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Discussion Starter · #1 ·
Hey all, I've recently noticed my sled (mxz adrenaline 600 ho sdi) had a different sound and wasn't working as well as it normally did so I decided to do a compression test on it, I did one about a year a ago with 140ish on both cylinders, this time my mag side was 142 psi but my pto side puzzled me with 220 psi, I checked them both 3 times with the same numbers, what the heck is going on here?
 

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Discussion Starter · #3 ·
I don't think so, my dad lives next door to me and I tried in on his sled (05 renegade 600ho sdi) and his showed 121 and 120 on both cylinders.
 

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With 220 in one cylinder it would have blown up real quick or blow out gaskets. Or not ran at all. You could have gotten oil in your tester. Clean the hose out, pull plugs out, pull engine over a few times to clear the cylinder and re test.
 

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Thats very strange and I don't see the gauge being the problem if you alternate between cylinders and the numbers stay the same. Also your dad should do a top end with those numbers. Do you have another gauge to test with just to verify?
 
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Like Burtamiss said, if the gauge reads good on the other cylinder and on another sled, I would focus on the high compression cylinder and not the gauge. What could increase the compression? For a performance sled this could be done by filling the head with weld and reshaping the combustion chamber for less volume to increase the Compression Ratio. Another way would be to lower the exhaust port height. Obviously no one welded the head or did any porting, so what else could have happen that would increase the compression?
 

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Oil pump stuck open?
 

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You said the sled runs and sounds different than previous. How about an exhaust valve stuck closed?

Time to take the head off and have a peak.
 

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Exhaust valve stuck open would lower the compression results dramatically. What colour is the smoke, black, blue or white?
 

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Discussion Starter · #14 ·
Smoke is blue, it kinda seems like it shoots an oil mist out of the spark plug hole, enough that it will leave oil on your thumb if you put it over the hole
 

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But what happens if it is stuck closed?
I think that I know what you are leading to with higher rpm, but for the sake of a compression test the rpm is kept very low somewhere around 250 rpm. With the valves closed it gives the right amount of psi for the common compression test.

I once had the engine out of the bay for a tear-down when I realized that I had forgotten to take a compression test. So I put the engine back in the bay with a buddy of mine to do the compression test. We started with no throttle bodies and the valves wide open. After seeing the compression was 140 psi which was far lower than what we had previously measured, we spend an hour mucking around trying this or that to see what affected the compression readings. We got varied results, but the most outstanding was 185 psi which was 20 psi better than the highest reading of 165 we had ever read. This was all with the same gauge. The other gauge we had read in the 175+ range was still an astonishing 20 psi over what the same gauge had read before. But on this gauge we could see the needle reach 180+ readings, it just didn't register (keep) this value as the highest reading. These are some of the reasons you never see me harp on different gauges because there is just too many variables. The same gauge on both cylinder should be accurate.
 

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Discussion Starter · #17 ·
So I spoke with a knowledgeable sled mechanic and he is pretty sure that the oil check valve in the base is faulty and its letting it fill with oil. I am going to replace it and see if it changes anything
 

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If that is the case then you can measure the oil level at the bottom of the crank chamber with a clear hose, or any clean hose. Remove the throttle bodies from the reed cage and tie them up, then remove the reed cage and push the hose in between the crank web (crank balancers) and pull out to get the level. I would then use a Mityvac to siphon all of the fluid and inspect to ensure it is indeed oil and not coolant or both. The tuned pipe can also be checked for fluids as it would run down the exhaust port when cranking. The engine will likely need to be taken out for service, but doing the most amount of verification with the engine in the sled will give a better chance at finding the problem. This way if you need to double-check your findings you can simply put the reed cage back in and connect the throttle bodies to run the sled then recheck.
 

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I had this issue in the past and it ended up being an inner crank seal which allowed the oil from the inner water pump cavity to leak into the cylinder which made high compression. I also agree that it could just be the check valve. I ran mine for a while like that but as it sat for a long time in the summer oil would actually start filling the exhaust pipe.
 
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