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Car Forum / Ford / Ford Trucks / October 2004No 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? Stuck valve? or compressiion rings? KEnny > 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 [quoted text clipped - 14 lines] > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > problem? Pulled valve cover, all valves working fine. > Stuck valve? or compressiion rings? > [quoted text clipped - 21 lines] > > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > > problem? Most likely you burned an exhaust valve. Bob > 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 [quoted text clipped - 18 lines] > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > problem? Would have guess that but the problem was instant, just like someone pulled a plug wire. Was running perfect to that point. > Most likely you burned an exhaust valve. > Bob [quoted text clipped - 20 lines] > > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > > problem? head gasket between # 6 and # 7 > 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 [quoted text clipped - 14 lines] > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > problem? 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 > head gasket between # 6 and # 7 > [quoted text clipped - 26 lines] >> side is hot. No oil or water at either exhaust pipe. Okay guys...what the >> problem? > If you blew the head gasket, then you'd have a compression problem in 2 > cylinders Right > Burnt valve burnt piston (sides), or even a hole in a piston > Steve A piston or ring problem serious enough to cause zero compression would cause massive blowby and oil usage issues as well. Bob > > If you blew the head gasket, then you'd have a compression problem in 2 > > cylinders [quoted text clipped - 7 lines] > cause massive blowby and oil usage issues as well. > Bob According to his original post, he does have compression problems with two cyls. H >> > If you blew the head gasket, then you'd have a compression problem in 2 >> > cylinders [quoted text clipped - 10 lines] > According to his original post, he does have compression problems with two > cyls. 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 Me too, Leaning towards a head gasket now also Steve >> > If you blew the head gasket, then you'd have a compression problem in 2 >> > cylinders [quoted text clipped - 11 lines] > cyls. > H > > If you blew the head gasket, then you'd have a compression problem in 2 > > cylinders [quoted text clipped - 7 lines] > cause massive blowby and oil usage issues as well. > Bob 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? >> > If you blew the head gasket, then you'd have a compression problem in 2 >> > cylinders [quoted text clipped - 10 lines] >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? It is not necessary that oil or water leak anywhere in order for a head gasket to be blown. I have seen this condition result in cutting a slot between cylinders in the block and/or the head in heavily loaded engines which required replacment of the block and/or head to repair. It is also not necessary that the cylinders have equally low compression for this condition to exist. If you bored the engine much when it was built, much of the surface area between the cylinders was removed making correct torque more essential. The fact that it was a sudden loss of compression under heavy towing conditions is not terribly surprising. If it were my engine, I would be either pulling the heads or checking into a replacement engine. With the FE engine, it is likely that the head will need to be surfaced. It is best to mill both heads evenly to maintain relatively even compression in the engine. At some point, it becomes necessary to also mill the intake manifold to maintain proper sealing around the intake ports. A good auto machine shop shoult be able to handle that chore easily. 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? > If you blew the head gasket, then you'd have a compression problem in 2 > cylinders [quoted text clipped - 31 lines] > >> side is hot. No oil or water at either exhaust pipe. Okay guys...what the > >> problem? 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.  SignatureStephen W. Hansen ASE Certified Auto Technician > 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 [quoted text clipped - 44 lines] > the >> >> problem? not necessarily the torn gasket can act as a flapper valve and allow passage of gasses in one direction more easily than the other > If you blew the head gasket, then you'd have a compression problem in 2 > cylinders [quoted text clipped - 31 lines] > >> side is hot. No oil or water at either exhaust pipe. Okay guys...what the > >> problem? > not necessarily > > the torn gasket can act as a flapper valve and allow passage of gasses in > one direction more easily than the other I guess anything is possible but I've never heard of that one before. However, I did see a friend lose a bet on whether or not a spark plug gap would increase when tightened in the cylinder. Your guess? > > If you blew the head gasket, then you'd have a compression problem in 2 > > cylinders [quoted text clipped - 37 lines] > the > > >> problem? > > not necessarily > > [quoted text clipped - 4 lines] > However, I did see a friend lose a bet on whether or not a spark plug gap > would increase when tightened in the cylinder. Your guess? A spark plug will stretch just like an overtorqued bolt will, changing the gap. H After 35+ years of wrenchin', i thought i had heard every BULLSHIT story in the book. I guess not. s > A spark plug will stretch just like an overtorqued bolt will, changing the > gap. > > H >After 35+ years of wrenchin', i thought i had heard every BULLSHIT story in >the book. I guess not. I think I'd like to see this 1st hand.. Anyone got a head off an engine they'd be willing to experiment with? Set the gap on a plug, then install it using proper torque and measure it again. Then overtorque it and measure once more. I don't see how the gap could change.. > After 35+ years of wrenchin', i thought i had heard every BULLSHIT story in > the book. I guess not. I'm surprised that after 35+ years of wrenchin', you've never seen a stretched bolt. Maybe you were just never a good enough wrench to notice. H > > A spark plug will stretch just like an overtorqued bolt will, changing the > > gap. > > > > H >> After 35+ years of wrenchin', i thought i had heard every BULLSHIT story > in [quoted text clipped - 3 lines] > stretched bolt. Maybe you were just never a good enough wrench to notice. > H Or just maybe he's good enough not to stretch bolts that weren't meant to be stretched......... In my opinion anyone who over torques spark plugs enough to significantly change the gap should have there tools taken away Bob > >> After 35+ years of wrenchin', i thought i had heard every BULLSHIT story > > in [quoted text clipped - 8 lines] > enough to significantly change the gap should have there tools taken away > Bob It's not as difficult to understand as one might think. Had I not witnessed it first hand many years ago I would not have believed just how much the gap did increase. Think of it in practical terms. The threaded portion of the spark plug is simply a hollow pipe with an "L" shapped electrode at the very bottom. It is much easier to stretch the threads of a hollow pipe than it is a solid bolt. The center electrode is held rigid by the upper portion of the spark plug seated against the plug opening. Once torque is applied to the upper portion of the plug the threads attempt to track further into the female threads of the opening. This creates a stretching of the "pipe" portion of the plug and efficiently and assuredly increases the gap in the plug. The increase is, although slight, much greater than you might expect. Find a loose head and see for yourself. A torque rating of 30 to 35 ft lbs will do the job. Our subsequent trials showed that a plug with fewest threads produced the greatest increase. > >> After 35+ years of wrenchin', i thought i had heard every BULLSHIT story > > in [quoted text clipped - 8 lines] > enough to significantly change the gap should have there tools taken away > Bob I think education is a better alternative, at least for those willing to learn. H > After 35+ years of wrenchin', i thought i had heard every BULLSHIT story in > the book. I guess not. Don't bet any serious money on this one Steve, cause you'll lose. See the post below. > s > > > A spark plug will stretch just like an overtorqued bolt will, changing the > > gap. > > > > H 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? > head gasket between # 6 and # 7 > [quoted text clipped - 20 lines] > > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > > problem? >whats the problem? 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) > >whats the problem? > [quoted text clipped - 4 lines] > > Dave S(Texas) 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. >> >whats the problem? >> [quoted text clipped - 16 lines] > sure at this point that #6 was producing power but it definitely misses at > idle. 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? SignatureDavid M (dmacchiarolo) http://home.triad.rr.com/redsled T/S 53 sled351 Linux 2.4.18-14 has been up 30 days 11:24 Head gasket blown between #6 and #7 and/or a hole in the top of #7 piston, most likely caused by prolonged pre-ignition? > 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 [quoted text clipped - 14 lines] > side is hot. No oil or water at either exhaust pipe. Okay guys...what the > problem? Okay. Time to reveal the actual problem. Everyone had pretty much the same solution but we all missed it...and me with more than 50 years experience in these matters, even taught the stuff at one time. Here's what happened, and the coincidence which sort of screwed up "normal" solutions. The valve seat in #7 came loose, turned slightly and lodged itself in a tilted position behind the exhaust valve. This permitted the valve to operate in a normal manner but never seated. For some reason the seat remained in a stable tilted position even with the valve opening and closing. The dimension between the valve seat and the tilted valve remained almost identical so it was impossible to determine by valve train movement that a problem existed in that area. The head gasket was in perfect condition between #6 and #7 cyls. However, the exhaust valve in #6 had a slight burn area and was "leaking" enough to cause a pretty dramatic drop in psi. This coincidence is what threw me (and everyone else) off. Had that occurred in any other cylinder (other than #8) there would have been no connection made. Didn't discover this until the engine was pulled and head removed. Everything else in #7 was fine with no damage...rings, top of piston, etc. One of the things that was very apparent was the lack of significant carbon build up as a result of unleaded fuel. And this engine burns no oil. In the past mechanics were able to tell exactly where rings were leaking because of the absence of lead/carbon deposits on the head of the piston. Many thanks to all who contributed their thoughts. Another one for the "knowledge warehouse". > Head gasket blown between #6 and #7 and/or a hole in the top of #7 > piston, most likely caused by prolonged pre-ignition? [quoted text clipped - 33 lines] > the > > problem? well, sh.t wrong again did excess heat cause this, maybe ? > Okay. Time to reveal the actual problem. Everyone had pretty much the same > solution but we all missed it...and me with more than 50 years experience in [quoted text clipped - 58 lines] > > the > > > problem? > well, sh.t > > wrong again > > did excess heat cause this, maybe ? I would normally expect excessive heat to cause this but the temp gauge did not go above the normal operating range. However, if the sending unit was bad (possible) that could help explain the resulting problem. The machine shop called me a few minutes ago and informed me the head was cracked (very small) between the valve seats which might explain the released seat. Anyway, I have to shell out $475 for a used replacement head, all new valves and other work associated with a head job. Thanks to everyone for your contributions. All very interesting. > > Okay. Time to reveal the actual problem. Everyone had pretty much the same > > solution but we all missed it...and me with more than 50 years experience [quoted text clipped - 62 lines] > > > the > > > > problem? You'll never catch engine damaging heat build up found during high load intermittent use (pulling a grade) by relying on the coolant temp gauge, takes too long for the coolant to increase in temp and when it does damage could already be done, or worse, you won't see any temp warning at all (like in this case). Install a cyl head temp gauge, I wouldn't fly in any plane that did not have one. > > well, sh.t > > [quoted text clipped - 98 lines] > the > > > > > problem? |
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