2005 Honda Civic fluctuating temp gauge, not heating well

2005 Honda Civic 190,000 miles. The car does not heat up fast like it used to. Thermostat has been replaced. On a couple recent occasions, the temp gauge fluctuated between normal, fully hot and cold (up and down and back up and down, etc.) over the course of a few minutes. Car seems to run fine. The repair shop has not been able to replicate the problem and initially seemed stumped, and now says it’s probably the head gasket. They have said the coolant remains pressurized hours after turning engine off. Before I agree to the expensive repair, what should I make sure has been checked? Thanks.

Time for a second opinion my thought.


I also thought of a bad head gasket as the cause of the issue. You can have the coolant checked for exhaust gases to prove that is the case.


There were some Honda engines that developed a cracked block. I’m not sure which ones, and whether the crack led to your symptoms.

The head gasket can be tested in several ways. Constant bubbles at the open radiator neck are a strong clue. An exhaust gas analyzer can sniff for unburned hydrocarbons at the open radiator filler neck. There is a chemical test that can detect changes in the coolant due to exhaust gasses.

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I almost wonder if the system either isn’t completely full and/or there’s an air bubble moving through the cooling system, but if it’s been to the mechanics, I’d think they would’ve checked that already…I think having the system checked for exhaust gases to check for a bad head gasket would be a good idea too


My Honda uses a water valve to control flow through the heater core. The heat needs to be set on Hot to open this valve all the way - which helps the system expel any trapped air out the filler neck.

When the engine is cold, remove the radiator cap.

Start the engine and let it idle while watching the coolant in the radiator.

If bubbles begin forming in the coolant, it indicates a blown head gasket.


Thanks, all. It was the head gasket. It has been replaced and the car is running well.

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