I replaced the exhaust manifolds on my 2007 Yukon XL the truck ran perfectly before but had a small leak due to broken studs. Now the truck is throwing a P0 300 code. The plugs and wires are less than…

Dqm999

I replaced the exhaust manifolds on my 2007 Yukon XL the truck ran perfectly before but had a small leak due to broken studs. Now the truck is throwing a P0 300 code. The plugs and wires are less than six months old I have not been able to find a vacuum leak and I appear to have spark at all cylinders. I did a compression test and have 180 psi on each cylinder. Both the coil packs and feel injectors seem to Have both power and ground. I do not hear any abnormal engine noises so I do not suspect a collapsed lifter the truck starts and runs first try but I can feel the misfire worst once the engine is warmed up and idling. If I rev the engine 1500 to 2000 RPM I cannot detect a misfire. Does anyone have any ideas? The truck has about 190,000 miles and a 5.3. Thanks

Comments (4)
No. 1-4
Dqm999
Dqm999

It turns out I’m a bonehead I reinspected in spark plug and found one with cracked porcelain stupid oversight on my part.!!

Dqm999
Dqm999

The symptoms only started after I replaced the gaskets and manifolds. I have since gine back and used one of those spark testers that lets you set a gap and two coils will not spark properly with a gap of more than about 10mm. The others spark out to twice that, weak coils I guess. I’ll let you know what happens when I replace those coils.

Techsus
Techsus

Why guess? Start here: https://youtu.be/OyvdWeyDHf4

Jackofall
Jackofall

U need to track the misfire may be just fuel issue, if none of the. Symptoms where they before u put on gasket