I Just did a compression test on my engine (83 F150 W/ 351W stock) and found 3 dead cylinders (25 Psi) And 4 dyeing (75 Psi)
Cyl 1 100 Psi
Cyl 2 25 Psi
Cyl 3 75 Psi
Cyl 4 75 Psi
Cyl 5 75 Psi
Cyl 6 25 Psi
Cyl 7 25 Psi
Cyl 8 75 Psi
I know they all are running low
1) What is the normal compression for this engine?
2) What could be the cause of the loss of compression?

I dumped oil into the cylinders and retested with no change in the results.

Thanks for all your help and Knowledge!
Scott

 

Average is about 150 or more however it varies per condition of engine. What is the mileage on this engine?

 

how do you do a compression test?

 

Bad valves. If the oil in the cylinders doesn't bring the compression up, you've got valves that are not sealing. They could be; flat cam lobes, valves sticking open, burnt, bent, or the lifters, rocker or other parts of the valve gear could have gone bad.

How loose is the timing chain? If you can watch the distributor while you rotate the crankshaft in one direction to take up the slack, then the other to see how far you can turn the crank before the distributor rotor starts turning, you can get an idea of the relative slack-ness of the chain.

I had a Chevy shove a pushrod right through a rocker when the timing chain jumped a tooth (or some teeth). The act of shoving the pushrod through the rocker must have been enough to get things back where they belonged because I was able to replace the pushrod and rocker and drive the car until it did it again. The second time it happened, I abandoned the car.

Check the valvetrain of the dead holes to see if both valves are functional, moving up and down a reasonable amount, like 4/10ths of an inch.

 

Currently I have 177000 mi. on the truck.

I pulled the Valve covers and found that the rockers on the dead cylinders are moving the same as all the others.

I haven't checked the timing chain yet but a few months ago we tried to adjust the timing and the truck would not even stay running ether way we turned the distributor.

I was talking to my mechanic (I got the truck from him) and when he got the truck he could never get it to run right,(like it had a miss at idle) that was 3 years ago. Is it a possibility that someone long ago put the cam from a 302 into it? Would a 302 cam even fit?

I also found that it always ran really hot, about 190 degrees at operating temp or higher if just sitting and idling.

 

Well, you might try moving the spark plug wires as if you're wiring a 302 instead of a 351 and see if that helps. The firing orders are different between the 302 and 351.

 

we tried that about a year ago used the diagram for a 302 , 351W , 351C , 351M and the only one that worked anywhere close to correct was for a 351W