How to check compression on motors

[Valv]

You can check compression yourself or have a shop do it for you.

It's a very simple procedure, go to Walmart (or any auto part store) buy a compression tester for $ 30.00 (automotive section), take all plugs out, thread tester in and crank motor keeping WOT (Wide Open Throttle), repeat process for every cylinder and record the results, you don't want anything less than 10% difference in between. The compression can vary from motor to motor, you can see from 85lbs to 175lbs depending on motor and year, but below 85 it's very suspicious. Now, if you find a cylinder that is almost flat, try to pour a little of motor oil in the hole, then test again, if it reads much more than previously (almost like the others) you MIGHT have stuck ring(s), caused by carbon, they will loosen up when you start motor.

This is normal, but very uncommon, 95% of the cases you have a bad cylinder...walk away regardless of how good is the deal (unless it's free) or how good loooking is the rig.

[marine_man]

Great tip Valv!

marine_man

[LaXCarp]

Valv...Does the frequency of bad compression occur more with older motors or is it something that just seems to happen to motors of any year?

[Valv]

LaXCarp, I don't understand the question. Bad compression can be on any motor, the chances of finding it on an older is higher due to hours of use, but you can have a 2006 motor with 2000 hours and 2 flat cylinders.

Newer motor have an advantage point which is that electronics regulate fuel delivery, but use, abuse, negligence, etc are always there to "void" the advantage.

[clamtrap]

hey valv dothey still do compression test or do they do alot more leak down test? Thats what they did to one of my older boats that I had had thanks

[Austin]

Post deleted by Valv

[Valv]

Austin,

can you please post another thread by itself, not attached to this, I am trying to keep it as a "Sticky" on top of page.

Thanks