rough idle

Having just recovered my 1989 from the repair shop (previous post) with planed heads, new head and misc. gaskets, new coil pack, new idler pulley, new serp. belt, new fuel sensor, new spark plugs/wires, several new belts/hoses, and new timing chain, the car has developed a rough idle. The mechanic knows not why.

Idle is roughest under load and maybe slightly low in terms of RPM (600 - 800). Seems to work itself out as engine speed increases but is quite irritating at stop signs. Any thoughts?

[Vincent Vega]

Bad Injector(s)? Fuel pressure? Fuel Filter? I was watching the 2 guys show on the Speed channel the other day. They had a car with a rough idle that was throwing a code(OBD II)
I realize yours is not throwing any codes but this amy be part of it. What they did to diagnose it was hook up a fuel pressure gauge. Then, one at a time they pulsed the injectors to monitor the fuel pressure drop. On one of the injectors, the pressure drop was far less than the others. They replaced that injector and it cured the rough idle and code. Worth a try.

[padgett]

Traditional means is to pull plug wires and see if there is one that had no effect but not certain if dual spark will take both cyl out. Guess modern way is to pull injector connectors one by one and look for same, is how I diagnosed son's quad 4.

With all those new components, is not unlikely that something experienced "infant mortality" or in the process, something got joggled that souldn't be.

First step is as above. If no luck, second step is to take system scan and look for something abby-normal.

[Gomer]

i hade a simaler problem with mine. i found the fan ate my plug wire. it would idle rough but at 40 miles it would smoth out. it only ate through half way so it was not noticable. just thought i would give my 2 cents [img]/ubbthreads/images/icons/smile.gif[/img] hope this is the same problem u have and i doesn't cost u any more [img]/ubbthreads/images/icons/confused.gif[/img] . i found with mine it took a ziptie to hold it far enough away from the fan so it wouldn't get eaten. 2 wires later [img]/ubbthreads/images/icons/mad.gif[/img]

[Orono]

It may be possible they did not have everything lined up correctly when they installed the timing chain. If they were off by one tooth the car would still run but not as it should. Not sure how to verify this other than pulling off the timing chain cover.

[padgett]

Never tried but suspect with a tooth off it would be down on power but still smooth. Possible but "rough" usuall means missing a cyl or two.

The ECM has a remarkable range of adjustment. Son's quad-4 with a bad coil ran relatively smoothly on 2 cyl. No power but ran.

If the problem is a miss you should be able to feel with hand over exhaust pipe opening.

Found the problem with the rough idle...bad valve guide on #5 cylinder. I'm now into this car for about three times its worth -- soon to be five as I am having the engine pulled for a rebuild (my luck is that as soon as the top end is new, a rod will fly from the block and probably kill the nun and several orphans who happen to be crossing the street) unless I can find a decent long-block crate motor. Any ideas as to suppliers???

Thanks

[Greg Ross]

Did you determine this (or confirm) with a compression test. I had a burnt valve that caused what you describe for symptoms.
Ist option is to simply replace the the offending valve and valve seal, thereby not loading up the bottom end.
2nd and only a slightly more expensive solution is a used engine. Can be bought with some guarantee.
3rd is the remanufactured route, GM GoodWrench can be had for around $1600. +++
4th and Megga Bucks is a link Robert had come up with a while back for the complete (New) 3800 Series II Crate Deal for $3200. +++
The decision is a simple function of the depths of your pockets. [img]/ubbthreads/images/icons/wink.gif[/img]

[Vincent Vega]

Please share your method of diagnosis. I'm curious.

Bad compression led to removal ov valve cover and play in the valve guide was fairly obvious after that (.25 inch or so). This is a set of heads I had just had planed and put back on -- wish someone would have paid more attention to the valve train before re-instaling as I could have saved the tuition.

The mechanic that found the bad valve guides is also the guy that did the initial tear-down and re-install of the top end. He's now telling me he can rebuild this motor for substantially less than the best long-block deal I could find (1779.00 delivered). I know the machinist he uses who is, in my opinion, one of the best in the country -- he did the machine work on two Pontiac 400s for me within the last couple of years (two cars, not the same engine twice) and they turned out flawless -- and the mechanic is straight up. I think he feels responsible for missing the valve problem.

Another couple of weeks I should be plowing snow with the Reatta.

[Vincent Vega]

Will you do my driveway?

[padgett]

1/4" play sounds more like missing valve guides than bad. With that much play, blue smoke out the tailpipe should have been very noticable. Something sounds strange.