5.3 misfires, can't find the problem!

[andrew04]

I just bought my 99 Silverado Z71 RCLB, 5.3L, automatic. Has 155k on her, and I think she's had a real hard life. I replaced the plugs and wires as well as running sea foam through the intake and feul to clean it up. Cyl 4 is misfiring consitantly and the oil pressure is unusually high, like 60 psi @ 2k rpms. Oil is mobil 1 high mileage less than 1k old.

Any suggestions would be greatly appreciated, I am fairly handy and would love to fix this myself. My buddy told me seafoam may have eaten the piston rings, is this a possibility?

I swapped plugs and wires and the misfire did not move, I plan to swap coils next. Thanks!

[andrew04]

also, previous owner was more of a dog person, he got rid of his cats.

[Frenchy]

Could be a lean misfire. Could be a coil, swap with another cylinder. Never once have I heard of Seafoam damaging rings, and it's pretty damn hard to screw that process up enough that you'd cause damage.

[country_3030]

Swap the coils 4x6 to see if the miss moves to 6. If that doesn't work swap the injectors 4x6. However, a compression test would be a good check also.

Never heard of sea-foam damaging rings, plugs yes but not rings.

[AFSP1211]

check the coils by removing the boot at the coil and see if it arcs. If you get a good arc, it's not the coils. Maybe post a sound clip of if from under the hood.

[Ray@NP]

perform a compression test, inspect for broken valve springs, use noid to be sure injector is pulsing

[screaminchicken]

check the coils by removing the boot at the coil and see if it arcs. If you get a good arc, it's not the coils. Maybe post a sound clip of if from under the hood.

You do not want to fire a coil without a load on it. A good way to check out a coil to is to swap it with another. To kill a coil but not damage anything you can put a piece of heater hose in between the coil and the plug and touch a grounded test light to it to kill spark at that coil, or just unplug it at the primary side. You could maybe do that and see if the misfire gets worse. If it does, it could be fuel related or valve or head problem.

[RedHeartbeat]

Seafoam can break the ring lands between the rings if poured in too fast. People have been known to bend rods when using seafoam.

[Chris@LSXPS]

If they are. 706 heads, the tend to crack more than any other head

[andrew04]

Sorry for the slow response, I got a noid to confirm spark and fuel, a compression gauge to do a compression test, and a service manual. I will post results, tonight if work goes smoothly. Will also confirm casting number on heads and look for crack if visible. Thanks for the help. -Andrew