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Discussion Starter #1
Over the past two weeks or so my '01 Insight (121,XXX miles) has developed an odd tendency to heat up, only slightly, during sustained 70mph+ drives.

My temp guage typically displays 6 bars and has done this since new regardless of outside temp, use of A/C, speed, etc. I have not made any modifications to the car and aside from a thermostat replacement about 15K miles ago have had no other problems. Coolant level is right on and the coolant itself looks clean (drained and re-filled when replacing thermostat). Lately, on my drive to and from work (47 miles one way) the temp guage will move one bar higher if I maintain 70+ mph (or thereabouts) for more than 10 or 20 minutes. If I keep the speed down in the 60 - 65mph range (where I typically run) all is well. The electric fan also seems to be running a bit more than what I previously remember.

I am at a loss as to what might be causing the rise in temperature. Any ideas? I thought the thermostat might be the culprit but if that were the case (i.e. opening at a higher temp than normal etc.) I would think that it would run 1 bar higher regardless of speed. I am tempted to take it to a dealer to run a diagnostic but REALLY don't want to go that route as I am certain, being a higher mileage car, I won't make it out the door without spending a ton of $$. I guess it may be best in the long run though rather than doing permanent damage to the engine.

If anybody has any ideas or has had this problem please let me know.

Thanks!
 

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Can you check the temp reading with an OBD tool? Borrow one for that? Maybe your gauge is not accurate, but the actual temperatures are OK.
 

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burp it

I strongly suspect air bubbles in the cooling system. Did you do the coolant filling yourself? There is a quite elaborate procedure for filling it that must be followed to the letter to get all the air out of the system. I wouldn't trust any shop mechanic to do that right!

You may want to try to burp it now:
Remove the engine cover, connect a piece of hose and a funnel to the coolant filler port on top of the water outlet, fill it with extra coolant and then run the engine (cabin heater on high, but fan off) until the radiator fan comes on. If there is any air, you should see it bubbling out in the funnel (may need to fill more coolant while it's running).

Bad for mpg, though 8-(
 

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UURRRRRRPPPPP!

Good point about possibly air "locking" the thermostat Armin. And in unusual cases it can take an overnight to appear. But 15K miles after effectively changing the coolant :?: Sorry I doubt it.

The radiator cap will slowly "purge" air from the system as long as the overflow tank has a minimum amount of coolant when cold (and of course the cap is working properly).

HTH! :)
 

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Keep a close eye on that temp gauge if your Insight has been running hot. During the winter this past year my Insight started running a little hot (in the dead of winter in Maryland, not a usual time for overheating problems), then the problem intensified after a while. I took it in, suspecting either a faulty thermostat or worse. Ended up that the dealer diagnosed the problem as a bad O2 sensor, and it was replaced and the problem seemed to go away. But, I kept going through coolant yet the dealer couldn't find a leak in the cooling system, and didn't find any coolant in the engine oil which would have indicated a blown head gasket. I didn't take any chances, the mystery of the coolant which could have been the early onset of a much more expensive problem, plus the fact that it had finally gotten to the point for me that two seats was just not enough, I traded my Insight in. Hopefully you'll find the source of the overheating problem and get it fixed. But definitely stay on top of it, by and large Insights tend to run very cool so overheating in any degree indicates something probably needs fixing.
 

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Discussion Starter #7
Thanks for the suggestions. I was also thinking about the possibility of air in the system but figured that would have caused problems immediately after changing the thermostat and refilling the system. I didn't follow the exact procedure mentioned by Armin but did something similar... ran the engine with heater on and filled the radiator until it stopped taking coolant. I have no leaks (no coolant loss) and the overflow is sitting right where it should be. I was wondering if an O2 sensor or possibly the cat. could cause this problem. I guess I may need to take it in just to be safe. I hope to keep the car until at least 200K so a bit of money spent now is probably better than a lot later... not that a cat. is a "bit" of money but it beats a new engine or new car!

I'll try burping the cooling system this evening and see if that helps. It is just strange... kept it at ~63mph-65mph this morning and had no problems whatsoever.
 
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