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GL1000 Thermostat housing leak

928 Views 3 Replies 4 Participants Last post by  toolcraft4100
Hi all,

It's been a while but I'm working on getting my GL1000 running again. I've bought the bike in the UK and used it for a few months but it never ran well. Decided to do some work on it to try and improve. When taking the bike apart I noticed there was tap water in the cooling system and not much of it. I've replaced the thermostat and the housing seal but as soon as I start filling the system with coolant it seeps out the bottom of the thermostat housing. It then runs along the block and drips down.
The housing is clean, the seal as mentioned is new and the thermostat is the correct type.
Any ideas what might be causing the leak?


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There is a thermal switch for the radiator cooling fan that is screwed into the t-stat housing. Is it possible it's coming from there? Other than the temp sender, the only other place it could be leaking would be the transfer tubes.
Are you sure it's not leaking from up higher and running down on the housing?
If there was tap water in the system, are there signs of corrosion? I have water in mine right now, but it is distilled water. "Tap" water here is loaded with calcium chloride. If there is no damage to the aluminum cover, or the mating surface on the engine, and you are sure it is leaking around the thermostat housing, you likely have the wrong 0-ring, or it is not seated correctly. I looked for 0-rings locally, but could not find anything that fit properly. Some were close. I got the oem Honda part, put some grease in the groove in the cover, pressed the 0-ring into it, wiped off the excess, and installed it. It doesn't leak. I never removed the temp sensor.

Dry everything off really well, have someone else pour water into the radiator while you lay down in front of the engine, and look for the leak. Another way to do it if you don't have a helper, is to stuff a piece of paper towel in above the cover, and pour water into the radiator. If it is coming from above, the paper towel will catch it, and get wet. You will have time to get down there and see where the drip is.
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maybe cracked

I had a thermo housing that leaked after I had taken it off and then reinstalled it. I am confident that it had a hair-line crack in the filleted area right next to one of the bolt-hole lugs. I could not see it from the inside, but from the outside that is where the drop seemed to form. Luckily, I had one from a parts bike and when I installed it the leak was gone.

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