#5 and #7 misfire

[rAiNTwist]

Hi, I have a 98 GMC Yukon (5.7L Vortec). Recently it started running pretty bad. Sometimes I would start it and it would drive fine for probably 15 minutes and then it would start misfiring… badly. Sometimes I would start it and it would misfire really bad and then as I forced it to drive it would smooth out about when it got out of open loop.
So I decided changed out the spark plugs, wires, cap, rotor and the fuel filter. After doing that it was running the same way. So I took it to a local shop and they thought it was the distributor, so they changed that and it didn’t fix it. They ended up trying to diagnose it for a few days and they changed out the EGR valve because it was full of carbon and the spark plugs again because they were also full of carbon after the short time they were in there from it running rich. They checked for vacuum leaks too. Still didn't fix it. I had them check the CMP Retard at 1k RPM from hearing stories about that after a distributor change and it only shows -10 no matter what RPM, open or closed loop. The misfires are on cylinders 5 and 7 and the fuel pressure looks good. The mechanic there was turning off the fuel injectors on 5 and 7 and there wasn’t any RPM changes. The only check engine code I’m getting is P0300 (multiple misfire). The intake gasket was replaced 4 years ago along with the fuel pump.
I’m at a loss at this point, so any help would be greatly appreciated.

 

[Ibustbravo]

I'd pull the valve cover & check the push rods to see if they're bent or you see a busted spring or something odd..

 

[rAiNTwist]

I'd pull the valve cover & check the push rods to see if they're bent or you see a busted spring or something odd..

I can give that a try!

 

[Fless]

Moved you here from the 2021+ section.

 

[rAiNTwist]

Moved you here from the 2021+ section.

Thanks

 

[exp500]

#5 & #7 are in the usual areas of intake leaks. Try a spray can test for external leaks. Also verify you have good manifold vacuum. Live data on scanner while driving is a big help also.
You seem to be at the point where normal scans do not exactly show fault, 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.
CMP retard is a strange symptom... Possibly a misdiagnosis/ adjustment by mech. Maybe scanner not set properly? Was timing reset to zero +-1degree?

Last easy items- Dist cap/rotor, Ign module, coil, Crank sensor are all Delco Delphi only or problems follow.

Good dignostics here- https://www.gmt400.com/threads/88-95-service-manuals.43575/


Cheap scanner- https://bafxpro.com/collections/automotive

 

[rAiNTwist]

#5 & #7 are in the usual areas of intake leaks. Try a spray can test for external leaks. Also verify you have good manifold vacuum. Live data on scanner while driving is a big help also.
You seem to be at the point where normal scans do not exactly show fault, 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.
CMP retard is a strange symptom... Possibly a misdiagnosis/ adjustment by mech. Maybe scanner not set properly? Was timing reset to zero +-1degree?

Last easy items- Dist cap/rotor, Ign module, coil, Crank sensor are all Delco Delphi only or problems follow.

Good dignostics here- https://www.gmt400.com/threads/88-95-service-manuals.43575/


Cheap scanner- https://bafxpro.com/collections/automotive

Thanks for the tips, the timing was set to 0 but for some reason the CMP retard wasn’t showing anything but -10. The distributor I got actually wouldn’t even let you clock it, it really only sits in the default position. The cap and distributor are both dragonfire, I got it off Amazon but the symptoms are the same as the old distributor so I couldn’t see that being an issue. The intake gasket is relatively new and the intake manifold pressure is showing around 18hg, so that’s pretty normal but I’ll check it myself with some brake cleaner.

 

[Pressureangle]

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.

 

[exp500]

Thanks for the tips, the timing was set to 0 but for some reason the CMP retard wasn’t showing anything but -10. The distributor I got actually wouldn’t even let you clock it, it really only sits in the default position. The cap and distributor are both dragonfire, I got it off Amazon but the symptoms are the same as the old distributor so I couldn’t see that being an issue. The intake gasket is relatively new and the intake manifold pressure is showing around 18hg, so that’s pretty normal but I’ll check it myself with some brake cleaner.

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!