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I have been quietly learning from everyone about the Pilot that we bought used 3 years ago. I have done most of the maintenance and minor repairs myself through reading the forum and watching youtube video. Therefore, by no means am I knowledgeable at all about cars. I just like to make things work again when they break and by doing it myself, I learnt a lot and save some money in the process.
Our pilot recently developed minor coolant leakage. The overflow reservoir gets empty every couple months. I saw some evidence that the radiator was leaking at the bottom on the driver side, although there was never any puddle on the ground. Given the age of the vehicle, I thought replacing the radiator was the fix. I installed a Denso radiator. In the process, I replaced the upper and lower hoses plus the thermostat while I was at it. When bleeding the air out, I saw no leakage of any sort, so I put everything back. So that was close to half hour of running the pilot. I thought everything was good. But after I pulled the Pilot out of garage on to the road way, I saw a puddle where the pilot was stopped. There was a leak somewhere! So I pulled it back into the garage and checked everything (all the hoses, thermostat, radiator etc). I did not see the source of the coolant but it was dripping down the driver side and later in the middle between the driver and passenger. I did see coolant in the area to the right (looking rearward) of the thermostat. But yet, I did not feel any wetness on the thermostat housing, top and bottom. All the hoses there felt dry. I am stumped as to where the coolant came from. I would appreciate if anyone can give me some insight. I felt so close to finishing the job and yet I might have to bring it to the pro.
 

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I forgot I was going to post a picture of the area near the thermostat. So here it is.
20191127_212759.jpg
 

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Get a cooling system pressure tester. If pressure doesn’t hold get a spray bottle with soap and water and spray around to see if anything bubbles.


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Pressure testing the system as mentioned above is a good idea.

When you pulled the thermostat out how much coolant came out? When I changed mine probably a quart ended up spilling on the top side of the tranny and onto my garage floor. Could the puddle be from that?

There are also coolant hoses going to the front heater core between the engine and firewall.
 

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Thanks for both of your quick suggestions. I am located in Canada so I will try Partsource to see if I can borrow a pressure tester. When I pulled the thermostat, there was not a lot of coolant came out. So I did not think the leakage was from that. Also, I forgot to mention coolant only started dripping about a minute after the car was started. I also wonder if it is possible I did not tighten the 3 bolts enough or had overtightened them on the thermostat housing, especially the one at the rear that I could not see, even though the housing felt dry where I could reach. Thanks again.
 

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When you replaced the thermostat did you also replace its gasket?

I’m a big proponent of using a torque wrench for tightening bolts to the proper torque spec. You can purchase a decent quality one for less than $50. I like that it takes the guesswork out of over/under tightening bolts. If you’re going to be doing lots of work on your vehicles I feel it’s a good investment.
 
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I too had the coolant passage gasket leak. I could smell it but it is kind hard to see without a mirror. The way I picked up on what was leaking was coolant staining on the front top of the transmission.
 

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Hard to tell from the angle of the picture. You might have to use a light and mirror to find the exact location.
 

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Discussion Starter #11
Again, thanks for everyone's input and advice. I tentatively (99% sure) the coolant is coming from the thermostat housing. I did use a new after market gasket. Maybe I did overtighten the mounting bolt. Just to be on the safe side, I will get an OEM gasket and reinstall the thermostat. I really don't want to do it again! I initially did not torque it to spec as it was really tight to use a torque wrench, but I will try this time or at least try to have a feel what the proper torque feels like. From my research, the torque spec for the mounting bolt is 12 N.M. or 8.8 ft lb. If this is incorrect, please correct me.
 

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Yes, 8.8 ft-lb is what the Haynes repair manual calls out for the torque spec of the thermostat housing bolts.
 

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I hope that thread is not stripped. Even if it is, helicoil can restore it.
 

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Yes, 8.8 ft-lb is what the Haynes repair manual calls out for the torque spec of the thermostat housing bolts.
Thank you! I replaced the gasket and torqued to spec as well as I can. It was a tight space to use the torque wrench to say the least. I have driven it a few days and all was good. So I concluded that I overtightened the bolts causing the leak. I am sure the new aftermarket gasket was fine but I didn't want to take a chance as it took way to long to do this job and it was getting cold outside. A rookie mistake. Live and learn, I guess. Again thanks for all the advice.
 

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I've had an issue with my Pilot where it only appears to leak coolant under higher temps/pressure on long highway drives and/or hills. Typically during warmer months. The reservoir will be completely fine for short to medium drives at lower speed but once I am on the highway going 65mph+ for a period of time the reservoir will become empty again. There is also always a heavy smell of coolant when standing near the engine bay. Replaced thermostat and the gasket doesn't appear to be leaking there. I'm thinking possibly pinhole radiator leak that only leaks under more pressure? Or perhaps the coolant passage gasket mentioned here?
 

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OP - Glad to heat that you got the issue resolved. It sucks having to do the same job twice, but like you said it’s a good learning experience.
 

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If your radiator is leaking there should be blue (if Honda coolant used) chalky residue on the radiator fins. Also look at the bottom of the radiator where the fins meet the body. That is where the majority of the leaking fluid puddles up on my previous radiator.
 

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Question: If non-Honda coolant has been used, when replacing a radiator should the same (usually green) coolant continue to be used, or should that be a good opportunity to switch back to blue Honda coolant?
 

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I never used Honda and do not know the difference. The common green stuff works fine in mine for five years soon. If the blue stuff costs any more than green than I would say it is not worth it.
 

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I would use coolant of the same chemical composition if I went with a non-manufacturer specific coolant. Given that the Honda coolant isn’t much more expensive compared to its equivalents I stick with OEM. I have heard mixing coolants of different chemical makeup is not a good idea.
 
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