Hi everyone. I am hoping someone can help me out with this one. I just recently replaced the lifters in my 00 5.3. I had the heads off too of course but just did gaskets and a cleaning though. Put everything back together with new plugs, ignition wires, filters, etc. The truck is running ok but misfires on 7 and 8 during idle and light load. At about 12-1300 rpm on a slight grade the truck will really miss and if you just pull the selector down to 3rd or step on the gas it is fine. I have switched the plugs, wires, coils, and injectors from 7&8 to 1&2 but the misfire stays with 7 and 8. The mass airflow sensor looks good and clean. I have been checking for a loose electrical or vacuum connection but all looks good. I'm sure someone out there knows what my next move should be. I'm thinking maybe wiring harness? Any help is appreciated. Thanks, 00Yuk

Hi everyone. I am hoping someone can help me out with this one. I just recently replaced the lifters in my 00 5.3. I had the heads off too of course but just did gaskets and a cleaning though. Put everything back together with new plugs, ignition wires, filters, etc. The truck is running ok but misfires on 7 and 8 during idle and light load. At about 12-1300 rpm on a slight grade the truck will really miss and if you just pull the selector down to 3rd or step on the gas it is fine. I have switched the plugs, wires, coils, and injectors from 7&8 to 1&2 but the misfire stays with 7 and 8. The mass airflow sensor looks good and clean. I have been checking for a loose electrical or vacuum connection but all looks good. I'm sure someone out there knows what my next move should be. I'm thinking maybe wiring harness? Any help is appreciated. Thanks, 00Yuk


Swap the injectors and see if the misfire follows the injectors.

I've tried that already. No luck. The misfire stays on 7 and 8.

A misfire at idle and light loads only is usually either low compression or a lean air fuel ratio cause by a vacuum leak. A leaking fuel injector can cause it also but since these are the two rear most cylinders on opposite sides of a V8 I believe there's some vacuum leak on the rear of the intake manifold or it isn't laying flat because something is jammed under the rear edge. The knock sensor harness would be the most likely prop but it could be anything.

Low compression could be caused by inadequate or incorrect valve timing among the usual worn cylinder stuff. Why did you replace the lifters? Any chance of flat lobes back there?

Vernon

Thanks Vernon, A guy at hockey last night just suggested exactly what you said. I'll check compression and he said with the truck idling i could spray around the back of the engine, intake, injectors, with propane,wd-40, etc. Any leak drawing in unmetered air should show up as a smoother idle and/or richer o2 reading. New one for me, I'll let you know how it goes. I changed the lifters because there was one that was sticking pretty bad. Thank-you for your help,00yuk

If your intake ends up being warped you can buy new ones at the auto parts stores in a kit from Dorman. It comes with gaskets and other hardware and the price is way better than GM's but still it's a good chunck of change.

Vernon