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2004 Mule 550, compression loss overnight!!


rmissildine
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Hello all.  I'm new to the forum and have read all the posts I could find concerning the 550 Mule, so far no one with my problem. So, I'll give it a go and hopefully get an answer.

I got this mule about three years ago, had to do some minor repairs, and it's been a great machine. Last fall, I noticed an oil leak coming from the left side around the crankcase cover.  I was able to slow the leak considerably, but it still leaked.  I never had the time to tear it down due to work. I retire in March of this year and decided to fix it with a new cover gasket. I ordered a new gasket and crankshaft seal and while waiting for them to arrive, I began the tear down and cleaning of the mating surfaces.  Got the seal and gasket installed and the cover back on and torqued. Decided to crank it up and let it idle a bit to check for any oil leaks before I put the converters back on. Fired right up, let it idle for about 10 minutes, no leaks. I'm happy at this point...

Next day, installed the inside converter cover, new belt and converters, not the outside cover though. I wanted to see how the new belt was going to work out. I still had the rear end jacked up so it wouldn't move.  Guess what, it wouldn't start up. Cranked over like always, just wouldn't start... Now I'm bummed. So I start checking gas, spark and everything is good. Even tried a squirt of starting fluid, and it did fire a couple of times but no start. Now I'm even more bummed. Time for a break.

Now I've worked on everything from small engines to tractors and have never had this problem.  After a little thought I decided to run a compression test. The repair manual lists 115 - 178 psi as the correct range, I had 30 psi!!!!! What in the world happened over night. I did spray a light oil into the cylinder and it came up to about 60 psi but that was all I could get. I checked the valve clearances, their good.

Sorry for the long post, but I hope someone can point me in the right direction cause I really like this little machine.

Thanks for your help,

Roger

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Thanks for the replies Travis. To answer your questions, yes the valves are operating properly and the plug is tight.  Yesterday afternoon, I made up a mixture of a little bit of new oil a little bit of diesel and a little bit of gas, all equal parts approximately. rolled the piston to bdc and squirted the mixture in with a syringe.  Screwed the plug back in and spun it over a few times to coat the cylinder walls and then attempted to start her up. It did fire several times but wouldn't start up.  So I bit the bullet and pulled the head off. First I inspected the valves, nothing out of the ordinary, just a bit of carbon on the intake valve. So, I screwed the plug back in, set the head upside down in my vice and poured in enough non areosol WD 40 and let it set while I inspected the cylinder.  The cylinder looked real good, I could even see some of the original hone marks. Rolled the piston all the way down, the entire cylinder looked good from top to bottom, no scratches or gouges.

I went back to inspect the head as it had been a good thirty minutes and everything looks good. Combustion chamber still full of fluid and the intake and exhaust ports are dry as a bone...

About the only thing I can come up with is that for some reason one or more of the rings stuck to the piston thereby allowing air to push past and not creating enough compression pressure. I also for got to add to my original post that this Mule only has 1622 hours on it.

Think I've missed something?? I do have a repair manual, but it doesn't have a trouble shooting section...

Thanks for all your help,

Roger

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Thanks Travis,

While a leak down tester will certainly tell that compression or pressure in this case is leaking down, I can't see that it will tell me where it is leaking. Can you enlighten me?  I've never used one so perhaps I don't understand exactly how it works.

Also, do you how the decompressor, located on the cam shaft works? Could this be the culprit? My manual doesn't cover it very well.

Thanks for the replies,

Roger

 

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The decompresser on the camshaft bumps the exhaust valve when the starter is rotating the engine, once it fires off, due to centrifugal force, the bump on the decompresser that opens the valve slings away from the cam lobe and stays out of the way. it's possible that it might be a issue, but not likely.

a leak down tester pumps air into the cylinder via the spark plug hole. Piston at TDC, with valves closed, you cut off the air, and remove either the exhaust or intake manifold first, and listen for air hissing out , if air is hissing from the carb side, intake valve or valve seat is toast. Sam for the exhaust.

If no air is heard in any of these spots, remove the dipstick cap, if you hear air hissing in the crank case, the rings are shot.

This video describes it well.

https://www.youtube.com/watch?v=l95j1pr7mg4 

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Thanks for the reply Travis.

What you described with the leak down tester is kind of what I was thinking. Seems the only way to determine where the leakage is, makes sense. While I was waiting for your reply, which I appreciate, I went out and checked the head for valve leakage from the WD-40. There was a tad bit from the intake but nothing serious. could be just a bit of carbon between the valve and seat which I will take care of, if I can find a small enough spring compressor. I don't recall if HF has one that small, but I'll take a look see.  Also while I was out there, I moved the piston to the bottom again and poured just enough diesel to come up to carbon line on the cylinder, covering about half of the piston, just to see if any leaks past the lower portion of the rings. From my experience, these engines with the slanted or horizontal  cylinders seem to wear more on the lower sides. I'll post what I find out.

I'm still curious as to why this loss of compression happened overnight when nothing had changed except for me reinstalling the converters and covers...

Thanks again,

Roger

 

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Yes, the wear more On the bottom sides, and top.

When I rebuilt mine, the machinist had to use a I think what he called a rigid hone to bring it back round.

This is the spring compressor I used, https://www.amazon.com/8milelake-Spring-Compressor-Automotive-Repair/dp/B01CQGZHY4

One thing I suggest doing, but you don't have too, while the head is off and the valves are out, I would get some lapping compound and  a lapper tool and lap the valves, it grinds the valve and seat together and provides a new,clean smooth sealing surface. You don't have to do it, but it's what I do when I'm that far into something.

It definitely is odd that it happened over night. Some engines have minds of their own!

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Thanks Travis, I'll be getting one of those compressors. I do have the lapping compound and will be lapping both valves.

Just a side note for you or anyone else following this thread, I learned from a mechanic friend of mine years ago how to easily lap valves if you have problems with those suction cup lapping tools.

He used a short length of poly tubing, like gas line, that fit snug over the valve stem and a drill bit. He used a variable speed reversible drill. He would run it with light upward pressure for a couple seconds in forward, then reverse for the same time. He'd do that a few times check the valve and seat and repeat as necessary. I've been using that method ever since with great results. Just don't get in a hurry and don't use a bunch upward pressure, just enough to get the job done.

Thanks again,

Roger

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Just an update. Went to HF yesterday morning to get one of their cylinder leak down testers, cheap one gone, it was a clearance sale and the only one they had in stock was $80.00... Too much for my pocket so I began an eBay search. Got the Valve spring compressor and the leak down tester brand new  for less than the price at HF for just the tester. Both should be here by Saturday. I'll do the valves first, then afterwards do the leak down test. I'm just hoping all the problem turns out to be the valves. I'm not looking forward to going back into the crankcase again. Wish me luck.....

Roger

 

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Travis,

Yes the head is still off. I'll try to post a picture of the cylinder bore if I can get a good shot. The engine is still in the frame and it may be a little difficult, but I will try. It's supposed to rain today, so it may be tomorrow before I can get out to the shop.

Thanks,

Roger

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Travis, here are a couple of pictures of the bore. Not the best, but the best I could get. Let me know what you think.

The spring compressor is due in today, don't know about the tester. I'm gonna try and get the head cleaned up today and hopefully back on and torqued, then I'll just be waiting on the leak down tester... I'll post the results from that as soon as I can.

The first picture is the bottom of the cylinder and the second is the top. Hard to tell, but you can see the hone marks with the naked eye.

 

Roger

100_1753.JPG

100_1757.JPG

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Yeah the bore looks pretty good. The only thing i see is how dirty and slightly discolored.... When I took mine apart at 1300 hours, it was still shiny and clean. 

other than that I see no vertical score lines... If course a ring could be stuck if it is the rings causing the problem.

The leakdown test will tell all.

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Hey Travis,

Yes, I thought the same thing when I looked at the pictures.  There is a bit of discoloration on the bottom, but in the picture some of that is shadow.  I took several pictures and that was the best one.  Not quite sure what you mean by dirty. It's been open since I opened it up, other than a rag over cylinder to keep junk out. I did wipe it down before I took the pictures. In the bottom picture, that could be a piece of lint. It'll be blown and wiped out again before I do the leak down test.

I'm not sure of the actual hours on this machine (1622 on the meter), but I do know it has been into before. I could tell by how tight some of the bolts were compared to other ones on the  component I was removing. Some of them were so tight it almost seemed like they didn't use a torque wrench, just tightened them down to what they thought was good!! I probably should use a bore gauge to check and see if the cylinder is out of spec.

I'm gonna do the valves tomorrow and do the leak down test before I go any further. Not sure when the tester will be here, but I'll keep you posted...

Thanks Travis, take care,

Roger

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