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JimS88
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Jul 17, 2012, 3:55 PM
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Compression Mystery
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Here is a problem I don't understand. My 4 cylinder volvo penta AQ151C 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.
Anyone have any ideas what is wrong?
Thanks,
Jim
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Hammer Time
Ultimate Carjunky
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Jul 17, 2012, 4:31 PM
Post #2 of 14
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Re: Compression Mystery
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Your testing procedure is making no sense to me.
What exactly do you mean it makes compression for about 5 or 6 revolutions, then for about another 6 revolutions it makes zero compression.
Are you relieving compression and starting over?
Does your compression tester not hold pressure with a schrader valve?
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JimS88
Novice
Jul 17, 2012, 4:43 PM
Post #3 of 14
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Re: Compression Mystery
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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.
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Hammer Time
Ultimate Carjunky
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Jul 17, 2012, 4:47 PM
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Re: Compression Mystery
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It was probably just collapsing the lifters from cranking it too long with no oil pressure. You only need about 3-4 revolutions for a compression test. You car has compression, you're done.
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JimS88
Novice
Jul 17, 2012, 4:49 PM
Post #5 of 14
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Re: Compression Mystery
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No Lifters, Overhead Cam.
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Hammer Time
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Jul 17, 2012, 4:51 PM
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Re: Compression Mystery
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Overhead cams have lifters too.
edit, I see this one has solid lifters.
Why are you checking this? What problem are you having?
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(This post was edited by Hammer Time on Jul 17, 2012, 5:03 PM)
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JimS88
Novice
Jul 17, 2012, 5:10 PM
Post #7 of 14
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Re: Compression Mystery
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Sorry, I was thinking pushrods.
The engine won't start so I need to find out why. If it ran fine I wouldn't mess with it further which is what I was hoping for when I put it back together. Now I have to diagnose the problem so I can fix it.
Are you suggesting that it is normal for an engine not to have compression intermittently? I've had this engine (actually twins) for about 22 years now and it never made this sound or had this problem before.
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Hammer Time
Ultimate Carjunky
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Jul 17, 2012, 5:16 PM
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Re: Compression Mystery
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If you say you can hear it through the intake, that means you're losing it through the intake valve for some reason. Pull the valve cover off and observe the valves and see if they are continuing to close on time.
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Discretesignals
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Jul 17, 2012, 6:14 PM
Post #9 of 14
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Re: Compression Mystery
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What is the year and model of your Volvo?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Hammer Time
Ultimate Carjunky
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Jul 17, 2012, 6:18 PM
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Re: Compression Mystery
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I believe this is a marine application.
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Discretesignals
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Jul 17, 2012, 6:23 PM
Post #11 of 14
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Re: Compression Mystery
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Just wondering because cylinder wash out can cause those symptoms. Especially if it is using EFI.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Hammer Time
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Jul 17, 2012, 6:26 PM
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Re: Compression Mystery
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I seriously doubt they are injected. he stated he's had them 22 years. Boats started using injection in last 10 years.
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JimS88
Novice
Jul 17, 2012, 9:30 PM
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Re: Compression Mystery
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I figured it out. the timing belt slipped. Still dunno why it had compression some of the time. Doesn't matter now.
Thanks for your help.
Jim
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Hammer Time
Ultimate Carjunky
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Jul 18, 2012, 3:05 AM
Post #14 of 14
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Re: Compression Mystery
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Yep, that makes no sense at all. Once it's out of time, the compression should be gone permanently.
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