Coolant temperature drop at idle - problem?

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Sep 6, 2002
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Mississauga, ON
Ever since I changed my coolant on my 2001 Honda Prelude 2 months ago, the coolant heats up fine and temperature stabilizes at a bit below mid-point on the stock temperature gauge. Yet when I stop and let my car idle for about 2 minutes, the coolant temperature needle drops about 2mm down. After I start driving again, the temp needle goes back to normal. The car runs very well, the engine seems smoother than when new. There's no coolant leak as I'm not losing any, and there's no coolant contamination in the oil. The coolant is premixed Honda coolant that I bought from the dealership, and I did the fluid change myself so I know what I put in.

Would that mean that I have some air bubbles in the coolant, or is there something wrong with the thermostat?
 
...Sounds better than normal to me. Your new coolant may just be transferring heat better than the old stuff. Air would cause a vapor lock and an overheating if anything. That engine isn't going to make much heat anyway at an idle. I wish I could say the same for my 70 Mustang Mach 1 V8. It goes the other way at an idle real fast. Your thermostat is probably ok but it wouldn't have hurt to replace it while you had things apart.
 
Maybe you have a low-speed fan that comes on whenever you stop. Is this incidentally with the a/c on?

I hate cars (GM) that wait until 230'F to hit the fan-- lots of heat stress on everything under the hood.
 
That's what I was thinking, there may be a fan that keeps cooling when it shouldn't. I opened up the hood once when idling and both fans weren't moving. The A/C is NOT on when this happens.

Since the engine runs well, I won't worry too much. Thanks for the advice though.
 
Given your location you're probably running the heater a great deal. My 2.5 Jeep TJ can't hold the thermostat setting at idle if I'm running the heater at full blower speed and the ambient temp is too far below freezing (we hit the single digits this weekend =rare for southeastern PA).

I must admit, however, that this is the first vehicle that I've had that has done this. Most just may get hotter in the summer ...but this is the first engine to actually cool down below the 205 thermostat setting. My wife's 4.0 doesn't have this traight.
 
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