I was about 30 - 40 minutes into a one hour drive and I noticed the temperature gauge needle climb to about the middle mark on the temp gauge. It wasn't there long (maybe one minute), then returned to its usual position (about the 3/8 mark) for the rest of the ride. I was cruising about 65 mph indicated, so cooling air shouldn't have been a problem. At those speeds, I would think that the cooling fan wouldn't even be needed.

Anyway, on the trip home home, the same thing happened on the freeway after about 30 or 40 minutes. The needle climbed up to half for a brief time, then settled right back down.

The ambient air temperature when this occurred was below 40F both times.

The cooling fans work and the radiator is two years old (new at the time I installed the JDM motor). The thermostat is not a Honda thermostat. It's a Napa/Stant that the previous owner installed and I used it when I put in the JDM motor. The thermostat and the water pump are about 3 years old with about 22k miles on them. The coolant level is on the high mark.

Has anybody seen anything like this and been able to find the cause? I don't know if this is a "real" problem or just something that the gauge is doing independently of the actual engine temperature. I've never seen anything like this before.

I'll probably replace the thermostat with the Honda part ($32, ouch). Other than that, I'm at a loss.

 

OEM thermstat is a good idea. When the engine is cold, remove the radiator cap and verify the radiator is filled with coolant.

 

Yes I have seen this before and there are two general causes, one the thermostat is getting stuck in the closed position or the head gasket is failing. 32 dollars should not seem like much of an ouch given the other potential. Rarely do head gaskets give advanced warning on Accords but sometimes they do... Civics will usually give some advance warning. If you were driving with the heat on did the temperature suddenly drop and then immediately come back?

 

Yep, did that. All full.

I'm used to seeing this behavior on steep hills where the thermostat doesn't open fast enough and there's some overshoot before the thermostat opens fully and settles down. I haven't seen this cruising at freeway speeds on level ground before, hence the head scratching. I know $32 isn't bad compared to a warped head. When I was ordering the thermostat, I cleared the cookies, reloaded the shopping cart and got a new price of $25. That will pay for about 1/4 gallon of the Honda coolant.

Don't say head gasket. That's why I put the JDM motor in in the first place. The only advance warning I got was some rough running for a few seconds at cold startup, which could have been anything. Eventually, it turned into a no-start and I found water in the number 2 hole.

The temperature selector was in the full heat position, but the fan was off. Air was not in recirculation mode, so fresh air was coming in through the HVAC, but not at a high rate. I didn't notice any sudden drop in temperature.

Thanks for the input, folks. I'll do the thermostat on Saturday and go from there.

 

Update: The OEM thermostat seemed to have fixed the problem. I haven't seen the problem since I put it in about 6 months ago.

 

Thanks for the followup. Glad it worked and a was a simple fix.