460 motor problem

[aigpd]

New to this forum. Need some help with an engine problem. Performed a compression test and all my cylinders were all 125-130 except for no. 5 and it measured 75. I run the engine for a few minutes to circulate the oil and rechecked no. 5 and now it read 25 on the tester. I assume a head gasket problem. No oil in coolant or coolant in oil. Thinking that when running that the fuel is washing the cylinder hence the additional loss of compression. Any thoughts on this? Thanks in advance for the help.

[MagicRat]

Oil does not seal a cylinder when running. I suspect that thermal expansion after its warmed up is causing either the rings or valves not to seal properly, which is why the pressure decreaes when warm.

Try pouring a bit of oil down into #5. turn it over a few revolutions and try another test. If the compression increases a lot, your rings are bad. If there is no change, its a valve or gasket.

Also try a leak down test. This is where you use an air line adaptor screwedinto the spark plug hole. Rotate the engine to TDC on #5 with both valves closed. Pressurize the cylinder with 100 psi of air or so and listen carefully.
Bad rings will allow air to leak into the crankcase and you can hear it run out the crankcase breather hose. A bad intake valve will blow air through the carb. Often a bad gasket will produce air bubbles through the coolant, escaping through the rad.
If you can hear air hissing out but can't find the leak, its probably a bad exhaust valve.

Finally, when its running, try removing the oil filler cap and have someone step on the gas while holding it with the brake in gear. Bad rings will send blow by shooting out the oil filler.

A bad intake valve will produce a backfire through the carb, or make it run poorly. A blown head gasket will blow all the coolant out the radiator. But if it runs okay without any of these symptoms, its probably a leaking exhaust valve.

[aigpd]

Originally Posted by aigpd

New to this forum. Need some help with an engine problem. Performed a compression test and all my cylinders were all 125-130 except for no. 5 and it measured 75. I run the engine for a few minutes to circulate the oil and rechecked no. 5 and now it read 25 on the tester. I assume a head gasket problem. No oil in coolant or coolant in oil. Thinking that when running that the fuel is washing the cylinder hence the additional loss of compression. Any thoughts on this? Thanks in advance for the help.

Magic Rat thanks so much for the information as this really helps me find the problem.

[MagicRat]

Originally Posted by aigpd

Magic Rat thanks so much for the information as this really helps me find the problem.

You are most welcome. Good luck!