Broken Exhaust Valves
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Broken Exhaust Valves

This is a discussion on Broken Exhaust Valves within the The Performance Tuning Shop forums, part of the Snowmobiles category; Last week my 2004 Polaris XCSP (4200 hours) engine seized up and I'm interested in your opinion of the root cause. The Running Conditions: I ...

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Thread: Broken Exhaust Valves

  1. #1
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    Jan 2010
    Last week my 2004 Polaris XCSP (4200 hours) engine seized up and I'm interested in your opinion of the root cause.

    The Running Conditions: I was racing across the lake at ~90mph on speedo about 2 hours prior. The top end speed seemed a little slow but not a big deal. I stopped for lunch for an hour and 15 miles down the trail we slowed up to about 20 mph for about 3 miles for some slower riders. Temps were about 28°F. My fuel consumption has been poor for the year, ~8 mpg vs 10 mpg last year. The choke throttle would pop out once an hour through out the day. Not sure if it would come out on its own or caught on my map bag. The engine seemed to run okay other than when the choke throttle would come out.

    Tear Down: After disassembling the engine, one exhaust value (VES) valve was broken in half and gone in a brittle type break (where you can see the crystalization of the casting in the remaining casting). Yes gone. No sign of the other half. The other cylinder's valve was broken at the stem and the remaining valve had aluminum above the piston melted at the top piston turnaround area. In this piston, the rings were sieze in the piston which looks like its due to the aluminum scraping up and down on the piston. The other piston (with the exhaust value broken looked reusable and the rings are free. Both plugs (NGK BP9R) were a nice mocho brown tone. I typically run BPR9 but needed to borrow plugs last week due to misfiring problems.

    Failure Analysis Theory:
    The crank and cylinders appeared to have good lubrication. I took the parts and engine into my local Polaris dealership for failure analaysis. He didn't think the failure was due to a lubrication either. He thought it might be due to running it lean... Another Polaris dealer commented that they typically only see broken exhaust valves when the valve hits the piston or when a ring break/chips and gets lodge in the piston and exhaust valve. (I discredited the broken piston ring theory due to both exhaust valves were broken.)

    1. The exhaust valves were serviced (by me) in 2008 with about 3200 hours. Both exhuast valves hit the piston. There is a 0.015 tolerance and the valve hit the piston.

    2. Wrong plug? I don't think so. But the BPR9ES are made with the resistor to reduce RFI that may cause interference with the electronics. I don't know enough on the snowmobile electronics to know if it would cause any detonation issues. There did not appear to have any detonation on the piston crown.

    3. Poor air/fuel ratio - Too Rich?: Poor fuel economy ~8mpg. The carbs were not serviced for 3 years. The sled appeared to be running great other than the misfire, choke throttle issue, and the stalled/siezed at about 20 mph.

    Engine Rebuild
    I'm rebuilding the engine with Wiseco standard sized pistons. I have an re-plated cylinder from Millenium (thanks!) and the other cylinder is honed. I will service the carbs and clutch.

    Feedback Request
    I'm a little nervous with the rebuild. I don't want the same thing to happen due to I'm not convinced I know what happened that caused the failure. I'm interested in your feedback of Do's & Don't Do's or possible root causes to the failure. Thanks!

  2. #2
    is Where da snow?
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    Jan 2006
    Your choke sticking open could cause the machine to flood and at high rpms its actually not that noticable. That would cause your poor fuel consumption. If your engine was flooding all the time it would always be washing the cylinders, but if this where the case you would notice a lot of "dry" scoring on the cylinders and pistons like it wasnt getting enough oil. You would also notice a bog on the bottom end. I dont know much about the polaris exhaust valves but I know on the skidoos if you put the exhaust valve in upside down it can hit the piston, usually catching in the ring land and nocking a chunk out of the piston. I would have the dealer make sure the exhaust valves are working right, I dont know how the poos are but the doos are vacume operated and are easy to diagnose.

  3. #3
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    Feb 2017
    I just had the exact same thing happen to my 2005 600 switchback MAG side - any update on the root cause? or more importantly how did your rebuild go with the Wissco pistons?

    Did you use New Polaris valves or something else?

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