08 Equinox 3.4 #6 misfire
Ok, equinox came in with obvious blown head gasket, Pulled heads had pressure tested, one was cracked, replaced. Resurfaced, valve job, etc... put it all back together and she runs again. However, I’ve had a cylinder #6 misfire that I can’t figure out. I feel like its a rich misfire because the exhaust has a heavy sulphur smell to it. if it was lean it wouldn't smell like that.
swapped injectors with #4 (problem stayed on #6)
swapped wires with #4 (problem stayed on #6)
spark plug tester, also pulled plug and fired to make sure the plug was actually arcing.
swapped in a known good coil pack as a test (problem persisted)
autel shows #6 is a consistent misfire.
All tests come back great. What am I missing?
Put a vacuum gauge on it and see if you have a valve problem. You said you did 1 head.... did I read that right?
No I did both heads. I don’t think there is a valve problem based on my compression test But I will pull a vacuum on it later today.
I wonder if it really is #6 that is the mis? With that smell and a misfire I would expect the CEL to be flashing. Fuel trims?
CEL is flashing and pulls a code for 0306. Don’t remember what fuel trims were but I will get those and post them as well. I’ve got a vehicle in the bay right now as soon as I get it buttoned up I’ll pull the nox back in.
Make sure the misfire counter is flagging the correct cylinder
It is and only flagging 6.
Cause a misfire on cyl 6 to confirm it is showing the correct cylinder
Pulled plug wire and no change, still calling #6, but the plug wire is arcing across the engine lift point like crazy when I had it off. Jumping a 1” gap no problem. But the arc looks a little red where I was expecting blue.
Sounds like broken valve spring or sticking valve on #6, just ran into the same exact problem on a reman 5.3 GM engine, new head resolved it.
Any chance you can run a tailpipe pressure sensor waveform?
No I don’t have the right tools to run a tailpipe pressure waveform. We had a shop fire a few months ago and we are slowly building our tool collection back up in the new building. It’s been a rough go.
Also, wouldn’t a stuck valve or broke valve Spring cause a backfire through the intake?
Do you know what caused the blown head gasket and was the cause repaired,Also what did the spark plug look like?I had an issue with a brand new AC/Delco spark plug had a hair line crack in a Chevy 4.3l after doing an intake.Couldn't find it until spraying the wires with soapy water solution.
EGR engine? PCV working correctly? There's also a TSB# 16-NA-383 for sticking/binding valves
Yes EGR engine and pcv is new replaced it cause my heavy hands broke it pulling it off... $73 mistake from chevrolet... also I read that tsb But I’m looking at an engine with less than 300miles on the top end complete rebuild. I watched the machine shop put the valves in clean as a whistle I can’t imagine that would be the case but I will take the rocker cover off and see what she’s doing…
No I don’t know the cause of the blown head gasket. someone else had attempted a repair before we got it so not sure what was done prior to us doing the head gasket.
Camshafts put back on the head properly (right or left)????
The 3.4 is not an overhead cam motor
May not apply to your 3.4, but worth a look. #PIP5528D: Information Assistance for HFV6 Re-Manufactured Engine Misfire Diagnosis - (Sep 6, 2019)
If you have a scope you should do an in cylinder pressure waveform. It may show a valve problem. Also scope the injector circuit to check for proper control from the pcm
Missed the lifter hole with a pushrod.
Man if that is the case I’m going to bang my head off the wall! I’ll check that first thing in the morning based on just now getting the vehicle back in the bay.
When does it misfire, at idle or driving down the road? Just wondering if it hydro locked and bent a rod. This could be checked measuring the piston height at tdc. The pcm might be flagging a misfire from seeing less contribution from that cylinder
Mostly at idle, driving down the road you’d never know it had a misfire except for the cel.
When doing compression test how many strokes does it take to get over 100PSI? Been my experience if the reading increases in say 40lb jumps you have a mechanical issue. I like to see 80 psi on first stroke. By stroke i mean cadence. Ugh, ugh, ugh, ugh is my minimum. But i want to see the gauge while cranking. Let's say 80 first ugh, 125 second ugh, 150 third ugh, 175 fourth ugh. If a cyl is…
Come to think of it I think it did jump in roughly segments of 40... I’ll check again!
My theory has one major hole in it. If a given cylinder just happens to be in just the right position valve/piston wise, it's going to get a really good "blow" on first ugh-a-dugga. So i do each cyl 2 or 3 times especially if I see a low first hit. BUT most engines stop in same spot nearly everytime if shut off at idle. So another hole in my theory...
Did you happen to look at the cam lobe at all? I imagine you had no reason to pull the lifter out, but most loves are visible they the oil return holes too. Those cam lobes don't tolerate much abuse and gall easily.
Have you considered that misfire may be located on companion cylinder and not actually on cylinder #6
Chris What were the results on the vacuum gauge test....
Install vacuum gauge and see if the needle bounces, if it does take a timing light and aim it at the vacuum gauge go from wire to wire when the needle stops bouncing you have found the problem in the valve train
we may have it figured Out. When I sprayed the upper intake with brake cleaner the exhaust started smoking. Going to install a new upper plenum set and I’ll let y’all know.
upper plenum gasket replacement complete. car runs good. Still feels like she has a misfire. No check engine light through the test drive but I’m getting intermittent codes for p0171 and p0300. but none have set a code. while I was working on it I started thinking about the possibility of a cracked intake manifold... one of the heads was cracked and maybe it got hot enough to crack the intake…