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Unstable/High Idle

Posted by sdp 
sdp
January 27, 2009 11:29AM
Hey there all... I was bummed when I ended up at that Russian site, but I found you all again and the new forum looks nice.. Kudos to those who organized/setup the new forum.

My sons E30 "died" this week end. Something "popped-banged" in the engine compartment and started running rough. He stopped the engine and we towed it home the next day.

I thought something went wrong with the rockers/head because we hadn't opened up the valve cover to readjust since we replaced the head a while back. As we started removing the nuts to the valve cover I saw that a spark plug cable was hanging down.. I was shocked to find that the #6 spark plug was missing. It blew out!.. Well we found the spark plug in the corner of the engine compartment, screwed it back in and Voila.. running again...

Question #1:
When we replaced the head (with a used/reconditioned head ) we found the threads on the #6 plug to be damaged. We bought and re-threaded the hole with the spark plug tool and everything was good. Now I'm wondering if the damage/rethreading has created a less than ideal fit with the plug and am I prone to having the plug loosen up again? Is there anything I can use to tighten the fit (i.e. a special Loctite) or will I need to have a shop put in a HeliCoil?

Question #2:
The engine has been running very rough for the past month or so (I wonder why? HAHA)
In addition to the loose plug we found a bad air leak due to a tear in the intake bellow.
With the plug back in and temporary duct tape work on the bellows, the engine is running nicely, but the idle is somewhat unstable and rather high.
My question is whether the computer "learns" the combustion conditions of the vehicle? Has the computer been compensating for the plug and intake and now that it is fixed it is now overcompensating? If so, is there a simple way to reset the computer? If it just takes some time, how long does it take to relearn?

Thanks
January 27, 2009 12:09PM
If you want to reset the ECU, disconnect the battery for 10 minutes. It will self learn its settings again. If idle isn't stable after a while, suspect something else.
January 27, 2009 12:13PM
You can reset the computer to factory defaults by disconnecting the battery for 5 to 10 minutes.
Then you may have to wait while the ECU relearns the engine and your driving style.

The ECU usually normalizes after several restarts so waiting isn't that big a deal.

I wouldn't bother with any of this until the new bellows has been installed; duct tape does a good job but there will still be small leaks.

The spark plug hole may have to have a helicoil put in because once the plug threads are damaged and any attempts to clean them are done, the factory fit will never be achieved. Even the helicoil approach isn't a guaranteed fix if it isn't done properly.

Much simpler to just check the plug every couple of weeks when you check the oil smiling smiley
January 27, 2009 12:24PM
Quote
sdp

Question #1:
When we replaced the head (with a used/reconditioned head ) we found the threads on the #6 plug to be damaged. We bought and re-threaded the hole with the spark plug tool and everything was good. Now I'm wondering if the damage/rethreading has created a less than ideal fit with the plug and am I prone to having the plug loosen up again? Is there anything I can use to tighten the fit (i.e. a special Loctite) or will I need to have a shop put in a HeliCoil?

I have no idea about loctite on the threads of the spark plug but I do know that the Bentley manual gives a specific torque value for the plugs themselves. Maybe you just didn't tighten that plug enough as it is a tight fit for that particular plug. I would torque it properly and drive it a while and check again to see if it has loosened.

Question #2:
The engine has been running very rough for the past month or so (I wonder why? HAHA)
In addition to the loose plug we found a bad air leak due to a tear in the intake bellow.
With the plug back in and temporary duct tape work on the bellows, the engine is running nicely, but the idle is somewhat unstable and rather high.
My question is whether the computer "learns" the combustion conditions of the vehicle? Has the computer been compensating for the plug and intake and now that it is fixed it is now overcompensating? If so, is there a simple way to reset the computer? If it just takes some time, how long does it take to relearn?

To reset the computer all you do is disconnect the battery for a couple of minutes and the computer will reset. If you still have a high idle after that then you probably need to take off the ICV and give it a good cleaning with carb cleaner. Also if your O2 sensor is bad it could cause problems once the car has warmed up and the ECU is using that input to determine whether or not to lean or richen the mixture. I would also check the other hoses that can let unmetered air in and replace if necessary...the one that connects to the valve cover especially!

Thanks
sdp
January 27, 2009 01:23PM
Good deal.. Thanks guys.
January 27, 2009 10:51PM
Funny thing, I just went through this on another vehicle I just bought (I knew it had this problem when I bought it). PO had messed up the threads on all of the cylinders by putting the wrong ones in. A buddy of mine (who I bought it through) had cleaned up all of the cylinders but one. One of them was just too bad and when he put the plug back in, the engine blew it out a short time later. I was able to helicoil that and it runs great now. BTW, you can helicoil it yourself pretty easily. You might even be able to do it without taking the head off.

On a brighter note, a friend of mine with an e30 had one fall out one time too. Actually, I think it was the #6 as well. Anyway, it just hadn't been tightened up properly by the shop he had working on it I guess. He put it back in and tightened it up and never had another problem! Did you take a good look at the plug? The one that blew out of mine was all banged up from hitting something when it came flying out, so make sure it is ok and that the gap is set correctly.

I would just tighten it up to spec and check it once a week or so. If you don't notice any change after a couple weeks I would say you are probably just fine. smiling smiley
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