Rough idle - P0306 Cyl 6 misfire - Gonna loose my mind

[scoob8000]

So very long story short.

1998 C1550, Vortec 5.7.

Did the intake gaskets due to coolant leakage. Ran fine for a few days then started with a really rough idle and p0306 code.
I threw all the usual stuff at it before pulling the intake plenum back off and replacing the sfi spider with the updated MFI setup. No difference.
Check for internal and external vac leaks, fuel trims seem normal as far as I can tell. Then one day it cleans up and runs fine.

So fast forward 500-600 miles. It's running great but I decide to throw new front O2s at it as PM. Right after the first startup it idles like poo, and now my P0306 misfire is back again.
Nothing changed besides the O2's


Just a mental checklist:
-New cap/rotor/plugs/wires
-Dizzy gear looks okay, no play or slop
-New fuel filter
-No vac leaks (internal or external)
-CAM retard is perfect at 0 @ 1500rpm
-Fuel pressure is good
-Cleaned MAF
-No coolant leaks
-Idle does not change if I unplug #6 wire. #6 has hot spark, plugs all look good
-Misfire totally goes away around 1000rpm
-Runs great, good power, just terrible idle
-New O2's seem to be working right, according to scanner anyway.


I did have a pass side bank cat inefficiency code a few months ago when I first bought it. I keep thinking maybe clogged cat on that bank, maybe banging around on the o2's knocked something loose.
But every clogged cat I've ever had before causes drive ability issues. Not so much idle.


Help before I loose my mind!

 

[hiredgun8]

"Nothing changed besides the O2's"

Did you try swapping the old ones back in to rule out bad out of the box o2's?

 

[scoob8000]

"Nothing changed besides the O2's"

Did you try swapping the old ones back in to rule out bad out of the box o2's?

Sadly I had the local exhaust shop do it and they pitched them.

I'm gonna try unplugging them and see how it runs. I think the vcm defaults to 400mv or something.

 

[twomanymontes]

O2s won't cause a misfire on a single cylinder. The whole bank, maybe if it was stuck lean. That being said, a single cylinder misfire can cause a false Convertor code.

If you have good spark on the #6 cylinder, try swapping the spark plug with another cylinder. I the misfire switches to a different cylinder, you know it's a spark plug. Wouldnpt be the first time I had a bad one out of the box.

If it doesn't change, then try doing a leak down test on that cylinder. Could have a valve that isn't seating properly

 

[scoob8000]

I forgot that in my mental checklist. I did swap plugs between #2 and #6 and no change. I don't have a leakdown tester, but I did do a compression test the first time I had the misfire and it came back okay.

The thing that gets me is how easily it comes and goes. I don't think I actually did anything that made it go away the first time, and magically it appeared after having the truck on the rack and doing the o2's. It's really reminding me of a loose wire or something like that.

 

[77Impala]

Since you have a hot spark on plug wire 6 but no change in idle when the plug wire is removed, and it acts like a broken/loose electrical wire, I would look for that loose wire being on the spider connection for cylinder 6.

The exhaust shop may have bumped/pulled on the harness when changing your 02 sensors.

 

[scoob8000]

Since you have a hot spark on plug wire 6 but no change in idle when the plug wire is removed, and it acts like a broken/loose electrical wire, I would look for that loose wire being on the spider connection for cylinder 6.

The exhaust shop may have bumped/pulled on the harness when changing your 02 sensors.

I don't know what the heck to do now. I just threw a whole new distributor at it, cap and all. Set the CMP retard perfect, and it's still ticking away #6 misfires.

After idling the #6 plug smells very faintly of gas, however it's not wet.

Going to pull the o2 on the pass bank and see if something is up there. It's so coincidental it started right after having new front o2's installed. I know O2's or a clogged cat should affect the whole bank, but I'm running out of options.

I also ohmed out the injector harness back to the PCM and everything there checks out. Injector is new MFI style and the resistance of all 8 look good.

EDIT:

I don't know what the heck to do now. I just threw a whole new distributor at it, cap and all. Set the CMP retard perfect, and it's still ticking away #6 misfires.

After idling the #6 plug smells very faintly of gas, however it's not wet.

Going to pull the o2 on the pass bank and see if something is up there. It's so coincidental it started right after having new front o2's installed. I know O2's or a clogged cat should affect the whole bank, but I'm running out of options.

I also ohmed out the injector harness back to the PCM and everything there checks out. Injector is new MFI style and the resistance of all 8 look good.

So I just redid my compression test. Previously they lokoed good, now the even bank all reads #180 except for #6. #6 is reading 120 dry, and 140 with a squirt of oil.

Running to the store to get a leakdown tester. I'm hoping it's a carboned up valve. If it were rings or burnt valve how would the problem go away then come back a couple hundred miles later?

 

[IOWNJUNK]

Valve could be bent, burnt, cracked or just sticking. Could also be a broken or weak valve spring.

 

[scoob8000]

Valve could be bent, burnt, cracked or just sticking. Could also be a broken or weak valve spring.

So got my leakdown tester and went to town.

It's sounding like the #6 exhaust valve isn't closing. Like you said stuck, burnt etc.

But what still really gets me is how did the problem go away for a couple hundred miles then come back. Unless it is carbon build up or just a rough spot on the stem or something.

I have #6 at TDC now and the cover off. I can spin both pushrods so it's not the rockers out of adjustment.

Valve spring seem stiff to my thumb, I can't compress them at all by hand.

 

[IOWNJUNK]

Turn motor over by hand, do rocker arms on #6 move like the rest? Might be hard to tell by the naked eye, but at least rule out that one rocker doesn't move.