Temp gauge jumps to the red zone

1995 Accord
12 midnight, 85 degrees. Driving home, the temp gauge jumps to hot (red zone). Notice some steam in the engine compartment. Stop and shut off the engine. This or similar has not happened before with this car.

Overflow tank (not pressurized) had reasonable amount of coolant, not high not low. No coolant on ground under car, no drips. No visible cooant in engine, no coolant odor. Radiator had no visible coolant but only took maybe 2 cups. Radiator fan works as required. Engine runs and sounds good.

I think of a defective thermostat as a possible cause. Cooling systems are not particularly complex - what else to consider?

Head gasket, possibly.


What did temp gauge do on rest of drive home?

Start with a cold engine. Make sure radiator is full.
Look closely with hood up as it warms up.
Look for drips or other seepage around hoses, radiator, water pump.
Top radiator hose should get firm after a few minutes as system pressurizes.

Could also be a pinhole in a hose. As the engine heats up and the coolant pressurizes the hole opens enough to emit a small stream of coolant. The resulting loss of pressure can cause a temperature spike.

Easiest way to find those is to open the hood and start the car. Wait for it to come up to temperature, then look for the stream of coolant.

Try a thermostat first. With the old thermostat out place it in a pan of water and bring it to a boil; monitoring the temp with a thermometer.
At some point up around the thermostat’s rating (call it 195 degrees) you should see it begin to open and stay open as long as the water temp is kept up.

1 Like

I’d also worry about the head gasket. But on a 1995 “anything”, there could be any number of causes.


check radiator cap

1 Like

I would agree the thermostat would be the logical place to start as long as the belt driving the water pump is still intact. Fluctuating hot/cold could also signal a head gasket problem but gotta hope not. Just because a problem has never happened before does not mean that the problem is not happening.

All good ideas. I’m guessing it is either a faulty radiator cap, external leak (e.g. water pump), or internal leak (e.g. head gasket). Replacing both the radiator cap and the thermostat as a first step makes sense, and might well do the trick. If not, and you are 100% sure the radiator fans are working correctly, a cooling system pressure check is next.

I had a similar experience one time on my Corolla, and the cause was the radiator fan wasn’t coming on when it should. Faulty thermo-switch. That same part has failed twice over 28 years.

Clogged radiators can cause this problem too, but they usually exhibit prior symptoms. Given what you say, my guess is a faulty radiator isn’t the cause. If it is a major cooling system part, a problematic water pump seems more likely.

Wow! Thanks.
A lot of good stuff I need to parse through.
The incident has not repeated but I’m going to go through the check points any way
I started with the radiator cap which I discovered to be defective - someone had removed the spring and seal. The T-stat is an unknown, but I have a new one anyway (they’re cheap).

Running the engine to temp did not reveal any leaks, plus even when the incident happened, the coolant was only 1 - 2 cups low.

I have seen no evidence of a leaking head gasket, i.e cross contamination or poor running.

The fan works and the control module is new.

That’s the first time I’ve ever heard someone reporting that. I’ve salvaged the spring from broken radiator caps, but I can’t imagine why someone would remove the spring and the seal then re-use a dismantled radiator cap.

I’ve seen where someone removed the seal to slow down a leak by preventing pressure buildup.

1 Like

Some Honda radiators use a radiator cap similar to the one used on Toyotas. Those caps fall apart, I find the pieces in the radiator.