compression test procedure.

[Reaction19]

was wondering if you guys do a comp test with engine dead cold or if you warm it up to operating temp then do it.

i know it would be different readings either way.

also do you do it with the throttle wide open or closed and would that make a difference.

also would running a cleaning aid like power tune make a difference in cleaning pistons and help rid the rings and stuff of oil buildup.

[Jeff_G]

I run the engine no warm it to operating temp, that distributed oil so you aren't doing a dry run and the pistons and block will reach their running shape. I don't bother opening the throttle at all.
Yes powertune make a difference it can free up stuck rings.

[Jay Smith]

IMO : The manner in which you do your compression test really doesn't matter as long as you do it the SAME way EVERY TIME.. Having the motor warm and the throttle open may alter the end results slightly but as I said how ever way you choose to do the test do it the exact same way to get a benchmark result. I do my compression tests all plugs out throttle open and ignition disabled but everybody has their own "cat skinning" ways. I use compression tests as a quick comparison IMO the best way to test bore/ring health is a leak down test..

I use and suggest Sea Foam as a regular "diet" to all my customers. I've found it to keep injectors serviceable longer and the carbon build up to a minimum on piston crowns.

my .02
Jay

[Dingie 2]

Yamaha compression test numbers are done on a warm motor/ all plugs out and grounded/ throttle in WOT position / you may have a lower compression reading with the throttle closed. The important thing is to have a strong battery and make sure that the cylinders are close not far off of factory specs. Caution read the manual and check the specs for your motor, some model yamaha 's have lower compression readings on the lower cylinders to keep those cylinders cooler. Jay's correct about taking notes and doing it the same exact way every time with the same meter that way you will know if anything changed. Joe R

[Reaction19]

mines a 3.0l merc w/ notches in the exhaust ports has cross hatches in the cyl's but im getting low numbers on my new snapon tester. 100 95 105 115... motor was dead cold when this was done i thought mybe the clearance were loose and a warmup might do the trick. also i thought maybe had some rings sticking due to alot of idle time and slow 3800rpm cruising. motor doesnt have any piston slap while running cold and all seems good just wondering. how would i do a good cleaning of the internals with seafoam.

[Jay Smith]

The Sea Foam dosage suggestions is 1 pt. treats 8 to 25 gals.If you feel you have a carbon build up and or a ring sticking issue I'd go with the stronger ratio ( 1 pt. of Sea Foam to 8 gals. ) I also get word from Linda Brucato that the injectors they service seem to give longer service with the addition of a small constant dosage say 1 oz to 5 gallons. When I run engines after a rebuild I have made it a point to run the engines with Stabil and Sea Foam added to the fuel oil mix. This gives the fuel system an amount of protection if the engines aren't installed and or ran trough the winter months..

Your gauge results could also be suspect. No matter who the manufacture is ( Snap-On,Mac, Proto,Sears, etc thay ALL vary and sometimes GREATLY) Also your altitude makes a BIG difference in the results , the higher the altitude from sea level the lower the numbers will be...

Good luck,
Have a better 09,
Jay

[Reaction19]

you think it could be the engine just has alot of clearance from the factory.

[Jay Smith]

Dunno what the factory compression specs were when the motor was new. Some of the engines were exhaust relief slotted style engines so they could be safe to run the lower octane gasolines and they showed less cranking compression. You might invest in a factory manual or inquire from a helpfull dealer and find out their target numbers on acceptable compression on your model.. BUT to find out the real health invest in a leak down tester. For most recreational engine 8 to 10% is runnable the more the leak down % are the more usable HP is left on the table. A FRESH rebuild should yield only 2% leak down. You could take that into consideration when you get your results.

Good luck,
Have a better 09,
Jay

[YELLOWSS]

i agree with jay.... that is how i do mine.....
MAKE SURE THE BATTERY IS FULLY CHARGED EVERY TIME!!!!!!!!!!!!!!!!!!!!!!!!! otherwise you will have false numbers.. a battery a tad low will give you lower numbers!!!!!!!!!!!!!!!!!

[150aintenuff]

also make sure there is no moisture build up in your tester if it sits in a un heatted garrage.. I had a bit of moisture in my gauge yesterday from non use and it altered the reading by almost 100PSI...(245psi on a stock XR6 block with prior readings of 110-115)