I have a misfire on cylinder 3 that i cannot figure out the cause. The idle is rough, backing out after starting is very rough, it seems to pick up at higher rpms but once the truck upshifrts to the next gear it shakes like a leaf in fall. within the past year the engine was rebuilt all new seals a new fuel pump, new injectors, new everything practically.
so far i have done the following:
compression test results:125-130psi
fuel pressure test results: 50psi once primed, 50psi with engine running, no immediate pressure loss after engine is shut off
new spark plug wires
new spark plugs
replaced all four o2 sensors
Your next check is if the coil is bad for that cylinder and to check the injector for proper operation.
50 PSI is low fuel pressure for this vehicle.
low fuel pressure would be indicative of a leaking injector? normal pressure would be in the 60 range correct?
130psi compression sounds kind of low, especially for a rebuilt
your fuel pressure should be in the range of 55-60psi
On your fuel injection system, fuel pressure just a few psi too low can cause misfires
What you need to do is have a shop perform an injector balance test. You need an oem-level scan tool and a fuel pressure tester to do this. I wouldn’t be at all surprised if #3 injector is partially plugged.
But your low fuel pressure also bothers me
Who did the rebuild? The compression number is too low and should be in the 180-190 range.
Squirt a small shot of motor oil into that 130 cylinder and run the compression test on that cylinder again. If it goes up considerably there’s a ring problem and back to square one.
A weak valve spring can cause a misfire and checking valve spring height and tension while rebuilding the cylinder heads is a must.
Ignition wires? I thought this vintage was already COP…
Spritz some carb cleaner into the intake, does the engine stumble or does it seem to run a little better? If it stumbles, remove the input connector to the coil for that cylinder. Does the idle/rpm change when disconnected? If not, just buy a coil for that cylinder. IMO, it’s the most likely suspect.
The way I understood the post, every single cylinder is 130psi . . . correct?
@TwinTurbo this truck has a distributor, cap and rotor
old school through and through
All three engines for this year have just one coil, not even multiple coil pacs. Those compression readings are not low enough to cause a misfire. Another possibility could be a leaking intake manifold gasket at the port for the misfiring cylinder.
Maybe they’re not low enough to cause a misfire on this particular engine . . .
But they are pretty low, considering the engine was recently rebuilt
And I believe they would be low enough to cause a misfire on some engines
That is my opinion
I did lose my train of thought and focused on one cylinder. The problem applies to all of them.
They all suck; just not enough if one wanted to look at it that way… 
Personally, I wouldn’t spend another dime on parts until the cause of the low compression is sorted out.
Generally speaking, when they’re low across the board this means a ring problem; assuming the camshaft is not out of time.
FYI for OP, compression test:
The OP is going to be going seriously in the wrong direction if they buy into Scotty Kilmer, the guy who dazzles the uninformed with sheer BS.
Kilmer is right about the 5 PSI difference as being irrelevant. He is 100% dead wrong in stating that 120-130 is normal. The only car engine that I’ve ever been around that had 120 or so and which could be considered excellent compression are the air-cooled VWs which have compression ratios of 6 to 1, etc.
"He is 100% dead wrong in stating that 120-130 is normal."
Please watch the video again. He never says that.
I went back to the original post and left it uncertain whether the engine was actually rebuilt. Clarification from the OP about exactly what was done would be very helpful IMHO.
Kilmer is a character, but in this video I didn’t see anything that bothered me considering that the gage and its instructions clarify that 120-130 is low. In the OP’s case, we still don’t know what the other cylinders were reading. If my suspicion is correct and the OP’s original statement that the engine was “rebuilt” really only means that the gaskets were changed, than he’s probably struggling with a tired old engine. If it really was rebuilt, and he’s still seeing 120-130, he’s got other problems. I’d then wonder if the rebuild included the heads. And I’d wonder bout the valve timing.
I don’t think there’s enough information here to make any assumptions. I’d personally like clarification on some of the questions that have been asked/implied.
I did watch the video again and there’s a number of things wrong with it. As to “normalcy” Kilmer uses the word “normal” in referring to the pressure differences.
The problem with that is the inference that all of those numers are normal and many people who watch that video will come away convinced that 120-125 PSI is just fine.
The OP has low compression across the board. This generally means worn rings or a cam out of time unless one subscribes to the theory that there is equal cylinder head valve leakage on every cylinder. The odds of that are very slim.
Lowered compression also has a tendency to kill spark plugs early with the ignition disease working its way down the line through the wires, coils, etc.
If the vehicle were mine I’d pin the compression problem down before spending money on part after part.
Equally misguided are some Chiltons manuals I have which state that 150 PSI on one cylinder and 115 on another is acceptable.
I see your point, OK4450.
I went back to the original post and I’m still unsure if the numbers stated were for just the misfiring cylinder or all the cylinders. I found the wording unclear.
I’d also appreciate from the OP clarification of the description that the engine was
“rebuilt all new seals a new fuel pump, new injectors, new everything practically”.
Whether the compression reading was for one cylinder or all, it left me wondering if by “rebuilt” the OP meant that the heads were removed and the bottom rebuilt or just that the engine had new seals etc.
I’m not comfortable that we really know what we’re dealing with here.
That part could be taken either way. Originally, I took it to mean that the compression had been tested on the misfiring cylinder only but in retrospect it could be taken to mean 125 or so across the board.
One or all though, any compression issue is going to mean tearing it all back apart if the OP wants to get to the root of the problem…
I wish the OP would clarify some of these questions. It would help us to help him/her.