Neighbor came over today to borrow my code reader. His 2002 4.7 Dodge Ram is throwing codes for a misfire on 1, 2, 3, and 4. Also has a code for a speed sensor which I haven't gotten into at all as I don't think it's related. He says the truck ran fine and then suddenly ran really rough with no power.

We pulled the plugs on 1 2 3 4 and 5. All the plugs were trash and looked like they hadn't been changed in the 140K miles the truck has ran. A compression test gave us 150 psi on all the cylinders except 2. Cylinder 2 wouldn't even move the needle.

My first thought was head gasket since 4 cylinders next to eachother were misfiring but now I'm thinking valve train/piston ring since it seems only 1 cylinder is actually bad. I don't have a big enough compressor to do a leak down test but I can get my hands on one if its worth it. Will checking the oil tell me if the head gasket is leaking? Any other quick checks I can do to diagnose? I don't think it can be electrical or timing chain since 2 from each bank are bad and all 4 showed misfire at once. Could a dead cylinder cause misfire on cylinders around it?

I think you're on the right track -- even with a blown head gasket, I'd expect it to build a little compression while cranking. The gasket is thin enough that forcing air through that gap should require a little pressure.

You don't need a massive compressor, just any old compressor and the spark plug adapter to send air into the cylinder (remove the Schrader valve if present). If there's zero compression you should hear (or even feel, maybe) the air leaking out somewhere: the intake; the exhaust (both would be valve issues); the oil fill cap (crankcase -- piston/ring damage), or the radiator (head gasket). Don't worry about getting 100 psi in the cylinder or measuring leakdown percentage yet -- right now, there's a massive leak somewhere that needs to be addressed first.

Thanks for the advice. I feel like it has to be a major valve issue. The needle is not moving at all on the compression gauge. When I had bad seats on my S2000 I still got 100-120 psi. Same thing with rings and HG. I think in order to not move the needle a valve has to be wide open.

I am guessing that this engine suffer from the plenum gasket failure that plagues the 5.9 and 5.2 v8's.

Ahh, the belly pan gasket problem.

4.7 is way different though intake design so it doesn't have the same issue.

Meant to say doesn't suffer from...

4.7 engines like to gum up like a nursing home.

Gotcha.

My '92 5.2L never had the issue even running about 9psi boost on it. My 5.9L later sucked oil through it like crazy.

If you swap the motor. I think it's only 2002 only u can use. Double check. I ran into that issue. Those engines are hit and miss.

Shop said its a PCM issue. How can that be possible if there is no compression on #2 and the rest are fine?

Did you double-check that your gauge hose was completely tight in the plug hole?

I've gotten zero on a cylinder, then taken the gauge off and retightened it only to have compression. Undo/redo again...and compression.

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Did you double-check that your gauge hose was completely tight in the plug hole?

I've gotten zero on a cylinder, then taken the gauge off and retightened it only to have compression. Undo/redo again...and compression.

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I tried it 3 times. Tried different cylinders in between and still nothing. One time it was on tight enough that the adapter stayed in the engine and the hose-adapter thread unscrewed.

Roger that.

I figured you had done it like that, but was just checking. I know how it can be sometimes when you're trying to figure crap out.

PCM wouldn't cause lack of compression, though. Although...it MIGHT be a two-fer.

PCM would throw a code wouldn't it? I guess that could be the misfire codes.

PCM could be jacked enough that it's not allowing X amount of volts, needed by the plugs, to fire correctly and that would cause the misfires, which would then trigger THAT code.

If it's totally F'd, yes, there's a code for that, too.

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PCM would throw a code wouldn't it? I guess that could be the misfire codes.

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Usually when a Chrysler PCM starts to fail youll have a Stack or Checksum error before it totally sheets the bed.