi.
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Re: Post interference valve-job poor idle not vacuum; 'thoug

Postby Yoshi » Fri Aug 28, 2015 11:51 am

You mean Tony? lol
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