compression test question

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alaskanassassin

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I am ready to test the compression on my 200hp merc. I have read how to do it and have disconnected my fuel line, and pulled my plugs. I understand that I have to disable my ignition. Can I just pull my emergency kill switch lanyard off my controls? Doesn't that kill the ignition? I don't want to fry any of my electrical system so any help is appreciated! It is a 2004 2 stroke carbed v-6 200hp Merc. Thanks-AA
 
H

huntin dawg

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Re: compression test question

alaskanassassin said:
I am ready to test the compression on my 200hp merc. I have read how to do it and have disconnected my fuel line, and pulled my plugs. I understand that I have to disable my ignition. Can I just pull my emergency kill switch lanyard off my controls? Doesn't that kill the ignition? I don't want to fry any of my electrical system so any help is appreciated! It is a 2004 2 stroke carbed v-6 200hp Merc. Thanks-AA
Click to expand...

I don't see why you have to kill the ignition but pulling the emergency kill will do it.

Remove all plugs. Open the throttle wide open to allow unrestricted airflow. Crank it 3 or 4 times on each cylinder. The compression guage should top out and not advance any further once you have reached the max PSI for the cylinder you are testing. All cylinders should be within 10% of each other. Actual readings are not as important as consistancy.

HD
 
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alaskanassassin

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Re: compression test question

Thanks for the reply, I read on here that you have to disable the ignition or ground all your plug wires or damage can occur to your ignition system. I will try it with the kill switch on unless someone tells me I shouldn't.
 
JB

JB

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Re: compression test question

With all plugs out when you turn it over to get a reading on one cylinder the rest of the cylinders are pumping fuel/air out of the spark plug holes.

Do you think you might want to add a spark to that mix?

The best way to avoid spark is to have the key OFF and jump the starter as described here:

http://forums.iboats.com/showthread.php?t=391323
 
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sschefer

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Re: compression test question

Plugs out with the plug wires GROUNDED !!!!!!!. Unless you like buying switch boxes.
 
saumon

saumon

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Re: compression test question

sschefer said:
Plugs out with the plug wires GROUNDED !!!!!!!. Unless you like buying switch boxes.
Click to expand...

I always tought that when you flip the kill switch there's no need to ground the sparkplug wires as it disable the ignition system...That was his original question i think: can he use the kill switch instead of grouding the plug wires? For me, that's what i've always done.
 
A

alaskanassassin

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Re: compression test question

I ended up using the kill switch and grounded the plugs. I had my buddy hold a plug wire with the kill switch engaged, and he didn't jump so I assume I can just use the kill switch next time. Thank you for all the replies! My compression was very strong 122-125 all cylinders, please check out my other thread and help me diagnose my problem. Title of the other thread is "now what"?
 
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sschefer

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Re: compression test question

alaskanassassin said:
I ended up using the kill switch and grounded the plugs. I had my buddy hold a plug wire with the kill switch engaged, and he didn't jump so I assume I can just use the kill switch next time. Thank you for all the replies! My compression was very strong 122-125 all cylinders, please check out my other thread and help me diagnose my problem. Title of the other thread is "now what"?
Click to expand...

yeah you actually only need to ground the plugs when you're checking the timing at cranking speed. I just got into the habit so I don't forget and accidently blow a switch box when a coil over builds the capacitor.
 
JB

JB

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Re: compression test question

sschefer said:
Plugs out with the plug wires GROUNDED !!!!!!!. Unless you like buying switch boxes.
Click to expand...

So you are saying that it is necessary to ground the plug wires when there is no attempt to make spark, like with the KEY OFF??

Please explain.
 
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sschefer

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Re: compression test question

JB said:
So you are saying that it is necessary to ground the plug wires when there is no attempt to make spark, like with the KEY OFF??

Please explain.
Click to expand...

To answer your question, No, not technically speaking no but just plain old no.:)

The reason I do it is because its a good habit to get into. I rarely just do a compression test. Its almost always followed by a spark test and I usually pull the harness connector and isolate the engine so I'm not trouble shooting the boat and the motor at the same time. It's mainly to protect me against my absent mindedness and old age.
 
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