2000 GMC Savana 350 5.3L random misfire

[Rondo Stech]

I have a 2000 GMC Savana with a 5.3L 350 in great shape but sat for a few years. It was running great until the fuel pump went out. Ever since then it has been misfiring. I had a full tune up. Ignition (full) wires, plugs, rotor cap, everything and still misfired. I replace all the injectors and it still misfires. I replaced the computer as it was acting up, and still it misfires. It is now random misfire code for #2 & #4 (or) just #2. I have brought it to 2 different mechanics and they are dumbfounded. I found a better mechanic who fixed and checked over everything and still cannot figure out why it is misfiring on cylinder #2. I have paid out $1000.00 of dollars to get my van fixed and it still misfires. What am I missing? As the code says misfire on #2 or sometimes it says #2 & #4. At times when the light is off it runs great, good gas milage, but the light will just come on to service engine soon, and the misfire happens, then I get crappy gas milage. I am a mechanic myself, but only know older vehicles and a little of the new stuff, and still cannot figure out what is going on with my van. We did a smoke test and found a air leak to the gas tank, fixed it, and it still misfires. I am at a loss, please help.

[Jsdirt]

You have a 5.7 with a distributor assembly, definitely not a 5.3 LS.

 

If I were a betting man, my money is on intake gaskets. I get a plastic bottle, poke a hole in the cap, fill with water & squirt it around the mating surface between the lower manifold and head while watching fuel trims on scan data (and also listening to the engine, and for the water being sucked past the gasket). Do this at first start up of the day, cold. Any changes are a dead ringer for a gasket leak. 

 

I've had many that don't reveal leaks on a smoke test, so I don't even bother with that anymore.

 

Another thought - those "pancake" style caps are absolute garbage. Make sure the new one isn't junk - it happens constantly these days.

Edited October 7, 2022 by Jsdirt