Vortec misfire

[crazy4mopar]

Hey guys
I'm getting a deal on a 99 vortec 4.3 for a new project. It has 21k miles on it. It has a misfire on the #5 cylinder. We have changed everything we can think of, 3 computers, wiring harness, all injectors, all sensors, 2 fuel pumps, wires, cap, 2 distributors, the only code that comes back is #5 misfire detected. It happens at about 2800 rpm. Would a bad valve spring make it misfire?
Any ideas would be GREAT!!!

 

[CMNCHE]

Could be a valve spring. Must be mechanical given all that you have replaced. Cam lobe wiped out??? Remove valve cover and see if the valve is opening when you spin the engine by hand.

 

[mikecooper]

Did you check the idle air control and does it have an adj tps?

 

[chris1044]

Try taking a propane torch - not lit, just so the propane comes out of it, and put it by the intake manifold gasket by cylinder number five while the things in the range that it typically misses. If the miss goes away, the intake gasket is bad. If you're ballsy you can use carb clean

I've seen intake gaskets on those things have just a pin hole leak that will cause a lean miss under higher RPM.

It's possible for the valve spring to make it miss, but that'd be way way worn out and would probably cause it to miss under other conditions as well.....the cam lobe theory could hold true for a partially worn lobe, but I'd find it hard to believe only one lobe is worn and the others are fine