Hi all, just did some testing on my 2015 800EX. Have a lot of oil in the air box. Compression on cylinder 1 was about 60psi, cylinder 2 was 120psi. So got a leak down tester. At 90 psi I can't build more than 20psi in either cylinder. I can hear air escaping, but can't find it. No bubbles in radiator, used a stethoscope to try to locate the culprit area, but I just could not find anything.

Motor starts and runs fine. No power issues, goes good. I have been up to 60mph with it and ran out of room before running out of throttle. Looking for any help I can get on this, because I am stumped at this point.

I bought this thing for cheap money last week. Prior owner was going to trade it in but dealer said it needed top end rebuild due to exhaust smoke. I have not seen any smoke. It does get quite a bit of oil in the airbox quickly, so I assume this is what they were talking about.

How hard are these things to rebuild? I am mechanically inclined, but don't want to fight with it if I need a bunch of special tools and such. For the price I gave, I can pay the shop to do it and still be happy with the investment. But it's always nice to DIY and put that labor $$ towards other toys!



Chris

[2015 Z Force 800 53/Trail EPS]

Hi all, just did some testing on my 2015 800EX. Have a lot of oil in the air box. Compression on cylinder 1 was about 60psi, cylinder 2 was 120psi. So got a leak down tester. At 90 psi I can't build more than 20psi in either cylinder. I can hear air escaping, but can't find it. No bubbles in radiator, used a stethoscope to try to locate the culprit area, but I just could not find anything.

Motor starts and runs fine. No power issues, goes good. I have been up to 60mph with it and ran out of room before running out of throttle. Looking for any help I can get on this, because I am stumped at this point.

I bought this thing for cheap money last week. Prior owner was going to trade it in but dealer said it needed top end rebuild due to exhaust smoke. I have not seen any smoke. It does get quite a bit of oil in the airbox quickly, so I assume this is what they were talking about.

How hard are these things to rebuild? I am mechanically inclined, but don't want to fight with it if I need a bunch of special tools and such. For the price I gave, I can pay the shop to do it and still be happy with the investment. But it's always nice to DIY and put that labor $$ towards other toys!



Chris

Well, the fact that it has half the compression in one cylinder over the other says there is a problem. There are only three basic places it can go- Through the rings, through the intake valves or through the exhaust valves. Of course there could be a blown head gasket or cracked piston or head. As it is excessive blow-by that pushed oil and mist into the air box I tend to think rings on the low cylinder are broken or warn prematurely somehow. Now I'm not suggesting you can't rebuild it yourself but..with all the machine work and other things to consider, it might be well worth your time and money to let a good shop do it. And being a CFM, I also suggest doing the bottom end as well because.. if it was heat that killed the top end, it usually does the rod bearings a number as well on these. Just a suggestion if you want to save some money and do the R&R yourself, talk to nFlow about having them do it. It will be right..in fact better then factory, warrantied and they will pay the shipping back. Their website doesn't show they do CFM engines...but they do.

Thanks for the quick reply. Don't think it was heat, more likely the STOCK AIR FILTER that is still on this thing, haha! Obviously the prior owner was not a member here.... I just bought it last week. Just now ordered the Uni filter, oil, and an outerwear pre-filter. At least I'll be set once I figure out what I want to do with this thing.

The thing that gets me is it runs like a top. Starts quick, idles fine, plenty of power. If it wasn't for this site, I'd never know there was an issue to be resolved. Did an oil change, just because I am unsure when it was last done. Now I just need to get this oil in airbox issue figured out.

The compression being so low on one cylinder yet still running smoothly has got me scratching my head. Both plugs looked decent, light tan color.

I am tossing together a parts list to see where I might be price wise. I would love to just go the NFlow route, but not sure if  I want to drop $3k on an engine. Dealer quoted $1500 to rebuild top end when the PO brought it in.

I'll have to see if I can locate a bore scope and take a look in the cylinders. Maybe I'll see something that helps swing my decision one way or the other.

One thing I want to add - As I did the leak down test, I sprayed a soapy water all over everything I could get to. Figured if I had a bad head gasket or a crack I would see some bubbles. Nothing found. Also, with 90psi on the cylinder, I rotated the engine by hand slowly. I was able to hear when different valves opened and I could feel the air coming from the exhaust. Not real sure, but I think it may have still been blowing out of the exhaust a little on the compression stroke. I need to redo this test tomorrow and see if I can pin point what is going on.

First, how many miles on this machine?

Second, was the compression test done on a warm or cold engine?

As for leakdown testing, You've got 4 places the air can go. Piston at top dead center and valves closed, apply air pressure and listen to the intake and exhaust ports separately for the leak. If none, top end is fine. Listen at the oil fill hole (might have to remove enough oil to uncover the bottom of the hole). If none, rings and piston are fine. That leaves the head gasket or cracked head.

1400 miles on it.

Thinking about it overnight, I ran the engine to warm it up, but likely cooled down too much while removing panels and belt cover. Needed to check belt and clutch anyway. Going to try again later, removing what I can ahead of time.

So tested again. Had the kids putt around to get good and warm.

Front cylinder has 135psi compression, holds 40psi at 50psi with leak down tester.

Rear cylinder has 120psi compression, holds about 30psi at 50psi with leak down tester.

Using a stethoscope. Hear a little air on the exhaust side of things. But would that allow oil to reach the airbox?

[2015 Z Force 800 53/Trail EPS]

So tested again. Had the kids putt around to get good and warm.

Front cylinder has 135psi compression, holds 40psi at 50psi with leak down tester.

Rear cylinder has 120psi compression, holds about 30psi at 50psi with leak down tester.

Using a stethoscope. Hear a little air on the exhaust side of things. But would that allow oil to reach the airbox?

There is only three things that allows oil and excessive oil vapor to get to the air box. 1-excessive blow by. Normal from warn leaking rings. 2- excessive vacuum on the air box. Normally from no one cleaning the bag prefilter over the intake inlet on a stock setup. and 3- oil levels way overfilled. Oil level is checked after running long enough to re-fill cooler with stick NOT screwed in.

If it were me I would ensure all valve lashes are correct. Do this cold...ie not run in over 6 hours. Then run it until normal operating temp, then pull the crankcase vent tube off the airbox and see how much its flowing at about 2K rpm. Perfect would be only a slight out-flow and a pulsing as the pistons move through their strokes. Might also see if there is much oil in the tube. If you do still have the stock setup on intake, pull the grill, pull the prefilter and clean it..if it hasn't been done lately.

Will check valve settings when I get more time. Headed to Vermont in the morn to look at a zforce 1000.

Here's something, I have oil on the crank sensor. That shouldn't be there, should it?

Cleaned useless prefilter. Can't wait to get the Uni on it.

Whole lotta air coming from the crankcase tube. Am I correct in thinking that is a sure sign of bad rings? Or possibly piston failure?

[2015 Z Force 800 53/Trail EPS]

Cleaned useless prefilter. Can't wait to get the Uni on it.

Whole lotta air coming from the crankcase tube. Am I correct in thinking that is a sure sign of bad rings? Or possibly piston failure?

Well..even at best rings don't seal completely. If nothing else the ring gaps see to that. There will always be a little..but if it's enough to allow oil and oil vapor to travel to the air box..then yeah there is something happening. Blow-by is what gets by or through the rings during compression and detonation. Warn rings are not only more rounded, but the gaps are also larger. Also abrasive stuff like dirt and dust that gets by the air filter rips scratches in the cylinder walls that makes for leaks too. You can also get a little blow-by from bad exhaust valve guides and seals..not so much from the intake valve-side cause there's no pressure on that side of the valves when shut. Yeah a cracked piston or ring can also do it but your tests are close enough warm to indicate it just wearing rings..IMO. 

NWK, after the time I have spent combing this forum I trust your opinion fully,  haha!!

So I see you highly recommend nFlow for reman engines. Do you know what their turn around typically is? Do they actually reman MY engine and ship it back? Or do I get one they have done and ship mine in as a "Core"? This is looking more tempting as I go, just for piece of mind and simplicity. Just don't want to be sidelined for months if I can avoid it.

[2015 Z Force 800 53/Trail EPS]

NWK, after the time I have spent combing this forum I trust your opinion fully,  haha!!

So I see you highly recommend nFlow for reman engines. Do you know what their turn around typically is? Do they actually reman MY engine and ship it back? Or do I get one they have done and ship mine in as a "Core"? This is looking more tempting as I go, just for piece of mind and simplicity. Just don't want to be sidelined for months if I can avoid it.

They have done engines for me in the past and have always be excellent. Same for others I know. For CF Moto engines I'm sure you will get your same engine back. They are very fast too but know it will not leave the shop unless it's right and if it takes a little longer to get some special parts..that's the way it is. Just call them. They will answer all your questions. Great people.