69 390 low compression
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69 390 low compression
Yesterday I did a compression test on my 69 F100 (390) and found about 60 psi on all but number 7 it had 0 psi are no compression. I had the truck about 13 years and did not build the engine so I have no idea of what type cam, pistons, compression ratio etc. It runs good and is strong but is has a miss that I can't get rid off. The plugs all look good. I removed all plugs, held the throttle plates open while checking the compression. Most websites say that 60psi is too low and is a sign of a bad engine. The engine doesn't smoke are burn oil and has never ran hot at least since I owned it. I did have to replace a push rod on number 7 about 10 years ago, could it be possible that the valve bent or stuck then? What should the compression ratio be of a stock 390? Thanks
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Joined: Mar 2013
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From: Phoenix, Az.
60 PSI on an engine that runs good sans the miss? Something doesn't sound right. At 60 PSI there'd be some obvious clue of an engine in bad need of repair. Blue smoke,blow-by,water in the oil,etc. On a 390 I'd expect 140 ish. Did you have all the spark plugs out at the same time?
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Joined: Jan 2007
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Pull the valve cover off and check if both rockers are moving on #7. ZERO compression is very suspect. Also, while 60psi is considered low, the fact that those cylinders are all the same is interesting. A lot depends on the cam shaft. If a cam has a significant amount of overlap, you can have what would be considered low compression even though you have a high compression ratio.
Look closer for the cause of zero compression on #7.
Look closer for the cause of zero compression on #7.
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Yes, all plugs were out. The throttle plates were open but I did not do a wet test (put oil in the cylinders). I was using a rental tool from Autozone which would not screw into the plug bores and my six year old daughter to crank the engine over. The results were pretty consistent. I am going to try and find a tool that will screw into the head to see if the results a better. I usually pull a Ford 3000 tractor with implements with no problem. Just yesterday I hauled for large round bails of hay, no problem.
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