Forcefed81

iTrader: (4)

I am having a misfire issue I just can't seem to get to the bottom of. All my mods are in the sig. I got in the truck Friday morning to go to work, and when I started it up I noticed it was running a little rough. I drove it to work. It managed to get me there well enough.

After work I decided I would try to diagnose it. It's not setting a CEL and there are no codes stored. I tried to pull up the misfire data on the tech 2 and it shows zero on all cylinders. I used a laser thermometer on the header primaries to determine cylinders #1, 2, & 4 are not firing.

Those cylinders have spark, fuel, and unless the intake manifold runners are clogged air. I have swapped coils, plugs, and injectors from good cylinders with no change. I have checked compression on those cylinders compared to a good one, and its consistent. I have checked cranking and running compression also. Those checked good as well. I also performed a leak down test which showed all was ok. I also swapped PCM's with a spare I have. No change. Fuel pressure is a solid 60psi, and the injectors are pulsing. Valve springs are intact and rocker arms all look good(no play).

What the hell am I missing?

Atomic

iTrader: (42)

Change anything since the last time it ran? Pulled the plugs and looked at them? Sure the wires arent burned/shorting to the headers and that they are plugged in all the way to the plug and coil?

Forcefed81

iTrader: (4)

The only thing I did recently was swap out the stock air filter for the K&N I had sitting around on my shelf. Which I had used before, but I took it out to clean and oil it and didn't put it back in right away. My wife and I drove the truck out to dinner Wednesday night with no issues at all.

The plugs were a little black, but didn't look bad. They weren't really even that wet for being in dead cylinders. I did swap plugs from known good cylinders and nothing changed. The plugs themselves have about 35-40k miles on them. Stock a/c delco plugs. I've had them in since the cam swap a few years ago. I also got a new plug and tried it on cylinder #4 with no change. The wires aren't touching the headers and do no appear to be burnt. I have also swapped wires from different cylinders with no change, and I don't see any arcing.

trxmxzx

iTrader: (7)

Atomic

iTrader: (42)

What temp are you reading for those cylinders to say its dead?

Forcefed81

iTrader: (4)

No oil on MAF sensor. I did recently get fuel, but I've been through 1/2 a tank already.

Forcefed81

iTrader: (4)

Ambient basically. By the time the cylinder 8 primary is at 240 degrees or so, cylinders #2 & 4 are around 115 degrees. Same thing on bank 1 with cylinder #1 compared to #3, 5, & 7.

It is possible I have some worn cam lobes not allowing full valve lift on those cylinders? It's not making any noises or anything. I'm really at a loss.

Atomic

iTrader: (42)

Well a compression or leakdown test is where I would go next if you are sure the ignition system is all working fine.

Forcefed81

iTrader: (4)

I did a compression test on cylinders 2 & 4 compared to a good cylinder, all normal. A leakdown test on cylinder 4, which is a problem cylinder, didn't show anything abnormal.

Atomic

iTrader: (42)

Well, thats the air portion...we are down to fuel or spark, and my money is on spark since 95% of the time on these engines a misfire is an ignition issue in my experience.