Help with Compression Test results
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Trailering
Joined: Feb 2012
Posts: 20
Likes: 1
Help with Compression Test results
I’ve got a 1973 F100 that is going through oil about a quart every 250-300 miles but otherwise runs pretty well after putting in a rebuilt carb and doing a tuneup. I know it leaks oil but did a Compression Test to see if it’s burning it also.
Heres the results
1 - 125 dry, 150 wet
2 - 125 dry, 145 wet
3 - 125 dry, 140 wet
4 - 122 dry, 135 wet
5 - 140 dry, 155 wet
6 - 139 dry, 160 wet
7 - 130 dry, 150 wet
8 - 130 dry, 148 wet
Any thoughts? I’m a little unsure about the results. The dry test seems to be good but my understanding is that when a squirt of oil is added it shouldn’t come up much more than 10%.
Thanks,
Andy
Heres the results
1 - 125 dry, 150 wet
2 - 125 dry, 145 wet
3 - 125 dry, 140 wet
4 - 122 dry, 135 wet
5 - 140 dry, 155 wet
6 - 139 dry, 160 wet
7 - 130 dry, 150 wet
8 - 130 dry, 148 wet
Any thoughts? I’m a little unsure about the results. The dry test seems to be good but my understanding is that when a squirt of oil is added it shouldn’t come up much more than 10%.
Thanks,
Andy
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Trailering
Joined: Feb 2012
Posts: 20
Likes: 1
I haven’t had the valve covers off. #4 is a bit lower but not that bad. But overall my main concern is whether I’m going to rebuild this motor completely or if i should go in the direction of replacing the leaking seals, re-ringing and doing valve stems/valves without having to pull the motor.
Senior User
Joined: Sep 2017
Posts: 194
Likes: 8
From: Union County, NC
*assuming both wet and dry measurements were done warm and under the same circumstances*
Your significant jump in compression shows that the chamber isn’t sealed as well as it should be, right? The tablespoon of oil you added through the spark plug hole helped to temporarily increase the seal during your wet test. And you can only lose compression past the rings, valves, valve seals, or head gasket (ignoring the spark plug hole).
Do you have the ability to perform a leak-down test? This could help you pinpoint the weak area; eg. hearing air escaping out the valve cover could indicate valve seals. Air (or oil?) escaping out the dipstick tube; rings. Air out the carb; intake valves. Out the exhaust; exhaust valves. Out the radiator; head gasket. You can probably rule out some of those based on other clues. Like milky oil, etc. But a leak-down test could/would/should give you a story about all of the above at the same time.
Seeing that most cylinders had a similar compression increase when wet, I would rule out head gasket and valve seats and think more about “normal wear and tear” like rings and valves seals.
Unless you have low oil pressure, you probably don’t *need* to rebuild the bottom end.
Your significant jump in compression shows that the chamber isn’t sealed as well as it should be, right? The tablespoon of oil you added through the spark plug hole helped to temporarily increase the seal during your wet test. And you can only lose compression past the rings, valves, valve seals, or head gasket (ignoring the spark plug hole).
Do you have the ability to perform a leak-down test? This could help you pinpoint the weak area; eg. hearing air escaping out the valve cover could indicate valve seals. Air (or oil?) escaping out the dipstick tube; rings. Air out the carb; intake valves. Out the exhaust; exhaust valves. Out the radiator; head gasket. You can probably rule out some of those based on other clues. Like milky oil, etc. But a leak-down test could/would/should give you a story about all of the above at the same time.
Seeing that most cylinders had a similar compression increase when wet, I would rule out head gasket and valve seats and think more about “normal wear and tear” like rings and valves seals.
Unless you have low oil pressure, you probably don’t *need* to rebuild the bottom end.
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tex_mex1911
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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