Thread: Compression test question...
05-03-2008, 01:22 PM #1
Compression test question...
Ive heard two different things, when doing a compression test are you supposed to take out all spark plugs or just the cylinder your working on?! I did one today and the results where WAY lower than they were last fall. 125, 100, 90, im assuming i did it right? Last year its 125, 120, 125, would sitting over the winter cause any issues or do it need to run a bit first?
05-03-2008, 01:49 PM #2
- Join Date
- Jul 2007
- near Toronto, Canada
If you fogged the engine, then the fogging oil can affect the readings until it gets burned off.
Also, if the battery is weak, then as you test the cylinders, it cranks more slowly, so the last reading can be lower than the first, even though the cylinders are actually in the same condition.
Remember to hold the throttle wide open, to minimize the load on the starter+battery, AND to maximize consistent compression readings.
If there was moisture in the engine/exhaust, or the rings are just sticky in the pistons, then you could have corrosion or other things that are affecting the readings, that wasn't a factor last Autumn.
Last edited by K447; 05-04-2008 at 11:22 AM. Reason: My practice is plugs out, well... sometimes
05-03-2008, 02:16 PM #3
Last edited by 32DegH2O; 05-03-2008 at 02:19 PM.
05-03-2008, 02:22 PM #4
- Join Date
- Sep 2006
i leave em in its says in all the compression tester instructions ,
now difference between a cold and hot engine can be tricky ,
what i would do if what happened to you happened to me i would mix fogging oil with a little bit of 2 cly oil and poor a little bit in the clys , place your thumb over the top , sheild your eyes then crank it ,
lube the walls and rings then do another test ,
05-03-2008, 03:10 PM #5
Yeah, i fogged it last fall, and today was the first time its been started since last October.... Battery is fully charged, same battery thats been sitting on a trickle charger over the winter...Ill try it again later with plugs in and see what numbers i get...
05-03-2008, 04:08 PM #6
- Join Date
- Oct 2006
- Cleveland OH
Make sure the engine is warmed up. Cold engine will read lower compression. Oil in the cyl will give false readings as it will fill in the voids and increase compression. Not an accurate reading IMO.
05-03-2008, 05:39 PM #7
05-03-2008, 06:38 PM #8
For what you're doing it doesn't matter a rip which testing method you use as long as the cylinders are within 10 percent of one another.
Since you have a differential, warm the ski, chose your compression testing method and this time check the compression in the opposite direction.
If your within 10 percent, call it a day. If you find that one cylinder is still low, perform a leak down and chcek piston wash. If they look good and the cylinder is still low, take it for a short ride and check it again. Careful with WOT runs as it may be a lean condition that was occuring at the end of your last ride. If you STILL have a low compression number pull the domes and check for signs of detonation and/or cylinder wear.
Keep us posted.
05-03-2008, 06:42 PM #9
also stay consistent when testing. If you are going to test hot, always test hot. if you are going to test cold, always test cold. Numbers can vary from hot to cold.
05-04-2008, 10:42 AM #10
I'm sorry to contradict(spelling?) anyone.
The Polaris manual recommends removing all spark plugs,using a good battery.Hold throttle wide open when cranking.
In my experience I like to have a battery charger attached if checking compression several times or on 8 cylinders to guarantee even battery charge/consistent cranking RPM.But that is splitting hairs.
The reason to remove all plugs,If 1 cylinder has more compression it will slow the cranking rpm's(lower the reading on other cyl) if 1 cyl has less comp,it will raise rpm,s(raise reading on other cyl)
Being consistant is the most important thing.
The amount and type of oil in cyl will affect readings.
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