No compression on one cylinder

Okay, here's one for the real experts. Have a '73 Ford F250 with a 390 engine bored to about 400 with a torque cam. Did the rebuild 74,000 miles ago and truck has been a real gem. Was pulling a pretty hefty grade a week or so ago, engine running good, warmer than normal because of the long grade pull. Suddenly engine began missing on one cylinder. Pulled over to check and, although temp was pretty high the new radiator seemed to be doing the job but pushed about a quart of water into the overflow container once the engine was shut off. Check water in overflow tank, no oil. Check oil, no water. Check exhaust no oil or water. Started it up, oil pressure normal, still missed like crazy but decided to drive remaining 35 miles home. Made it fine with no further problems other than steady miss. Pulled all plugs and did a compression check. Cylinder #7 had virtually NO compression, even with just my finger in the plug hole. Cylinder #6 showed only about 60 psi while all other cyls had 120-130 psi. Still no water in oil or oil in water. Truck starts easy and runs fine except for steady miss. I have dual exhaust and the driver's side exhaust is actually cool at idle while the passenger side is hot. No oil or water at either exhaust pipe. Okay guys...what the problem?

Reply to
The earnest one
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Stuck valve? or compressiion rings?

Reply to
Mercury

Reply to
Bob

head gasket between # 6 and # 7

Reply to
TranSurgeon

Bad head gasket or stuck/bad valve. The best way to diagnose the problem, without a tear-down, is an air-leak test. Compression tests are good, up to a point. Air-leak test can pin-point the problem, whether/not it's a bad valve, head gasket, ring problem.

Dave S(Texas)

Reply to
putt

If you blew the head gasket, then you'd have a compression problem in 2 cylinders Burnt valve burnt piston (sides), or even a hole in a piston Steve

Reply to
Steve

Right

A piston or ring problem serious enough to cause zero compression would cause massive blowby and oil usage issues as well. Bob

Reply to
Bob

According to his original post, he does have compression problems with two cyls. H

Reply to
Hairy

Sorry I missed that......and thanks for bringing it to my attention. Considering that info I would guess Transurgeon is right about the head gasket. Bob

Reply to
Bob

Pulled valve cover, all valves working fine.

Reply to
The earnest one

Would have guess that but the problem was instant, just like someone pulled a plug wire. Was running perfect to that point.

Reply to
The earnest one

Problem with that is that there is no water in the oil, no oil or bubbles in water and the compression between #6 and #7 was very different...about 60 psi on one and none on the other. Any other suggestion?

Reply to
The earnest one

Correct! But this thing leaks virtually no oil, no water, no bubbles in water, etc. Exhaust is cool on driver's side. Any other ideas?

Reply to
The earnest one

You're probably getting close Dave. Sounds like a good idea. You're saying to insert something into the plug hole and apply air pressure to see what happens I assume. Will give that a try. Let's also assume it's not a blown headgasket, no hole in piston, rings good, valves operating properly but air blows out the exhaust port. I'm assuming it's not a problem on the intake side because that would create hell on the intake side...manifold, carb, etc. However, there was virtually no noise that would indicate a broken part of any type (valve head snapping off and dropping into cylinder) and the engine ran fine except for the missing cylinder. I'm not sure at this point that #6 was producing power but it definitely misses at idle.

Reply to
The earnest one

Reply to
Steve

Again, this was an instant problem...running perfect one moment, complete miss the next, like a plug wire had been pulled. A hole in the piston would throw a lot of oil through the valve system and out the exhaust. That's not the case. Any other ideas?

Reply to
The earnest one

Regardless of how, You've lost compression. You could do a wet compression and confirm or eliminate the rings, But due to the loss of compression in 2 cylinders, I'd lean towards the head gasket also. Unless you have a borescope, you can't see until the head was off. You can use a air compressor to listen to air leakage, but regardless of what you find, the heads will probably need to come off. All good compression repairs start there.

Reply to
Steve

Head gasket blown between #6 and #7 and/or a hole in the top of #7 piston, most likely caused by prolonged pre-ignition?

Reply to
Tyrone

On Mon, 18 Oct 2004 04:06:27 +0000, The earnest one rearranged some electrons to form:

If you didn't want anyone else's ideas, then why did you bother asking? A head gasket burned through between two cylinders would not necessarily cause bubbles in the water.

Rather than keep on guessing, why don't you try some testing and troubleshooting?

Reply to
David M

not necessarily

the torn gasket can act as a flapper valve and allow passage of gasses in one direction more easily than the other

Reply to
TranSurgeon

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