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Hi everyone,

I have a 2018 RZR 900s with 3664 kms on the clock. On hot days after a few hours of riding it will begin to loose power, back fire and eventually die. After a few minutes of sitting I can get it started again for a few minutes until the problem returns once it warms up again. No codes are present. I have checked the CPS, coil, spark plugs, valves (one intake valve was out by .001), cam chain, Intake and fuel pressure. I have looked at the wiring harness and there are no signs of wear on the black outside insulation. I am running regular fuel. The machine is stock. Anyone have an idea of where to look at next?

Thanks for any help,

-Andy
 

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Andy, sounds like it's vapor locking. Could be low fuel pressure. Next time it does it....pour some cold water over the fuel rail and see if she fires back up.
 

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Just take the gas cap off and if it is vapor locked and it will release any tank pressure that has built up. Also check your vent line on your gas tank. This is what I did to mine down here in the Florida heat and humidity.
 

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Just take the gas cap off and if it is vapor locked and it will release any tank pressure that has built up. Also check your vent line on your gas tank. This is what I did to mine down here in the Florida heat and humidity.
I'm not sure I follow your thinking on this solution. Vapor lock is the boiling of fuel in a fuel line. When fuel boils only vapor reaches the carburetor or fuel injector causing a lean condition. Pressurizing a liquid increases it's boiling point (which is the reason for a pressurized cooling system). Any pressure in your fuel tank should add to the pressure a fuel pump produces thereby reducing the possibility of vapor lock albeit over pressurizing the system. Pressure in the fuel tank basically acts like two pumps running in series, which is one method commonly used to increase pressure when pumping liquids. If releasing pressure from your fuel tank solved a running issue I submit you have a problem other than vapor lock. In fact as near as I can tell from my service manual the tank is vented through an open hose which should prevent the buildup of any pressure in the tank. Surprisingly, the Polaris service manual requires vent line replacement every two years. I have no idea why that would be a requirement unless they expect the line to collapse easily and prevent proper venting of the tank.

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kreiser42, RJ is correct, you may indeed have a fuel pressure problem which is allowing vapor lock. A fuel pressure gauge will confirm the problem if it exists. You mentioned having checked fuel pressure but did not report results in PSI. I believe your unit should have fuel pressure around 58 PSI +/- 2 PSI. You also mentioned having "checked" the spark plugs but did not mention whether you replaced them. I have seen spark plugs that appear good when inspecting the electrode and insulator but which broke down under running conditions causing the same symptoms you have described.

My advice, if you haven't done so, is to replace the spark plugs and see if the problem is resolved. If not, you have a new set for when you need them in the future.

Another possibility is water in the fuel. You stated you are running regular fuel but did not mention whether is is E-10 or not. If you are running E-10 and the fuel is not fresh it is possible that humidity had been absorbed by the fuel and allowed phase separation to take place. Humid summer months add to this possibility. E-10 that has undergone phase separation can cause many issues with how an engine runs, or doesn't run. How can you tell if phase separation has taken place? Drain the fuel tank and allow the fuel to sit overnight. You will see distinct layers in the fuel, a layer of mostly water near the bottom. (it may not even take overnight to see the layers which can sometimes be seen in minutes in severe cases)

Ethanol in fuel has several negative aspects other than phase separation. It can cause deterioration of fuel system rubber and some metal parts and corrosion. Ethanol fuel also loses it's octane rating much faster than non ethanol fuel which means that engine are more subject to detonation and pre-ignition both of which are harmful to the engine's internals and can cause severe damage. Hot weather can bring on detonation and pre-ignition faster than milder temperatures, particularly in engines with higher compression (I believe the 900 has 10:1 compression ratio). Whether ethanol in fuel is the root cause of your running problems or not I strongly suggest running non-ethanol fuel and a fuel stabilizer which will keep it fresh for a minimum of a year. E-10 begins to undergo phase separation and loss of octane rating in as little as 30 days in storage, and believe it or not, these changes can begin even in a closed plastic container.

Ignition coils can cause the symptoms you describe as well and they generally break down when hot. I would not say this is a common problem with modern coils but it can happen.

Backfiring can be caused by lean mixtures which is another reason why fuel pressure is important. If your fuel tank vent system is not functioning properly your fuel tank will develop a vacuum after running for some period of time as fuel is used and not replaced in the tank by air. This can cause fuel starvation and backfiring. You may think that heat is the cause when it is merely time running which eventually removed enough fuel from the tank to created a vacuum and cause fuel starvation. If, as FL Ranger states, removing the cap alleviates the problem you have a fuel tank vent system problem which is preventing air from entering the tank to replace the fuel used. FL Ranger mentions relieving pressure by removing the cap but I believe what he is doing is hearing a rush of air when he removes the cap but rather than air rushing out as it would if the tank were under pressure it is rushing in to fill the vacuum created by fuel used. Vent lines can become clogged by dirt, insects or pinched shut.
 
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