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Author Topic:   Compression Test
jimh posted 12-15-2001 10:36 AM ET (US)   Profile for jimh   Send Email to jimh  
The condition of older outboard engines is often evaluated by performing a compression test. This procedure is often mentioned and the test results are often discussed, but the actual technique of performing the measurement has not been well documented.

Several questions come to mind:

--do you crank the engine by hand or by electrical starter?

--do you have normal fuel flow or crank dry?

--do you suppress ignition sparking with the KILL switch?

--do you test a cold engine, a warm engine or a hot engine?


Advice from the outboard engine mechanics among our readers is welcomed.

JBCornwell posted 12-15-2001 11:30 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Some of the variables you mention, Jim, can make a significant difference in compression readings. In addition, throttle setting can make a difference.

Different manuals state different conditions for compression testing, but the most common are: Warm engine, ignition disabled, throttle wide open, all plugs removed, crank with starter. Run through at least five revs per cylinder.

However the conditions stated may differ, almost all manufacturers seem to agree that cylinders should not differ by more than 10%, and that range is more important than absolute readings.

Red sky at night. . .
JB :)

bigz posted 12-15-2001 12:40 PM ET (US)     Profile for bigz    
Answers to your questions JimH,

1. Either way is fine. 4 or 5 pulls per cylinder, or 5 to 8 seconds with electric starter make sure equal cranks for all cylinders

2. Normal fuel flow, as much as you can open the throttle in neutral

3. Yes use kill switch if supplied, or disconnect gang plug from ignition module, or if neither available ground each spark plug wire with a clip to the engine. Don't need any stray sparks with the fuel mixer being sprayed into the open cylinders which might exit the open spark plug hole

4. Done on a cold one or slightly warm --- probably burn yourself on a hot engine

lhg posted 12-15-2001 05:40 PM ET (US)     Profile for lhg    
This is less scientific, just what I have observed when a Mercury mechanic was checking out one of my V-6's. Nothing was done at the engine controls, simply left in Neutral, Idle. He removed all of the spark plugs, and plugged in (somehwere on the engine!)a tester unit about the size of a TV remote control.
The compression testing device screwed into the spark plug hole. He pushed a button on the remote control to operate the starter, crank engine and read compression. I assume ignition also disabled by this tester device.

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