Pontoon Boat & Deck Boat Forum

So, I started up the new to me 2007 mercury 90hp 4 stroke and it started right up. No problems. Decided to pull the plugs and run a compression test. Plugs looked good. Compression test was 210psi on all 4 cylinders. What I think is weird is that I've checked two service manuals. One says minimum 125psi, the other says minimum 138psi for this engine. 210psi sure is a long ways from the minumum, and I can't find a maximum number. I guess I shouldn't complain or worry since it was consistent. Anybody had a test be that high above the minimum like that?

Not sure if this will help but, here's a link that also mentions a "leak down test."

http://www.marineengine.com/boat-forum/ ... p-4-Stroke

Could be your gage is out of calibration or you are reading the wrong scale.

[quote="illinoid"]Could be your gage is out of calibration or you are reading the wrong scale.[/quote]

That's what I'm thinking...

Although, the 454 V8 in my airboats compression ran from a high of 190 to a low of 170. Now, the cylinders were no doubt "washed" from a bad carb dumping so much gas that it was running rough and smoking, plus the lower readings were on the last couple of cylinders checked, and the batteries were pulled down a bit by the rime we got to them.

I'd be worried if the numbers were as low as the manual states, most larger displacement 4 stroke outboards should be at 170+. My DF140 tests out at 205-215 on a digital gauge. 125psi Minimum sounds like 2 stroke specs.

I know the gauge is good. I just used it on my 3 cylinder kubota lawn tractor and it was right at the specs. And I'm reading the right dial location. Sorry the pic is upside down. The gauge was hanging off the motor that way.

I did find there are two models of mercury 90hp outboard 4 strokes. One is a lower cc and mine is the higher cc. Like 1732cc and is the same as the 115hp and another model that was higher hp. But that model had the 138psi minimum. I wish they would list normal numbers instead of just a minimum.