Compression Test

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bvlmv

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Hi,

What is the proper way to perform a compression test on a 5.0 Gi ?

Thanks in advance!.
 
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skydiveD30571

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Re: Compression Test

Same as most any engine block. Remove spark plugs, unhook coil to prevent sparking, open throttle to WOT, make sure battery is fully charged.

Checking one cylinder at a time, screw compression tester into spark plug hole and turn the key so that the engine makes 5 complete cycles. Check your manual to make sure your numbers are within tolerance.
 
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skydiveD30571

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Re: Compression Test

Either. You just don't want all those plugs having high voltage current traveling thru them while they are out in the open.
 
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muc

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Re: Compression Test

I assume you have a 5.0GIPWTR
If you do, the correct procedure for disabling the ignition system is covered in the workshop manual and service bulletin 05-1-7 version 2.
The wires you should disconnect are at the coil (#22)

Fuel Injected - GM Throttle Body Injection
Remove both the (grey) 2-wire connector, with
purple and grey wires and the (black) 2-wire connector,
with pink and brown wires located at the
ignition coil.

If you have a different engine the recommended procedure might be different.
All spark plugs out.
Have the throttle all the way open.
Count the “puffs” on each cylinder – Volvo recommends 4 compression cycles.
After doing all 8 cylinders go back and redo the first cylinder to confirm you get the same reading --- this is to make sure your battery isn’t getting weak because the speed the engine is rotating at will affect your readings. That’s why Volvo doesn’t publish a PSI reading. They want the lowest number to be within 75% of the highest number.
 
achris

achris

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Re: Compression Test

BTW, warm engine....
 
bruceb58

bruceb58

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Re: Compression Test

You can just unplug your man overboard switch and it will also disable the ignition.
 
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bvlmv

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Re: Compression Test

If the man overboard switch you are referring to is the image attached how will I be able to turn the engine? The ignition is completely dead without this connected.b_27296B.jpg
 
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skydiveD30571

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Re: Compression Test

Yes that's what he is referring to.

You don't want ignition, and ignition isn't necessary for the crankshaft to turn. You just want the piston to travel thru a few cycles and build up air pressure inside the combustion chamber. No fuel or spark necessary for that. The starter will turn the engine, it just won't catch and actually start, which is what you want.

Plus, since you'll have every spark plug out of the engine, what good is ignition going to do anyway?
 
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muc

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Re: Compression Test

bruceb58 said:
You can just unplug your man overboard switch and it will also disable the ignition.
Click to expand...

NO
That only works on some engines.
There are ignition systems that will still have spark during cranking with the tether pulled. That's why when you don't fully understand the system your working on, it's best to follow the service - manual.
 
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skydiveD30571

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Re: Compression Test

Sometimes I really hate the way that iBoats automatically hyperlinks text. I hate taking away sales from iBoats, but OP, if you want a shop manual you can find it here. It is the OEM manual and not a cruddy Seloc manual.
 
bruceb58

bruceb58

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Re: Compression Test

muc said:
NO
That only works on some engines.
There are ignition systems that will still have spark during cranking with the tether pulled. That's why when you don't fully understand the system your working on, it's best to follow the service - manual.
Click to expand...
So you are saying the OP's engine will still have its ignition operable with the man overboard lanyard switch open? It gets powered through the cranking circuit as well?

Here is the Volvo doc you referenced in your other thread:
http://marinemechanic.com/volvo/volvo/2010/05-1-7_A[1]disableignition.pdf
 
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muc

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Re: Compression Test

bruceb58 said:
So you are saying the OP's engine will still have its ignition operable with the man overboard lanyard switch open? It gets powered through the cranking circuit as well?

Here is the Volvo doc you referenced in your other thread:
http://marinemechanic.com/volvo/volvo/2010/05-1-7_A[1]disableignition.pdf
Click to expand...

No I’m not saying that. The OP never posted what engine he has.
But the fact is that some ignition systems will still have spark with the tether cord pulled. That’s why none of the engine builders recommend pulling the tether cord to disable the ignition during a compression test. Will it work on some engines? Yes it will, but why take the chance? When it only takes one spark to ruin your day.

Thank you for posting the link to the service bulletin.
 
achris

achris

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Re: Compression Test

If you don't want loose sparks flying around, just pull the centre wire from the distributor cap and lay it on the engine block... It's easy and fool-proof.... EOD...
 
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muc

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Re: Compression Test

achris said:
If you don't want loose sparks flying around, just pull the centre wire from the distributor cap and lay it on the engine block... It's easy and fool-proof.... EOD...
Click to expand...

The OP asked the “proper way” to do a compression test.
Thank you for pointing out I missed the warm engine part.
But I do disagree with your fool-proof method. A lot of coil wires have the terminal recessed into the boot by at least ?”. That means you will have at least a ?” spark jumping. Also just laying it on the block could allow it to move around and maybe create a bigger gap for the spark to jump. With your method the fuel injectors will still be putting out fuel. The easy way isn’t always the best way.

Why take chances? The engine manufactures are more than happy to tell us the right way to do the job.

P.S. a lot of the engines I work on don’t have a distributor; they have one coil per cylinder.
 
achris

achris

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Re: Compression Test

muc said:
The OP asked the “proper way” to do a compression test.
Thank you for pointing out I missed the warm engine part.
But I do disagree with your fool-proof method. A lot of coil wires have the terminal recessed into the boot by at least ?”. That means you will have at least a ?” spark jumping. Also just laying it on the block could allow it to move around and maybe create a bigger gap for the spark to jump. With your method the fuel injectors will still be putting out fuel. The easy way isn’t always the best way.

Why take chances? The engine manufactures are more than happy to tell us the right way to do the job.

P.S. a lot of the engines I work on don’t have a distributor; they have one coil per cylinder.
Click to expand...

:facepalm: I completely forgot it's an EFI engines.. Man, that was 5 days ago, and I think I've answered about 140 post since... I gotta get out more....

As per the book...

I would like to add one comment about fuel. If it was a carbed engine, it would still be pulling fuel in... (and that can't be stopped ;))
 
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muc

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Re: Compression Test

achris said:
I would like to add one comment about fuel. If it was a carbed engine, it would still be pulling fuel in... (and that can't be stopped ;))
Click to expand...

I don’t believe that to be true.

The reason for setting the throttle to wide open and blocking the choke plate wide open is that that’s what puts the carb into “clear flood” mode. So no or very little fuel will be going to the cylinders.

If you try doing a compression test with the throttle closed and also with the throttle open you will see that the PSI numbers don’t really change.
 
Fun Times

Fun Times

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Re: Compression Test

Well since we're describing the proper way to do this in full detail, it seems that some of you amateurs:p:D have forgotten to mention that it would be best to have the stern drive lowered down along with water running to the engine so that you will help avoid damaging the sea water pump impeller while turning the engine over so many times.;) Or remove the belt/s on some engines.

Just messing with you guys. :)

Thanks Muc for sharing your professional experience here on iboat for so long, it is truly appreciated. And by the way, congratulations for your success with becoming an AMTECH Technical Specialist, Nice job.:thumb:

To Bruce and everyone else that contributes their knowledge here, the thanks is extended out to all of you as well.:)
 
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