Compression Testing Questions

glockdoc

Cadet
Joined
May 29, 2007
Messages
8
Newbie 3rd Class questions:

Where can I buy a compression guage set? I am looking for the type with a hose and connector for the spark plug thread, and not the press-on type.

What is the correct procedure? Do you remove all the spark plugs or leave them in place and check the cylinders one at a time? Seems like it might make a difference whether or not the other cylinders were open vs closed up and compressing... Maybe it does not matter at all, I don't know...

DO you check compression by just turning over the engine with the starter?

What pressure range is acceptable for a 2000 Johnrude 175? I understand that they should be very close to the same pressures on all cylinders, but how far can it vary and still be acceptable?

Thanks in advance.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,226
Re: Compression Testing Questions

Auto parts stores have gages. Take all the plugs out. Crank with the starter till you achieve the highest reading.

It will start an argument to say this but it is NOT necessary to block the throttle open on a two-stroke. It is on a four-stroke. But it won't hurt anything to block them open if you insist.

Usual 10% difference between cylinders is customary.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,226
Re: Compression Testing Questions

Forgot to mention this important thing: Prevent sparking at the plug wires. There are several ways to do this. One easy one is make up a set of wires with alligator clips on both ends. clip one end into the boot and ground the other end.

Sparking can start a fire. Just ask me.
 

BoatBuoy

Rear Admiral
Joined
May 29, 2004
Messages
4,856
Re: Compression Testing Questions

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92697

92697.gif
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Compression Testing Questions

If you operate the starter with a jumper on the solenoid with the key OFF sparking will not be a concern.

It is best to use a fully charged battery and warm the engine to operating temp before taking readings. Cold engines will read lower and engines not run for a long time even lower than that.

To get useful comparisons between cylinders turn each one the same number of strokes. 5 or more will do it.

Absolute readings are not a big concern as long as they read over about 90psi on that big V6. What is important is the range for all 6.
 
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