CONTINUOUSWAVE

I'd like to perform a compression test on Mercury 90 FOURSTROKE. The procedure seems straight forward. Please let me know if I have it correct.

• Bring engine to operating temperature (some say cold is also fine)
• Remove all four plugs
• Disable fuel delivery fuse
• Install gauge
• Crank for three to fives seconds while in neutral and wide open throttle
• Repeat for each cylinder
• Each cylinder should be consistent or no greater or no less less than 10%

Where is the specification for a Mercury 90 FOURSTROKE?

What is an acceptable range?

Thanks for any input.

For advice on how to perform a measurement of peak cylinder pressure see my article in REFERENCE at

Assessing Used Outboard Engines
http://continuouswave.com/whaler/reference/outboardCheck.html
and see the section under the subheading Compression Check.

bmat5 wrote:Where is the specification for a Mercury 90 FOURSTROKE?

The specification for the normal peak cylinder pressure is not normally provided, but if it were provided it would probably be stated in the OEM service manual. If you plan to perform service on an outboard engine, a good start on that project is to obtain the OEM service manual.

bmat5 wrote:What is an acceptable range?

If you mean what variation among measured peak cylinder pressure is usually considered normal, the normal variation among cylinders for measured peak pressure is usually not more than 5-PSI to perhaps at most 10-PSI for the range of ALL cylinders between highest measured pressure and lowest measured pressure.

Check the OEM service manual because in some engines there is an intentional design that results in some variation among cylinders that might be greater than 5 to 10-PSI.

Again, my thanks Jim.

A Yamaha technician informed me that on a current model F300 you are fine with a range of [variation among cylinders of up to] 10%. He was working on a 2014 model year when he informed of this range.

[Moderator's Note: I had to re-write this post because the original violated several forum rules.]

Where on-line can be found a resource for [an engine described as a Mercury 90 FOURSTROKE]?

I'm trying to determine an acceptable compression PSI [measured value for peak cylinder compression without combustion using a sensor threaded into the spark plug hole].

Hello BMAT5--most outboard engine manufacturers do NOT publish on-line for free downloading their factory OEM SERVICE MANUAL, which is where a specification for peak cylinder pressure measured without combustion in the cylinder using a pressure sensor inserted into the spark plug threaded boss would be likely to be specified

You notion that you can find this information on-line is probably too optimistic.

If you want to SEARCH for information on-line that is not part of the information on this website, I recommend you use the BING search engine instead of posting here and asking readers to search for you.

Perhaps you can contact Mercury customer support and ask them for this information.

Perhaps you can contact your local Mercury dealer that has a factory-trained service technicians and ask them for advice on the range of compression in your specific engine.

By the way, so far the engine has only been identified as a "Mercury 90 FOURSTROKE." That description might encompass several different models.