Mercury Cougar Owners banner

1 - 6 of 6 Posts

·
Registered
Joined
·
60 Posts
Discussion Starter #1
I finally did a compression test on my '69 with the 390:

1: 140psi
2: 115-120psi
3: 110-115psi
4: 110-115psi
5: 130-135psi
6: 130psi
7: 120psi
8: 120psi

The plugs in cyls 3 & 4 were a nice bright white - all others were a bit of tan/brown/black (looked pretty normal to me).

I squirted a bit of oil in cyls 7 & 8 - the compression came up to 135psi. Since the engine wasn't really at operating temperature - I imagine most cylinders would do the same.

Finally - on cyl 3 - I did get some lower readings. Maybe 90-100. But after testing it again - it came up.

Any concerns here - or just drive it and hope for the best. The idle is set a little high - about 600. Otherwise it likes to miss sometimes when it is in gear.
 

·
Registered
Joined
·
1,160 Posts
Compression Method

Around 1980, the last time I ran a compression test on my '68 390GT S-code, my readings varied between 125 psi to 150 psi on 7 cylinders, with one cylinder having a reading of 25 psi. This caused a miss at idle and you could hear a putt-putt-putt out the tail pipe. Pulling the cyl heads revealed a severely burnt exhaust valve. New TRW S/S valves and a complete valve job made the engine run perfectly smooth. Huge difference.

If I remember correctly, you want no more than a 10-15% max pressure delta reading between all 8 cylinders. Your 140 psi max vs. the 110 psi min has a 21% reduction in pressure, so that's outside the limits for acceptable range for sure. So the other cyl which gave you the 90-100 has even a greater delta. But I can't confirm that requires a ring job.

When I conduct a compression test, I run 2 cycles. First cycle involves removing all plugs, then run the compression test on all 8 cylinders.

Then squirt some motor oil into each cylinder, crank the engine over a couple of turns to allow the oil to coat the entire cyl wall, then run the compression test again.

If the 1st and 2nd reading remains constant for a cylinder, that typically means your rings are OK.

If your 1st reading is low, but your second reading is considerably higher, the rings in that cylinder are weak.

If your 1st reading is low, then your 2nd reading is equally low, you have a leaky (most likely exhaust) valve...or possibly a hole in your piston.

By conducting the compression test in this manner, you can usually narrow down whether you have a leaking cylinder (rings) vs leaking valves. Valve jobs are considerably easier, faster and less expensive than tearing down and rebuilding the entire engine. Something to consider.

Good luck.
 

·
Premium Member
Joined
·
13,577 Posts
One other comment/suggestion. The clean white plugs might/would be of concern, that usually happens when coolant gets into the combustion chambers. Couple that with those cylinders being a little low, not too low, but low just the same. The quick and easy test is to run the engine up to temperature (with the radiator cap on). Then remove the plugs, 3 and 4 first since they are of concern. Look for steam coming out of any of the spark plug holes.

Not trying to be an alarmist, but this is an easy check which could be very telling about a couple of really clean plugs. If nothing turns up, nothing lost.

Regards,

Bob
 

·
Registered
Joined
·
11,950 Posts
Those 2 approaches are going to give you a good health assessment. I don't think you have to be in any hurry if it's running OK. Just be aware that you will want to rebuild or look at other options in the future. Be sure to keep checking your oil and use oil with correct additives (zinc or substitutes) and I bet you will get much more out of it yet. That gives you time to gather up what you need when the days comes.
 

·
Registered
Joined
·
97 Posts
I had some fairly similar systems with a recently rebuilt 289. After a lot of fussing I found some gremlins in the ignition; a weak coil and poor connections in the distributer cap. A new coil & cleaning up all plug & coil wire connections smoothed everything out. Just a thought.
 

·
Registered
Joined
·
1,044 Posts
i'd drive it---the miss @ idle could be many things---try pulling the plug wire when idling--if when you remove a wire the idle quality does not change much , you probably have a vacuum leak
 
1 - 6 of 6 Posts
Top