To answer your ? about the plenum, yes it can because it causes a vaccum leak. Besides that it's a 98 and if the gasket has never been replaced it's most likely atleast one of the probs. And if it was bad for a long time it can screw up the valves.

Take the TB off and get one of those litle round mirrors and a flashlight and look around inside the intake and see what you come up with.

If you want to know there is also a way to check the plenum with a vacuum test.

 

Part 1 I will soon find out how bad the plenum leak is because I will probably take the engine down to the heads this weekend. Are you saying the leak could be around the area near the intake runners for #6 and #8?

 

Part 2:Is the valve damage caused by the leak making the engine run lean?

 

#8 is definitely low, but it's not that low, and a leak down test isn't likely to show you your problem in this case. Before you do major disassembly, don't forget what I said in your other thread about the valve springs; A broken one can still close the valve, but not fast enough. Pull the valve cover. It's easy to do and will eliminate this possibility.

It's not your plenum gasket. Vacuum leaks cause misfires because they lean out the A/F ratio. They WILL NOT cause lower compression.

If the valvesprings check out, then you're looking at a burnt or severely carboned valve (maybe cracked seat), or worn rings. Either one means teardown.

 

I agree with with you sixtysix ! I should have stated that the plenum alone will not cause the lower compression. But, it is most likely the origin of the prob. It goes bad first and depending on how long it's bad , the next thing is the CC gets F'd up , then all that extra heat transfers to #6 and #8 ,and all kinds of bad things happen to the motor the longer the problem goes unfixed.


I got lucky on mine 6 years ago when I bought it, not knowing how long it was bad, I replaced the plenum gasket , CC , injectors and still had the misfire. I kept dumping Techron in it and a month later the misfire stopped. So it was just carbon build up on the valve and I was just lucky to get it at the right time.

 

test. test. test.

 

Ok, DF doesn't want to cooperate. Won't let me post my write-up.

 

Cliff notes: Leak down test on #8 causes air to leak into plenum. Likely cause: cracked head.

 

Plan to pull heads this weekend. Will know for sure by then.

 

Sixtysixdeuce, I did as you suggested before pressurizing the cylinder and the valve springs are OK.