I worked on a #8 cylinder misfire tonight and have more questions than before. I checked the plug wires, (good spark to the plug) and the plug itself, (looked clean, then swapped with another cylinders plug), and still #8 misfires. Even after several minutes of running, when water immediately flashes off the header primaries of the other 7 cylinders, water only slowly evaporates of #8's. That together with the rough running leads me to believe #8 is not firing for some reason. Since I pretty much eliminated it as being a spark problem, and since a bad injector on TBI wouldn't cause miss on one specific cylinder only, I started looking for other explanations. One of the ways to check for excessive blowby caused by bad rings is to feel for heavy pulsing from the breather hole in the valve cover, so I checked that. There was absolutely no pulsing at all, and I've been under the impression that there should at least be some any time an engine is running. I checked the oil level, added about 1/2 a quart to get it right, and shined a flashlight in the oil fill hole while the engine was running. Shouldn't I see some oil splashing around in there? It looked wet like there had been oil there at some point at least, but nothing else. What am I looking at here? Clogged oil passages? Fragged valve train on #8? This weekend I'll do a compression test on it, hopefully that'll shed more light on this...

 

Have you checked the distributor cap for a crusted contact? That would be a reason for the one cylinder to misfire. I would be unsure as to exactly how only 1 would do that though.

 

The spark is fine as far as the plug itself, I checked it. (I'll double check this weekend) I even swapped plugs with another cylinder with no change...

 

What might "look" like it's a good spark, doesn't guarantee it's a complete or timely spark, or that it fires everytime it's supposed to. I'd get a new distributor cap anyway, and maybe replace that ignition wire anyway, even though it "looks" fine. I've learned the hard way, several times, that things aren't always what they look like


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[This message has been edited by Blade (edited March 08, 2001).]

 

Best thing to do now is a compression test on that cyl to see if you have a burnt valve or something like that. Also a broken valvespring will cause this.

One old school trick that is kind of accurate(cant hurt to try) is to take a piece of paper and hold it over the tailpipe. If the paper intermittantly sucks in against the tailpipe, it could be a sign of a burnt valve. Pretty stupid, but I actually was able to diagnois a couple cars that way. Doesnt work all the time

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[This message has been edited by Kevin G (edited March 08, 2001).]

 

2 words....Compression check.

If the comp. checks out, take Kevin Gs advise and look for a broken valve spring in that cylinder. This will usually show good comp. cranking but no comp. running.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
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1981 corvette
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I had already been planning a compression check for this weekend. I'll reply with the results of that. Now about the other part of it: Shouldn't you be able to feel some pulsing from the breather when the engine is running? Also shouldn't you be able to see some oil splashing around under the valve cover?

 

Pulsing is a "some do, some don't" thing. Not all engines produce a noticable "pulse" out of the valve cover.

As far as the oil splashing around, oil comes up the pushrod and just kind of rolls off the tip of the rocker arm. It get splashed when the rocker arm bounces.

I have seen engines that won't splash any oil up off the rocker until you rev it up.

Try reving it up and looking inside. If you weren't getting any oil up there you would know it.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R

R.I.P. #3
My favorite quote about D.E. "He can see air"