1970Williams

Hi,
I recently bought a 93 F150 4.9L from a friend who hadn't had it started in 4 or 5 years. After replacing the bad gas tanks and pumps, I got it running but it has a miss. I checked compression and all cylinders except #4 had about 120psi. #4 cylinder was hard to get a reading on, but sometimes I did see 120psi. When I pull the spark plug on #4 cylinder it is wet with gas. I swapped spark plugs between #1 and #4 cylinders but still had the some problem. I also verified the spark plug wire was good ( arcs good to ground when truck is running). I suspect since the truck had been sitting so long that a valve may be stuck or sticking. Is there any good way of checking this without removing the valve cover? Someone told me to try some Seafoam in the intake to try cleaning the intake valves. I would think if it was a ring problem I would see oil at the spark plug and wouldn't see the 120 psi during the compression test. This truck has about 90,000 miles on it. Any help is greatly appreciated! Thanks

Chris_1

iTrader: (1)

You need to get a good compression reading. If it's low, squirt a few drops of oil in there and take another reading. If it suddenly jumps up, the rings are shot. If it doesn't you have a bad valve (or head gasket).
Also check resistance on that plug wire against the others with an ohmeter.
A spark plug that will throw a spark in open air may not perform as well under compression (harder to arc through 120 psi compression than open air).
Unless you have a bad cylinder (rings, valve, h/g) it's most likely electrical as it sounds like your injector is working. - cap, wires, plug ?

1970Williams

Thanks. I got a good compression reading of 120 psi several times on that cylinder without adding any oil, so I don't think it's rings. I just couldn't get a good reading every time I tried. That's what makes me think valves. Plug, wire, and cap are good.

96bb6truck

Had the same issue on my 96 4.9. Cylinder 1 developed a misfire on the highway and then went dead 30 miles later. Compression revealed low on cylinder 1 dry and wet. Pulled the head and found white carbon built up on the exhaust valves on all cylinders, but #1 was the worst and had caused it to stick open. This was at 300,000miles. Put a new head on it (without the AIR holes) and all cylinders read at 150psi now.
these motors are notorious for carbon build up from the egr and air pump not operating properly.

1970Williams

Thanks for the info. Would there be any benefit of trying something like Seafoam to clean up the valves before I tear it down?