Low compression

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Apr 23, 2011
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Pulled off my exhaust manifolds as I had a problem overheating. While the manifolds were off figured I'd check compressions. Checked the compression on my 1986 volvo 225 and all readings were between 45 and 55 psi. does the engine need to be warm to get accurate readings. Engine has been running strong with no apparent engine problems besides the overheating. temperature on the boat gauge read steady at 160 though a temp gun at top of the engine read above 220 in some spots. Any ideas?
 

Don S

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Re: Low compression

I doubt you could even get the engine started with that kind of compression.
did you have a fully charged battery? I doesn't have to be a warm engine, but it should have been run within the last few days.
 
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Re: Low compression

Just ran the boat yesterday. One of the exhaust manifolds was not getting as hot as the other or I would not even realized there was an issue. Engine starts right up, idles smoothly and has good power getting on plane. I even got a new compression tester as I thought mine was defective. I only checked it being the manifolds were removed for replacement. What do you think? And yes the batteries are turning the engine well.
 

Don S

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Re: Low compression

How do you do your compression test?
 
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Re: Low compression

went through each of eight cylinders cranking four or five times till readings topped out. Had throttle wide open. having the manifolds removed would not make a difference, would it? I guess I could just put the new manifolds on and see how she runs. I just didn't want to buy the manifolds and risers only to find the engine needs an overhaul. Any other ideas?
 

captmello

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Re: Low compression

My manifolds don't cool equally, but neither overheats. How hot was the hotter of the two? Could you hold your hand on it for any period of time?

What were your second compression test results?
 
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Re: Low compression

I could hold my hand even on the hotter of the two. the temp gun showed 160 on the hotter one and 115 on the other. Adding oil on a second compression test showed no real change. Could I have two bad compression testers? Seems unlikely.
 

captmello

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Re: Low compression

It doesn't sound like it's overheating.

Typically if the oil doesn't help the compression, it means your valves are leaky. All of the cylinders are 45-55 psi??
 
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Re: Low compression

45-55 on all 8. They just seem too even. Usually seems there are one or two way off. It doesn't make much sense the engine would even run properly with it so low. I'm lost as to how to proceed with this. The gauge on the boat says the engine is 160 boat the temp gun the read over 220 at the engine manifold on one side.
 

captmello

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Re: Low compression

The block may get hotter than the water temp the gauge is measuring. Did you check the temp right where water temp sensor is located to check its accuracy?

Not sure what to say about the compression. Like Don mentioned, it seems to low to run or perform.

How does it perform? What RPM are you reaching at Wide open throttle (WOT)?
 
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Apr 23, 2011
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Re: Low compression

shot the thermostat housing with the gun and it was 160-165. no telling how accurate the gun is but that is what it read on the housing. Never ran the boat past 3400 rpm. It planes out nicely at 2600 and goes 22mph at that setting so I leave it there. Only had the boat a couple of months so I don't have much history with it. She starts right up, idles smoothly at 800rpm and doesn't hesitate when you give it throttle.
 

captmello

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Re: Low compression

Well, I don't know if you have a problem.

Give it a little throttle next time you're out. See if you can get up to 4000+ rpm. Do you know your prop size?
 
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Re: Low compression

I don't know the prop size. I"ll take a look tomorrow and guess at this point I'll go ahead and order the exhaust manifolds as they were in pretty badshape. Put it together and see how it goes. The compression thing just confuses me. I should get the manifolds on in a week or so and I'll post results. I'm committed at this point to make this thing work. Thanks for all your input. Really appreciate all your help.
 

captmello

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Re: Low compression

Check your impeller and outdrive if you just got the boat. Make sure everything is good to go. Maintenance is key. have it checked by a mechanic if you're not sure.

I look forward to you test results.:)
 

Don S

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Re: Low compression

Something is wrong with your gauge, or how you did the test. There is no way you will get that engine to run with 50 lbs of compression on all cylinders. You aren't holding the release button on the compression tester down while cranking are you?
 

paultjohnson

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Re: Low compression

Something is wrong with your gauge, or how you did the test. There is no way you will get that engine to run with 50 lbs of compression on all cylinders. You aren't holding the release button on the compression tester down while cranking are you?

LOL :facepalm:
 
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Re: Low compression

OK guys. Believe it or not the second compression gauge although new is defective. Put the gauge on my toyota truck and read 60psi. I know the truck is OK so problem is with the gauge. I'm doing the test by the book so I'm going to get the new manifolds and go boating. And yes I'll bring the new gauge back. Crazy stuff had me losing sleep. Thanks for all your help.
 
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