cam failure symptoms?
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Laughing Gas
Joined: Nov 2015
Posts: 950
Likes: 73
From: San Diego
cam failure symptoms?
Hi all. My 72 f250 has a 390 and Motorcraft 2v.& C6. I am feeling an vibration through the gas pedal when I hold speed starting at 20mph. Shifting to NUETRAL the engine roughness feeling stops.
Its prgressed to a miss at idle and on acceleration under a load like pulling a hill.
The spark plugs, Pertronic full billet distributor and its cap, along with taylor. 8mm ignition wires were done 14k miles ago.
My engine has a fairly substantial rear seal oil leak
So I dont know if Im burning oil.
So its probably time for a wet & dry compression test and a leak down test...to see what's what...correct?
Its prgressed to a miss at idle and on acceleration under a load like pulling a hill.
The spark plugs, Pertronic full billet distributor and its cap, along with taylor. 8mm ignition wires were done 14k miles ago.
My engine has a fairly substantial rear seal oil leak
So I dont know if Im burning oil.
So its probably time for a wet & dry compression test and a leak down test...to see what's what...correct?
Tuned
Joined: Apr 2023
Posts: 363
Likes: 108
nothing wrong with a compression test in general, but a wiped lobe wouldn't show with one because the valve wouldn't be opening (or as much)
you could pull the valve covers and turn it over to see the rockers move. another option would be to allow the engine to just heat up and drip some water on each of the exhaust primaries (not sure if that's what they are called on a manifold) to see if one or more cylinders aren't getting as hot as the others. if a lobe is wiped on the either the exhaust or intake, one cylinder shouldn't be able to get hot exhaust gases out because there no where for it to escape or there is no fuel rich air to burn.
you could pull the valve covers and turn it over to see the rockers move. another option would be to allow the engine to just heat up and drip some water on each of the exhaust primaries (not sure if that's what they are called on a manifold) to see if one or more cylinders aren't getting as hot as the others. if a lobe is wiped on the either the exhaust or intake, one cylinder shouldn't be able to get hot exhaust gases out because there no where for it to escape or there is no fuel rich air to burn.
More Turbo
Joined: Sep 2012
Posts: 739
Likes: 256
From: Zimmerman, MN
Do you have a caliper? Seems to me you could check if a lobe was wiped by simply pulling the valve covers, and with the spark plugs removed, turn the engine over by hand and measure the actual depression on the valves when at full or max lift. It may not give you the exact lift as per Spec, but at least you'd know if any of them were "off" by comparing one to the others (intake to intake and exhaust to exhaust, of course).
Lead Driver
Joined: Jul 2017
Posts: 9,601
Likes: 1,174
From: San Jose, CA
The missing and popping upon acceleration is one symptom of a wiped lobe, but you get more of the popping through either exhaust or carburetor.
However, the miss is also a sign of fouled spark plugs.
that 14,000 miles might not sound like much today, but is a long time for an old engine.
If you’re used to modern vehicles with hundred thousand miles tuneups, you might consider checking the spark plugs, and the wires too while you’re at it, if you have an ohm meter.
Which any owner of a an older vehicle should have.
And d if you haven’t already, check your ignition. Because you can see if there have been any changes or deterioration in the ignition components, or the timing chain and gears.
A compression test is a legitimate test of a cam, because if an intake valve is not opening, you’re not pulling in the volume of air needed to create the compression in the first place.
So while it might not be a consistently accurate test, it is quite valid. If it’s an intake valve, it’ll certainly lower compression. If it’s an exhaust valve, I’m not sure what the reaction to be per.
Perhaps that’s when it would not show much difference.
However, the miss is also a sign of fouled spark plugs.
that 14,000 miles might not sound like much today, but is a long time for an old engine.
If you’re used to modern vehicles with hundred thousand miles tuneups, you might consider checking the spark plugs, and the wires too while you’re at it, if you have an ohm meter.
Which any owner of a an older vehicle should have.
And d if you haven’t already, check your ignition. Because you can see if there have been any changes or deterioration in the ignition components, or the timing chain and gears.
A compression test is a legitimate test of a cam, because if an intake valve is not opening, you’re not pulling in the volume of air needed to create the compression in the first place.
So while it might not be a consistently accurate test, it is quite valid. If it’s an intake valve, it’ll certainly lower compression. If it’s an exhaust valve, I’m not sure what the reaction to be per.
Perhaps that’s when it would not show much difference.
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