Crystalflow

I didn't check all of them as the misfires are always on the same cylinders. I'm checking cylinder specific issues.

Plugs - ok
coils- ok
wires - ok
compression - ok
injectors - ok

I may smoke the intake before I tear into it.

does the 4.8l intake manifold have coolant passages?

mountainmanjoe

Wouldn't a bad gasket cause loss of compression?

LCAC_Man

A bad intake gasket wouldn't cause any change to compression, it is a possible cause for this but typically not on a wide scale (multiple cylinder misfire) it usually occurs on a single (maybe two side by side) cylinders. Failed catalytic converters are a possibility and bad gas/low fuel pressure/air leak in fuel system are as well. Cam and crank position sensors usually cause you to stall all the way out so not as likely..

Crystalflow

I plan on checking the fuel pressure as well when I get it smoke tested.

I only checked the compression on 1 of the bad cylinders. #5. It has the most misfires. I got 150 psi.
I know I'm supposed to check the others and compare results, but I felt 150 was ok.

LCAC_Man

you could swap coil packs between the cylinders as well...it's not typical to fail more than one at a time but it isn't difficult to do..

Crystalflow

I already swapped the coils. No change.

Ron6519

Check the wiring going into the cam sensor and the crank sensor. The cam sensor plug in can get corroded by the issues the distributor has. My mechanic changed the harness about 9 years ago due to hard starting/no start. Wires were all corroded. Now here's the kicker. He put in make shift connectors that loosened over time. The intermittent signal (2 wires were loose)caused, P0300 and P0304 misfire coded. Along with blinking daytime running lights. Finally checked the repair and found the issue. Soldered the wires and the headlights stopped blinking and the "check Engine" light has not come on for 5 days of numerous test drives.
.

Crystalflow

I actually got it figured out, but forgot to post. I got a local mechanic to perform a crank position sensor relearn and it worked. This was several months ago and no more check engine light.

Kael

Extra bit for checking cats. The chemical process involved results in more heat, one can check that a cat is working correctly by driving for a bit, then measure the temperature at the in and out tubes of the cat. Make sure to check about the same distance from each end with engine still running and fairly quickly, as idle will even the temperatures.

The air should increase in temperature going through, so the out temperature should be higher than the in temperature. How much, depends on a lot of things, but for my 4.8L the working cat was about 20F higher at the out. The other cat, not working, it actually cooled the air, which means it's not doing its job. Plan to replace as part of my P0300 chase.