The truck is a 1990 f150 with the 4.9 liter inline 6, with about 75k on a rebuilt engine. I performed a compression test on the truck today and the results were as follows,

1-140
2-125
3-5
4-5
5-140
6-150

Reading the plugs, the truck looked to be running really rich on number 3 and fairly rich on number 5. All the other cylinders looked good. 3 and 5 didn't seem to have any oil on them or look like they had been burning oil. Plus the truck really doesn't burn more than a quart every 3000 miles. Do to the fact it doesn't burn any oil and it really doesn't look like there is any oil getting past the rings into the combustion camber, I'm thinking this is a valve train problem of some sort. There doesn't seem to be any oil in the coolant either. However, the cylinders that are low on compression are right next to each, could it be a possible headgasket?

I'm not too familiar with these engines, but I was wondering if I could get some opinions of what the possible/likely problems could be. Would it be unlikely that it's a valve train problem if compression is low on two cylinders? Could some thing like a pushrod be stuck or bent? Could the truck have broken a valve spring? Thanks for any help!

 

depending on firing order, maybe it jumped timing by a tooth on the timing chain. Might also be worth pulling the valve cover and see if you have proper movement of the valves on those two cylinders.

 

Thanks for your response, but jumping a tooth on the timing chain wouldn't cause there to be no compression, unless doing so actually bent a couple of valves, but I kinda doubt it.

 

Sounds like either the headgasket leaking between those two cylenders or burned valves on those two cylenders. In either case, the head will have to come off to determine which it is.

 

I'm leaning more toward a burnt valve. The blown head gaskets that I've seen usually has SOME compression. And it isn't burning any coolant. Would running too lean cause the truck to burn a valve?

 

If the mechanical timing is off then the piston will come up with a valve open. Why wouldn't that cause a no compression situation?

 

Even if the piston was only half way up or down when the valves came fully closed there would stil be cylinder pressure. Besides how common is it that the timing chain actually jumps a tooth?

 

There is another way to tell if the head gasket is blown between the two cylinders.
Apply air to one with it's valves closed and listen with a hose in the others plug hole.
Listen in the intake and the oil fill for air escape in large quanities
Sounds like the head wasn't torqued properly.
These OHV inline sixes have been around a long time and are not known to be bad for gaskets.

 

Thanks for the suggestions bluegrass, I'll have to try and see if I can locate an air compressor to borrow. It's actually my mom's boyfriend's truck and I think he has around 330,000 on the body. I know he had the engine replaced about 75k ago, but I'm not sure if it was a long block or a short block. If it was a short it would certainly explain a burnt valve or an improperly torqued head.