#5 and #7 misfire

[rAiNTwist]

You need at least a compression check, and preferably a leakdown test. Head gasket is suspect. All of your symptoms support a head gasket failure, and one of the key diagnostics for head gasket failure is that all of your other efforts have not made any change.

I have the same suspicion. I bought a CO2 detector that goes on the coolant reservoir that tells you if your head gasket is bad or not (that’s if exhaust gases are getting in the coolant) and that should be here later today, so I’ll let you know what I find on that. It’s at least worth a shot before I start throwing parts at it.

 

[rAiNTwist]

Many of the aftermarket distributors come with the non-adjustable dist hold down. Swap with the factory one and that should remedy that issue, as well as CMP being one of the most misdiagnosed as something else problems. A little more testing and we should be able to pinpoint your problem(s).
Minimum tool list- scanner, fuel pressure guage, spark tester. Total under $100.
First suspect on misfire is dist cap on these. Just sayin. Good Luck!

Wouldn’t CMP affect all cylinders though, or at least a bank and not just 2 cylinders?

 

[Fless]

I have the same suspicion. I bought a CO2 detector that goes on the coolant reservoir that tells you if your head gasket is bad or not (that’s if exhaust gases are getting in the coolant) and that should be here later today, so I’ll let you know what I find on that. It’s at least worth a shot before I start throwing parts at it.

CO2 is everywhere; you probably mean CO (carbon monoxide).

 

[exp500]

Wouldn’t CMP affect all cylinders though, or at least a bank and not just 2 cylinders?

A few assorted quotes from me unanswered-
"so the next steps are to eliminate possibilities one by one starting with the most obvious or the easiest diagnostic. I think verify that misfires are ONLY on #5 &#7."
" as well as CMP being one of the most misdiagnosed as something else problems."
"Dist cap/rotor, Ign module, coil, Crank sensor are all Delco Delphi only or problems follow."
"Minimum tool list- scanner, fuel pressure guage, spark tester. Total under $100."
"First suspect on misfire is dist cap on these"
"Live data on scanner while driving is a big help also"
"Good dignostics here-"

Live Data- not stored codes- monitor ALL cylinders for misfire, along with as many other items you can fit or watch. Missfire codes are set after a specific number and circumstances, a guess is somewhere in 150-300 range

I think change hold down is 1st step repair. Don't overthink it.You stated it is -10 deg NOT "0"

Was the injection updated/replaced when intake gaskets changed?
Manifold vac appears to be reasonably good, I don't know how you tested or if you monitered missfires at various steady RPMs, so 1st guess is intake gaskets ok.

A 6 foot hose on fuel pressure guage allows you to watch fuel pressure while driving.

Have you read the troubleshooting section of manual for misfires?

 

[east302]

I think change hold down is 1st step repair. Don't overthink it.You stated it is -10 deg NOT "0"

I’d agree, check that one off of the list. A lot of aftermarket distributors come with those fixed clamps. In case you need it, the GM part number is 10096197. If you’re junkyard shopping, the same part number is used on the 305 and 350. Not used on the 4.3.

 

[West 1]

I had a nightmare issue helping a friend with his 99 5.7 vortec engine. After trying everything I pulled the valve cover and examined the valves while running. When it missed the #3 exhaust valve was not closing all the way. So hard to find since it was random, would miss and then work perfectly for a while. We pulled the heads and a friend at the machine shop told me he sees this on the Vortec heads. Carbon builds up on the valve stem and they stick a little open. When it gets worse it will be stuck open all the time and is easier to find. Hope yours is simple.