I have yet to see the bypass valve be the problem. It is always either a worn cooling fan and_or a dirty engine and cooler. It is critical to wash this enigne the right way at least once a year. it takes a lot of time but safes a lot of down time later. You need to unbolt the air cleaner housing...while you have it off you need to wash it.......keep washing it, you may not see dirt at first but it takes some soaking and some continued washing to get it all out and clean. Wash from all directions and use air from time to time.......then finish with air. Be sure it is dry before you cut . Then remove the rest of the engine shields. You need to unbolt the upper oil coolers so they can be moved around for better access........dont forget the shields on the out side of the jugs. You then need to start washing with high pressure.......a garden hose wont do it...........you need to wash the engine and the coolers in both directions until nothing else comes. At this point you are still a long way from done. Be sure you wash the engine compartment out as this will help you decide when the water is clean.....Then take high pressure air and blow the coolers and the engine in all directions until nothing else comes. Keep doing this......it will take many times before it is truely clean. Most people wont beleive how dirty things really are. While you have all this off it is a good time to inspect the cooling fan. If the leading edge is rounded or the trailing edge is sharp or starting to peel back it is liekly the fan should be considered for replacment. I know a lot of people rely on a water column test to determine the condition and this work well....but if the fan is starting to wear and is very deep beyond the coating it will not last very much longer. If you have any doubt about the fan have a mechinic that knows Deutz engines look at it. Whe working on the fan besure they do a complete rebuild of the fan and not just a fan replacment. Good luck! Van