jackrabbitZ71
Newbie
Hi all, relatively new to the forum but spend a good amount of time lurking. I have a 1999 Tahoe Z71 5.7 Vortec. I bought the truck last February, and around September it developed a misfire at highway speeds, but only when ascending a hill trying to maintain 65-75mph. The truck had roughly 183k on it when the problem first occurred, and I wanted to do a full refresh anyway. Since then, I've replaced:
Plugs, ACDelco Rapidfire 14
Wires, SMP Blue Streak 8.8
Cap+Rotor, UMP Vented/Brass contacts
Distributor, UMP Aluminum
MPFI injectors, Delphi
Fuel Pres. Regulator, Delphi
Ignition Coil, Delphi
ICM, Delphi
Fuel Pump, Delphi
Air Filter, Wix
Fuel Filter, Wix
Alternator, ACDelco
Upstream O2 Sensors, Denso
MAF Sensor, ACDelco
MAP Sensor, Delphi
IAT Sensor, ACDelco
I have also tested for Vacuum leaks using the propane method AND a proper smoke test, no issues there. All Cylinders hold compression at roughly 180 cold, tested twice with two different gauges. No codes for this misfire and I'm absolutely stumped. It runs AMAZING at anything below 70mph but the moment I try to maintain highway speed going uphill, it misses like crazy. If I give it more throttle and let it kick down, it runs fine, but only when ACCELERATING up the hill. I for some reason can't simply maintain my speeds without the truck misfiring. My thoughts would either be a weak spark, or maybe some sort of EGR issue? Any help is appreciated.
Plugs, ACDelco Rapidfire 14
Wires, SMP Blue Streak 8.8
Cap+Rotor, UMP Vented/Brass contacts
Distributor, UMP Aluminum
MPFI injectors, Delphi
Fuel Pres. Regulator, Delphi
Ignition Coil, Delphi
ICM, Delphi
Fuel Pump, Delphi
Air Filter, Wix
Fuel Filter, Wix
Alternator, ACDelco
Upstream O2 Sensors, Denso
MAF Sensor, ACDelco
MAP Sensor, Delphi
IAT Sensor, ACDelco
I have also tested for Vacuum leaks using the propane method AND a proper smoke test, no issues there. All Cylinders hold compression at roughly 180 cold, tested twice with two different gauges. No codes for this misfire and I'm absolutely stumped. It runs AMAZING at anything below 70mph but the moment I try to maintain highway speed going uphill, it misses like crazy. If I give it more throttle and let it kick down, it runs fine, but only when ACCELERATING up the hill. I for some reason can't simply maintain my speeds without the truck misfiring. My thoughts would either be a weak spark, or maybe some sort of EGR issue? Any help is appreciated.