We have here a problem with a customers 400 Ranger. will start easy and idle perfect but has no power and misses at high rpm. I personally own a polaris ranger 400, mine has 500 hrs, this problem utv has 300 hrs. Cleaned and checked carb. Switched slide/diaphram/needle with mine. Put in a 135 main jet (had a 150) Switched CDI box with mine. Checked cam lobes for wear, .008 valve clearance, ball in decompressor is sticking out of the cam at rest. Valve timing marks are correct. Mine shows 140 lbs compression with throttle open while cranking. Problem utv has only 80 lbs. Oil in spark plug hole increases his compression to 140 for a couple of tries, then goes back down. All these tests point to bad rings/bore/piston. These old sportsman 500 motors are usually bulletproof especially if oil is changed regular. No air leaks in intake. I guess it is time to pull cylinder off and have a look, Unless you guys that have rode Polaris a long time have run into another problem that we don`t know about. PS He had run with almost a quart too much oil. Could this has caused the damage? Any advice? would be appreciated! Thanks, Robert W
2013 Ranger 400 lost compression
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I don`t know of any seals that would affect top end compression, but I also don`t have experience with overfilling oil. Most of my customers who bring me atv`s have little or no oil in them, dirty or missing air filters, no oil changed for years. This customer said he changed his oil every 100 hrs. If I tear down the engine in a couple days, will report back the findings. I still can`t believe 300 hrs needing a rebuild, that`s a first for me. Thanks, Robert W
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The compression release would've been my first suspicion, but sounds like you've already ruled that one out.
If adding a drop of oil through the spark plug hole makes the compression go up, it sure sounds like bad rings to me.
If adding a drop of oil through the spark plug hole makes the compression go up, it sure sounds like bad rings to me.
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248 Posts
If you have checked out the camshaft and that is good shape, then sounds like piston and rings. From what your describing sounds like the common issue they have with the exhaust lobe wearing off. www.nflowmotorsports.com
Yes the center cam lobe. I fixed about 6 of those bad cams mostly in the old 425 magnums. The first one of those was hard to figure out. Now thats one of the first things to check.
This Ranger 400 actually runs good at idle and a burst of power on takeoff, no backfiring at all. then the faster you go (more throttle) the power goes away and engine misses. Another thing that don`t make sense on this 400: The 70 or 80lb compression reads with throttle both open and closed. My personal Ranger reads 80 with throttle closed, 140 with throttle open.
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248 Posts
Sounds like Piston/Rings. Have you pulled the top end off yet? www.nflowmotorsports.com
We pulled head and cylinder today. The cam and its compression release look and works proper. The cylinder bore is great and the piston is right on size. Miked both like new measurements. The rings are not broken or stuck. The ring gaps are about .009. Now I`m getting concerned. Ordered a gasket kit and hoping it was a bad head gasket. I suppose some oil on top of the piston might raise compression a bit since its a flat top piston. The Ranger was not smoking and the coolant level was good. I sure hope the head gasket fixes it. Maybe someone has an Idea or maybe I can think of something else before the gaskets arrive in the mail. Will inspect the head real good while it`s off. Thanks for all the comments.
The head gasket did not fix it. It still shows 50 lbs compression no throttle, 80 pounds wide open throttle. Pulled the muffler. No better. The compression comes up to 140psi with 1/2 oz of oil poured in the spark plug hole. Only last a few tries, then goes back down. This oil/comp test should point to bad rings/bore and or piston. The ring gap, piston size, bore size all measure out. No visible damage or cracks anywhere in the cylinder or head. No water in oil or oil in water. We always tell customers that if we can`t fix it, no charge. Had to eat my words this time.
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171 Posts
There are a lot smarter people here than me, but since you still have the problem, could valve springs be weak ? Also, head gasket replacement, rings, pistons checking good, could there be a hair line fracture in head, cylinder bore that opens up with heat ? I know you said no fluid exchanges, but something is off on compression, increases with heat. Hair line fractures hard to see. Because of the compression issue, focusing on mechanical issues, not those pesky Electrical Gremlins.
Of course, any restriction on Intake air has been ruled out, so am following this with great interest.
Of course, any restriction on Intake air has been ruled out, so am following this with great interest.
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54 Posts
could you do a leak down test? apply air pressure to the cylinder with cylinder TDC on compression stroke? might have to hold the motor in place if the air wants to turn the motor. then listen for air to escape. a little is normal out the crank case, but the problem should show up with air leaking from the intake, exhaust, crank case or around the head some where. with say 100-120 psi applied the leak ought to show up.. valves should have no leakage, same as head gasket. with compression that low the air should escape petty good some where.
later Chris
later Chris
We bought a leak down tester from harbor freight for $40. It is well worth the money and worked great but probably not as good as the more expensive Motion Pro. With the intake manifold off, and 15psi you could hear and feel the intakes leaking. Shown 45% leakage. Poured wd40 into the pockets above the valves, Bubbles like crazy. Off with its head. Pull the valves (1 at a time) and put in a milling machine collet, they were not bent. Polished with fine emery paper at 800rpm. Lapped the valves measure valve springs OK. Now I only have 16% leakage. Ordered a new head gasket. It has not came in yet so I went ahead and put it together and tried it. Compression jumped back to 140 with wide open throttle. I rode it about 30 minutes on the farm and all of a sudden, the coolant is being pressurized and squirting out the overflow bottle or radiator cap Back in the shop the leakdown test (25%) clearly shows pressure coming from the rad cap. It will try to eject all the coolant from the radiator. Still waiting on head gasket. Yes I am still worried about a crack somewhere although none can be seen with a magnifer and bright light on the cylinder or head either. Will report back later when the Ranger is totally fixed and running like new. Thanks to all you guys for help. Notes: should have bought a leakdown tester years ago 2) try to hold the crank at tdc so the 15psi don`t push the piston back down. A medium flat screwdriver can be used as a lever against the 3 camshaft sprocket bolts (from the top valve cover) to rotate the engine. If you ever get perfect TDC, piston will stay put by itself with the 15psi. The exhaust pipe springs are extremely strong, what a pain.
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