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Old 10-06-2016, 11:50 PM   #1
PKile
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Anyone have an M20 that idles perfectly with no audible misfire?

I've just finished the complere rebuild on my 92 vert...new pistons, bearings, balance, all new gaskets and seals, full valve job, new cam and rockers, all new lines and hoses, new FPR, new blue injectors, new intake boot, all new ignition, etc. Before the rebuild I had an erratic hunting idle, dying when cold, dip and recovery in the idle when coming off the throttle, all the things that have been discussed to death on this forum and others with no one actually coming back and documenting a specific cure.

Now that the restoration is complete, all those symptoms have gone away...except one. The idle is steady on the tach, no more dying, dipping or hunting. But when it is idling when warm there is a slight audible irregular miss rrrrrrrrrrrr-m-rrrrrrrrrrrrrrrrrrrr-m-rrrrrrrr-m that can also be felt in the seat of your pants. I saw a post somewhere that said this may be an endemic Motronic trait. The only things I can think of that might be responsible are the throttle body, AFM, ICV, or the ECU.

So my question is does anyone have a car that doesn't do this, or better yet one that had the symptom before but has been corrected?

Cheers,
Paul
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Old 10-07-2016, 05:32 AM   #2
ahlem
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There are several vacuum hoses and the related fittings that can be slightly loose and allow a combined vacuum leak. The fittings that attach to the throttle body are suspect as well as the idle control valve. I added sealant at the metal to metal fitting connections and hose clamps at the hoses that didn't have hose clamps and solved the problem for now anyway.
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Old 10-07-2016, 09:17 AM   #3
Downeaster1
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Ironically, I was just about to post something similar. Same exact situation. Car runs flawlessly, however after replacing damn near the whole engine, I get these ever so slight misfires. More so noticeable when standing right next to the exhaust pipe, but can also be felt in the seat of your pants. I have new plugs, wires, cap, rotors, basically everything and am equally as lost.
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Old 10-07-2016, 10:22 AM   #4
LateFan
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I'm no expert, but could it be that huge cam and high compression you built in? ..because race car?

But seriously....when I was a kid and rebuilt a 302 Bronco - when I knew nothing about fixing engines - I would go across the alley to the neighborhood mechanical wizards constantly. "What's that noise?!" "Did you hear a miss?!"

They would shake their heads and say, It's fine. It's normal. It's an idling V8. It's brand new and it'll settle in.

As long as it runs smooth to redline, and has steady pull, and doesn't stumble or bog, it's probably fine. And like these guys have said, these seem to be susceptible to little vacuum leaks here and there. A lot of fittings and hoses connected to the system. And the valve cover gaskets. Just check all of them.

Did you replace the ICV and rebuild the AFM? We sent ours to the r3v member in CA who rebuilds and calibrates them. (Lost his name...)
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Old 10-07-2016, 12:51 PM   #5
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I do not have an e30 that idles perfectly
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Old 10-07-2016, 01:25 PM   #6
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If it's the sudden stumble it's normal I can't think of what does this to the m20 but mine does it and everyone I know


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Old 10-07-2016, 03:00 PM   #7
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I think that's just a characteristic of the cam and the flapper afm.
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Old 10-07-2016, 05:07 PM   #8
PKile
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The ICV is not new, but I cleaned it. I took a can of carb cleaner and sprayed the vacuum fittings and the throttle shaft on the throttle body...no discernible change to the idle. The AFM is not new, but a buddy of mine and I have pooled our junkyard AFMs and we are sending them off to Bavarian Restorations to get a couple of rebuilt ones.
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Old 10-07-2016, 06:17 PM   #9
jeffnhiscars
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Paul, I wish I had a lucid answer for the main question since I have 2 such e30s. Neither has a miss, stumble or hiccup and they both pull all the way without missing a beat. I know you well enough to know you've checked all then usual suspects.

The only thing that comes to mind is that both my cars have stock internals, fresh injectors and sssquid chips.
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Old 10-07-2016, 08:04 PM   #10
PKile
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Jeff, my engine is the same, minus the sssqid chip. Bear in mind that the misfire is very mild, not ignition related (tach is rock steady), and something I can live with for awhile. That being said, my anal nature requires that I get to the bottom of it!
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Old 10-08-2016, 05:06 AM   #11
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I've never heard a 6 cylinder E30 with a perfectly smooth idle. They all seem to miss on a cylinder once in a while. Sometimes nothing more than a single miss over the course of a minute with an otherwise even idle. Maybe they did it from the factory. Strikes me as just something you're going to run into with what may as well be batch fire fuel injection and a not exactly state of the art (even for the time) engine management system. I'll bet with a sequential EFI setup and a good tune you could make it idle perfectly at 800rpm.
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Old 10-08-2016, 06:34 AM   #12
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"Dip and recovery" after a throttle blip = rich mixture.
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Old 10-08-2016, 06:48 AM   #13
wilholl
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I actually do have an m20 that idles as smooth as butter!
No Joke!
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Old 10-08-2016, 07:41 AM   #14
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Quote:
Originally Posted by wilholl View Post
I actually do have an m20 that idles as smooth as butter!
No Joke!


One more here!
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Old 10-08-2016, 09:52 AM   #15
jeffnhiscars
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I had an 01 Lexus that had a stumble. After much fuss the service manager told me most of them have that and they didn't have a fix. Great !!

My 550i did it too. I knew the dealer people very well and they said it had to do with an EPA mandated change in fuel additives and they hadn't written the software to compensate for it yet. A few weeks later they reflashed my ecu and it was fine.

I gave up hoping for a smooth idle yet here I am with 2 29yo cars that have one.

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