My 390 has problems
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Freshman User
Joined: Nov 2001
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From: US
My 390 has problems
I purchased a '73 F-250 with a stock 390, as in never rebuilt stock. the truck was sitting for about 2.5 years not even being turned over every so often and i bought it outta love of my dad's old ford... but on to the problem, i have a dead cylinder and i don't know why. by dead i mean no compression, plenty of spark, no compression. i did a standard dry/wet compression test and discovered this little problem this mornin, and am wonderin if there is anything i can do to figure out if it is a head, lifter, or whatever problem... all the other cylinders have good pressure ranging from 85 up to 100, not bad for a beater with a couple 100,000 on it i think.... but i was just wonderin if anyone knew of a test or two because this is my daily driver and i need as little down time as possible, so maybe a more experienced wrench either knows the problem or a test i can run to save time and money... thanks for any responses.
Cargo Master
Joined: Jul 2000
Posts: 2,281
Likes: 18
From: Northern California
My 390 has problems
I'd remove the valve cover and see if one of the valves are sticking. I've heard that varnish can gum up the valves and lifters.
You could remove the rocker arm and see if it gets some compression, that would tell you that the valves are not seating properly.
The best test is going to be a leak down using compressed air into spark plug hole and listening for the hissing sound in the intake, exhaust, valve cover...
You could use some carb cleaner on the valve train then flush and drain the oil. but it sounds like sticking valves.
You could remove the rocker arm and see if it gets some compression, that would tell you that the valves are not seating properly.
The best test is going to be a leak down using compressed air into spark plug hole and listening for the hissing sound in the intake, exhaust, valve cover...
You could use some carb cleaner on the valve train then flush and drain the oil. but it sounds like sticking valves.
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Freshman User
Joined: Nov 2001
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From: US
My 390 has problems
i do believe there is something up with the valves, and now i am wonderin if there is anything wrong with rebuilding the top end and not the bottom? i had a chevy-mechanic curse me for daring to do so, and now i ain't real sure... last time i had the oil pan off to replace the oil pump there was nothin wrong under there, and i checked it all. so i am just wonderin, and has anyone ever heard of using a little diesel in your oil for like 10 minutes to clean out the "gummy crap" in my block? i was told it might free a hanging valve, but i ain't sure.... any response is helpful, and thanks to all who already responded.
Elder User
Joined: Aug 2001
Posts: 962
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From: Collierville, TN
My 390 has problems
If you literally have zero compression in one cylinder, it means one of two things: (1) a disintegrated valve; or (2) a hole in the piston. IMHO, the first possibility is more likely. It's also easy enough to check. Either do a leak-down test (with no pressure holding in the cylinder, all you really need to do is hook up a compressor and listen for where the air is escaping) or just pull the head on that side and take a look. Pulling the head isn't that difficult, and you should probably do it anyway. If the valve is shot, just have the head rebuilt. You should do both heads at the same time. Around here (Memphis), it costs about $125 per head for a basic rebuild if you farm it out. If there is a hole in the piston, of course, you'll need to do the bottom end. In any case, 80-100 psi cranking compression is actually pretty low. I'd bet that your rings are on their last legs. If it was me, I'd plan on doing a full rebuild soon regardless of whether you have a hole in the piston.
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Freshman User
Joined: Nov 2001
Posts: 34
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From: US
My 390 has problems
i have done the test, and the results all point to a burned out valve.... but the weird thing is that my FE is still layin it down and movin on out... kinda got me messed up, if the one cylinder is dead why is it still kickin butt and takin names??? just can't wait to get the heads rebuilt and on to really start movin.... thanks for all the help
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Cargo Master
Joined: Jul 2000
Posts: 2,281
Likes: 18
From: Northern California
My 390 has problems
It's ok to do the heads and not the bottom end, I've done it before but that was because I know exactly what the problem was and it ran for 6+ years after that and still going. It really depends on the condition of the bottom end.
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From: Gainesville, FL
My 390 has problems
z,
There is one option. If the truck runs fine, then your valve may not be that bad. Remove the rocker arm. Remove the retainer and keepers. Place a drill on the valve stem and spin the valve while pulling up on the valve. It is possible to relap the valve if its not too bad. This is an act of butchery and I do not condone such behavior in my house but, your not at my house.
Good Luck, Have Fun,
KingFisher
There is one option. If the truck runs fine, then your valve may not be that bad. Remove the rocker arm. Remove the retainer and keepers. Place a drill on the valve stem and spin the valve while pulling up on the valve. It is possible to relap the valve if its not too bad. This is an act of butchery and I do not condone such behavior in my house but, your not at my house.
Good Luck, Have Fun,
KingFisher
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