cylinder misfiring

type of truck? year?

Run a compression test on #8 yet?

Doc

Well, have at it and then post back! If you slipped a ring or have a cracked valve (or maybe bad head gasket) it would case #8 to crap out pretty damn quick. You're looking for 150 psi or better.

Doc

the compression test wont solve the problem ....but it will help u isolate it.

1. vacuum leak will not cause plug fouling
2. i doubt the carb is your problem since you stated that the plugs are white when running good (LEAN)....?

3.sounds like you are oil fouling the #8 plug VIA a broken oil control ring or worn valve guide or maybe the valve guide seals are riding the stem and letting oil run down thru the guides.

1. run a dry compression test (per DOC) and record the reading, then inject a few cc's of your favorite petroleum based lubricant into the cylinder and run a second compression test and compare the readings, if the wet test is higher than the dry test than you have ring seal problems. Test all 8 cylinders and compare readings amongst them...they should be within 10% of each other on a well used engine.......much less variance on a rebuild
2. Hope this helps.

6.hello Doc, still runnin the big roads.....got a 4" lift kit waiting on me at the house! Ye Ha

we can also rule out that the intake is sucking oil outta the lifter valley cause this affect all cylinders.....not just #8. The only time that becomes an issue is when the heads have been milled excessively creating a gap at the bottom of each intake port. angle milling remedies this phenomenom.

for a moment i was thinking that maybe the vacuum line for the power brake booster could be plumbed into the #8 intake runner and when combined with a defective brake booster/master cylinder would draw brake fluid into the #8 cylinder causind said fouling and loss of brake fluid as well.........Christ almighty

Ran into a problem like this on an Old Buick. Was the transmission modulator. Killed only one cylinder.

Al

Yep, early gm's had the vacuum modulator and brake booster plumbed n2 the same intake runner...... but i can,t r'member whether it was #7 or #8

No, what you need is to give us vacuum NUMBERS. How the hell are we supposed to know if it's "pulling too much vacuum" without a figure to go by?

Doc