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Old 07-11-2015, 02:35 PM   #1
marc silva
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Default Low compression in one hole

I am getting ready to check my rod shims for clearance because I have slightly noisy knock when I first start the engine after it sits for a few days.After 15 or 20 seconds or so it goes away, I assume after oil is getting to the rod bearings. Should I be concerned about a 15 lb difference in 1 cylinder? I warmed it up , went through 3 times in each hole,throttle open. Came up with front to back at 75,60,75,70...or, am I over thinking this thing?. It seems to run fine other than that. Just took a 750 mile trip with no issues. Thanks, as usual I appreciate your experience and opinions.

Marc
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Old 07-11-2015, 02:42 PM   #2
Patrick L.
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Default Re: Low compression in one hole

You didn't mention if the pressure came up or not after a little oil was added.
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Old 07-11-2015, 02:43 PM   #3
marc silva
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Default Re: Low compression in one hole

I didn't try it with oil.
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Old 07-11-2015, 02:44 PM   #4
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Default Re: Low compression in one hole

I will go out and try that right now.
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Old 07-11-2015, 02:44 PM   #5
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Default Re: Low compression in one hole

Add a little oil to that cyl and if the compression comes up, its a valve problem, and will get worse in the future.
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Old 07-11-2015, 02:47 PM   #6
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Default Re: Low compression in one hole

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if the compression comes up when oil is added to cyl, its not a valve problem, its a ring problem, worn, weak or broken, I could also add, badly worn cyl, badly groved cyl,, or a cyl that is out of round

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Old 07-11-2015, 03:08 PM   #7
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Default Re: Low compression in one hole

I have added oil on many perfectly fine motors and the compression always went higher. I would rather to a leak down for better results
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Old 07-11-2015, 03:09 PM   #8
marc silva
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Default Re: Low compression in one hole

Just did it . I gave them a couple good squirts wth a bendable tip little pumper can and it didn't change any.
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Old 07-11-2015, 03:10 PM   #9
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Default Re: Low compression in one hole

I'm thinking a leak down will probably be the thing to do.
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Old 07-11-2015, 03:26 PM   #10
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Default Re: Low compression in one hole

A leak test is great, I didn't think you had access to one. But, I'm thinking you have a bit of a beginning valve problem. What is the lash/clearance ?
But, if can do a leak test, by all means do it. Actually I just make an adapter and feed a bit of air in the hole.
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Old 07-11-2015, 03:35 PM   #11
marc silva
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Default Re: Low compression in one hole

I have never checked that. I have a new set of guides and valves I may put them in after the leak down. I do know that there are some Ford embossed valves and some un-marked ones in there though. This thing has been bored 60 over and re-built by someone in the late 60's.
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Old 07-11-2015, 04:39 PM   #12
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Default Re: Low compression in one hole

A knock when cold that goes away as the engine warms up can also be a worn/cracked piston. It is loose in the bore when cold and expands as it warms up so gets quiet. This may be apples to oranges (although still Ford), but years ago I had a '68 Ford 302 that started having a slight tick when cold. It gradually took longer and longer to go away. When it got to the point where the noise never went away, I finally tore it down and found a cracked piston skirt. Apparently the crack slowly spread and the skirt kept collapsing bit by bit.
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Old 07-11-2015, 05:00 PM   #13
Chuck Sea/Tac
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Default Re: Low compression in one hole

Do you have sludge in your oil? You could be starving an oil port. Another thought is if you have holes in your dipper trays or one just one tray, then it takes a little bit of time to fill that tray back up so the rod can get a oil.
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Old 07-11-2015, 05:18 PM   #14
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Default Re: Low compression in one hole

It could be piston slap, which goes away as the piston warms. You will get oil to the rods immediately on start from the pan tray. A rod knock does not usually go away when warm. Short out the spark plug on the suspected piston and see if the knock goes away.
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Old 07-11-2015, 06:01 PM   #15
marc silva
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Default Re: Low compression in one hole

if I were to disconnect the plug wire from the low cylinder before i started it up would i get the same result?
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Old 07-11-2015, 06:13 PM   #16
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Default Re: Low compression in one hole

no difference in compression when you added oil to the cyl usually means you have a weak valve (burnt or burning), if the engine is still running put your hand over the tail pipe or hang a rag over the end of the tail pipe, a burnt valve will cause the rotating engine to try to suck your hand or rag into the tail pipe
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Old 07-11-2015, 06:44 PM   #17
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Default Re: Low compression in one hole

Quote:
Originally Posted by marc silva View Post
I'm thinking a leak down will probably be the thing to do.
Do the leakdown test, Air pressure in the cylinder and listen for air escaping at the exhaust, intake, oil fill and radiator.
This will tell you if you've got leaking valves ( which one intake or exhaust ) or rings, It will also tell you about the head gasket and cracks in the head or block.
An excellent test, Be sure to put it on TDC and in high gear and block the wheels or have someone hold the brakes.
Bill
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Old 07-11-2015, 06:51 PM   #18
Mitch//pa
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Default Re: Low compression in one hole

Quote:
Originally Posted by bbrocksr View Post
do the leakdown test, air pressure in the cylinder and listen for air escaping at the exhaust, intake, oil fill and radiator.
This will tell you if you've got leaking valves ( which one intake or exhaust ) or rings, it will also tell you about the head gasket and cracks in the head or block.
An excellent test, be sure to put it on tdc and in high gear and block the wheels or have someone hold the brakes.
Bill
x100
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Old 07-12-2015, 12:21 PM   #19
marc silva
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Default Re: Low compression in one hole

Thank you, I will try all of the above suggestions.
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