I need motorhead logic troubleshooting.
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Senior User
Joined: Mar 2004
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From: Spokane
I need motorhead logic troubleshooting.
A quick history. 95 F-250 4x4 351 210K miles...... Bought it a year ago and has run good (flawless) and strong. No history on the engine. I did not do a compression check because it ran so good and quiet. Then one day in town I started it and got no power, missing, unburned gas smell from the tail pipe, almost no power. It takes about 1/4 mile to get up to 35 MPH. I limped home. It starts right up but runs rough. Runs quiet. I checked the codes.. got no problems. Took it to a local shop and they got no codes and the mechanic says #6 cylinder has no compression. Brought it home and did a compression check. they look like this:
Front
#5 150 PSI #1 110 PSI
#6 45 PSI #2 90 PSI
#7 45 PSI #3 140 PSI
#8 50 PSI #4 130 PSI
Trobleshooting thoughts:
1) timing chain has slipped a tooth.
2) some kind of restriction in the intake... no air in.. then no compression
3) exhaust restriction..... doubtful
Any other ideas before I tear into this thing? I hate doing this in the winter. Thanks for thinking about it.
Front
#5 150 PSI #1 110 PSI
#6 45 PSI #2 90 PSI
#7 45 PSI #3 140 PSI
#8 50 PSI #4 130 PSI
Trobleshooting thoughts:
1) timing chain has slipped a tooth.
2) some kind of restriction in the intake... no air in.. then no compression
3) exhaust restriction..... doubtful
Any other ideas before I tear into this thing? I hate doing this in the winter. Thanks for thinking about it.
Post Fiend
Joined: Nov 2001
Posts: 14,947
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From: Topsfield, MA
Moved to appropriate forum.
Any time you do a compression test, do a dry and wet test. Dry is just as is and wet is to put some oil down the cylinder to see if the compression figure improves. It helps single out whether or not it's a part of the valve or ring sealing.
A teardown is probably your next step - either the heads need rebuilding or the engine does or both.
Any time you do a compression test, do a dry and wet test. Dry is just as is and wet is to put some oil down the cylinder to see if the compression figure improves. It helps single out whether or not it's a part of the valve or ring sealing.
A teardown is probably your next step - either the heads need rebuilding or the engine does or both.
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Senior User
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From: Spokane
It seems to me that the low compression is just the outcome of whatever failed. 30 seconds before this problem occured I would have put a 30' boat behind it. It ran strong. An engine with these compression numbers wouldn't pull an empty trailer. I will tear into the timing chain before I pop the heads. That is my best guess of the problem. Anyone know how to verify the timing chain is correct without tearing into the engine. Thanks
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Remove the #1 spark plug. Slowly bump the starter or turn the engine by hand until the #1 piston reaches its highest position on the compression stroke. If the timing chain is okay, the tip of the rotary cap should now be pointed near the #1 spark plug terminal under the distributor cap.
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Senior User
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From: Spokane
I don't think a slip of one tooth can actually be seen. It would have to be some kind of measurement. That does give me a thought. If I check the timing with a timing light, it should be off of the correct mark if the timing chain has slipped. OH!! we might have something here........ I will check it tonight and let you know. Thanks
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Senior User
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From: Spokane
I checked the timing mark with a timing light and it seems to be in the right place. It is a few degrees BTDC. Well, I still don't understand the sudden lose of power. Something must be broken somewhere.
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From: Wetaskiwin, Alberta
Pull the valve cover off and have a look at your valve springs - could be a broken one. My 400M broke a valve spring and compression in that cylinder went almost to 0 - ran really rough etc. etc. check for any slop or obvious signs of damage in the springs. Also do a Wet compression test as Mr. MustangGT221 advises - actually, do this 1st before you pull the valve cover off.
Good Luck wth it!
Mike
Good Luck wth it!
Mike
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MSEE
Joined: Apr 2004
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From: Austin, TX
Low compression won't have anything to do with your timing chain. You're looking at blown head gaskets, burnt valves, or worn rings. I'd do both the wet and dry tests like they said, and you might want to run a leakdown test too.
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From: Spokane
From the first message I wrote. This happened like a light switch. One minute it runs like a new engine and then the next second no power. I might buy off on the blown head gasket but rings or valves are not very likely. I was thinking that the timing chain had slipped so the valve timing was incorrect which would cause bad compression.
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Fordberg,
I just had exactly the same thing happen to my 93 F150/302 with 180k.
it actually happened pulling a trailer. Started smoking at full throttle, and missing really bad. The compression test showed that all were within 10 lbs, except #8 which only had 35 lbs. I assumed I'd burned a hole in a piston, and tore it apart. Turned out that there was a giant chunk of sludge (I'm thinking out of the intake) which was lodged on all three rings, causing them to sieze, allowing compression & fire to leak by, and melted the edge off the piston.
I had just treated the intake with some Spray stuff from BG products which supposidly would clean the inside of the intake, and ran two tank fulls of the accompaning fuel treatment through the truck which was supposed to clean the intake.
I think I owe a debt of thanks to penzoil.
Good news was, cylinders were still within specs!
Ken F
I just had exactly the same thing happen to my 93 F150/302 with 180k.
it actually happened pulling a trailer. Started smoking at full throttle, and missing really bad. The compression test showed that all were within 10 lbs, except #8 which only had 35 lbs. I assumed I'd burned a hole in a piston, and tore it apart. Turned out that there was a giant chunk of sludge (I'm thinking out of the intake) which was lodged on all three rings, causing them to sieze, allowing compression & fire to leak by, and melted the edge off the piston.
I had just treated the intake with some Spray stuff from BG products which supposidly would clean the inside of the intake, and ran two tank fulls of the accompaning fuel treatment through the truck which was supposed to clean the intake.
I think I owe a debt of thanks to penzoil.
Good news was, cylinders were still within specs!
Ken F
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Posts: 158
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Cam timing will effect cylinder pressure, but if the chain jumped, I think the change in cam timing would effect all cylinders equally. Try turning the engine back and forth by hand while watching the rotor. If the crank turns much before the rotor moves, the chain is sloppy. If it's really sloppy it could have jumped, but as I described above, I doubt that's your compression problem. By the way, if that chain is original (210k), it should be changed no matter what. The dry/wet test is a good idea before you open anything up. Then I would pull the valve covers. I lost compression on a cylinder once because the intake rocker arm loosened up and fell off (nothing to compress). Just like you said, one minute it seemed fine, the next minute, it ran like crap. This used to be an old GM problem, but a few wiped out cam lobes might cause this. Kind of a long shot, but just another possibility. Unless you find something under the valve covers, I'd say the heads probably need to come off.
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