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Fluctuating Compression Test

J

JimS88

Member
Here is a problem I don't understand. My 4 cylinder volvo penta AQ151C engine quit running the other day. 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 tens 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.

Anyone have any ideas what is wrong?

Thanks,

Jim
 
Have you installed the timing belt correctly? And most important, did you install a new timing belt? Your old one may have some broken teeth and it may be "jumping" in the crankshaft timing sprocket.
 
el_pescador said:
Have you installed the timing belt correctly? And most important, did you install a new timing belt? Your old one may have some broken teeth and it may be "jumping" in the crankshaft timing sprocket.
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I was careful to line up the timing gears and I've done it sucessfully many times before so I believe I installed it correctly. Yes, I put on a new 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.
 
I still think it may be the timing belt slipping, or perhaps (not so likely but also possible) sticky valves.

Here is an easy test. Remove the valve cover and crank a few times at random, then put #1 piston on TDC at the end of the compression stroke and see where the cam lobes are pointing. If one valve is open, or if the position of the cam lobes change during repeated tests, then you know for sure the timing belt is slipping. You could also see if some valves are sticking.
 
el_pescador said:
I still think it may be the timing belt slipping, or perhaps (not so likely but also possible) sticky valves.

Well I checked the timing and you are correct. Thanks for your help. It's nice to have an objective viewpoint.

Now if I can just get spark (that should be easy but I ran out of light. Hopefully tomorrow.

Thanks again

Jim
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You are welcome, that's what we are here for. :)

You may want to follow kimcrwbr1's advice and check the condition of your gears. If pitted/corroded, replace them even if with a used set that you can buy from an autowrecker. They are exactly the same as automotive, but the ones used in car engines are not subject to the corrosive effects of salt water (that is, if you run your boat in salt water).
 
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