Troubleshooting a TH18

[Chris727]

I'm working on a Kohler Triad right now. Its certainly a different animal. Tractor appears to have around 400-700 hours on it. It will start and run with decent RPMs immediately after cleaning the plugs but will only run about 10 seconds then slow down and die. Runs rough when it does run. The plugs are not fouling, it just seems to be very sensitive to plug condtion. There are no mix adjustments on the carb, fuel appears to be good and the carb is clean inside. I compression tested it and one cylinder has 71 psi, the other has 81 so its a little out of balance but I wouldn't think its enough to keep it from being runable. These compression figures will seem low because the engine has automatic compression release. I'm wondering if the timing belt is my problem? Anyone ever worked on one of these that hadn't blown up? Thanks in advance. Chris

[PhanDad]

Sorry I can't help you with troubleshooting the engine. Hopefully Al will respond. However, the Kohler TH-18 was an award winning engine :O Found this while reading the August 1998 Issue of Popular Mechanics (I'm a little behind in my reading sm03:

[Al]

Hi, I have just written 3 paragraphs. and due to my shaking, I lost it all. I will redo it tomorrow or Wed night. Al Eden

[Chris727]

Thank You Al. As always your help is greatly appreciated. There's no rush, whenever you get time and feel up to it. Thank You. Chris

[Al]

HI, I think you are heading in the right direction. The early THs had some problems with the cam timing gears and the belts, with failures occurring in as few as 100 hrs if the units were ran in sandy or abrasive environments. I think 75 lbs sounds low for compression, if the timing is off one tooth, This is an option, Kohler brought out a kit with new materials for the gears etc, and this really fixed this issue. The kit contains new gears, belt and oil pump assy. One thing if you are setting the tension on the timing belt. NOTE It says to tighten the belt to 35 (I think) inch pounds with a torque wrench in the square opening in the tensioner arm and lock the bolt to hold it. DO NOT CONFUSE THIS WITH FOOT POUNDS OR YOU WILL HAVE A BROKEN SHAFT ON THE OIL PUMP IN LESS THAN A COUPLE OF HOURS!! I have observed this. There is a tool to set the timing with, but it is no big deal to do it without. Basically the locating holes have to be in a straight line when the crank pulley is aligned with its mark. The Kohler service manual covers this very well. By the way these kits are quite reasonable and if you have trouble obtaining one if you need it , let me know. Our distributor has them. Next I would put full synthetic oil in the engine. Check the cylinder fins for oil film and dirt. The valve cover with the fuel pump if prone to leak along the bottom edge. This oil them migrates down to the fins and dust collects on them. Then the engine overheats, and eventually the heating and cooling of the aluminum the aluminum doesn’t return to the original size. Then the sleeves come loose. I believe the lost foam castings are more prone to this than die cast castings. They are less dense. We no longer use the gaskets. We use silicone material and coat the top of the head with a thin coat. Next we coat the inside edge of the valve cover with about a 1/8 inch thick of silicone. Then we let it “skin” up and then put the valve cover on and the screws in. We think this seems to work well, especially if the valve cover is warped from the actuation pressure of the fuel pump. Next I would rework the PTO end of the air cleaner to allow the dirt to blow througj instead of being trapped around the filter. We cut “mouse holes” in the cover , These air cleaners are very prone to getting packed with dirt and chaff in very little time. The cooling fan blows air into the air cleaner and the dirt can’t blow on out. This fixes this problem. My night guy went from buying 6 air filters a year and cleaning them every mowing to using one all year. Don’t cut all the way to the top or you will lose residual strength in the cover. We have a number of these engines out that have a lot of hours on them, and we have seen a lot of failures.. They had a lot of great engineering firsts, and me being an old race guy was very excited about them when they came out. I believe with minor changes and with the technological advances that have occurred in the lost foam casting process that this could be built today and be a very reliable engine. We sold a brand new one last month to the city of Danbury Conn. Kohler referred them to us for help with a “Sandtrap Rake” using a TH with an internal splined crankshaft. While trying to run down a Command with a spec # that would have the needed closure plate and crankshaft and the other items that would have needed to be changed. Working from pictures and measurements. I ran across a brand new TH, and a number of short blocks for this application. The original engine has lasted for 12 years and gave good service. If the new one goes for ten years, the unit will probably be replaced by then. It saved them a lot of work doing something that they had little experience with and we probably would have added to the value of ATT stock.. Am adding a picture of how we revise the air cleaners. Al Eden

[Chris727]

Thank You Al! Both of the valve cover gaskets appear to have been leaking for a while and it was on my list of repairs to do. Looks like the next step would be to pull the engine and upgrade the timing gears and belt. If that doesn't fix it I'll probably try to find a used command for it. Thanks Again. Chris