Mechanics

COMPRESSION TEST MYSTERY

1989 Volvo 240 • 100,000 miles

Here is a problem I don't understand. My 4 cylinder volvo penta AQ151C marine engine quit running the other day. (It's the same basic Engine used on countless Older Volvo cars through the years} I thought the problem was the head so I got a rebuilt one and put it back together. It seems to be doing the same thing:
The motor won't start. I did a compression test and When I turn it over it it makes compression for about 5 or 6 revolutions, then for about another 6 revolutions it makes zero compression. This make no sense to me whatsoever but I suspect the problem is in the lower end due to the head rebuild.

I was careful to line up the timing gears and I've done it successfully many times before so I believe I installed it correctly. I put on a new timing belt.

It does it on all 4 cylinders although #4 has lower compression. All plugs were removed for the test. I turned the engine over about 15 revolutions on each test attempt. I frequently hit the button to release the pressure on the gauge when it built up. And when it was in the group of revolutions where it made compression the needle would climb but then after about 5 or 6 revolutions the engine would spin faster and the needle would stay at zero for about 5 or 6 more revolutions then it would repeat the process.

Also, and this may be a key clue, when it is making compression air blows out the spark plug holes (obviously) But when it isn't making compression, air blows out of the carburetors, which seems odd to me.

I can't understand how any engine could alternately make compression then make none. I've done compression tests before on this and other engines and always the needle rises with each revolution until about the fourth time where it hit maximum.
Anyone have any ideas what is wrong?

Thanks,

Jim
Avatar
JimS88
July 17, 2012.



If you have compression comming out the carbs then your valve timming is off. Because if both valves are closed on the compression stroke how compression come out of the carbs?

Saturntech9
Jul 18, 2012.