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.
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.
