First off, hope everyone is doing well. I'm new to the page as I have been googling for a bit and have seen some similar issues but nothing the same as mine so sorry if this is already posted somewhere. I have a 1999 honda accord 3.0 coupe and I'm experiencing over heating but it's only when I'm stopped AND if I turn the heat on or AC it immediately drops in temperature back down to where it should be. I changed coolant temp sensor and when I turn on the AC both fans work so I know the circuit is good. From what I'm reading online it could be water pump on its way out? I'm hoping it's not if it is ill do the job myself anyway but I really want to get this fixed ASAP as it's my daily. Just under 300 thousand kms it's been a great car so I would like to keep it going. LONG STORY SHORT:
Car overheats unless moving going pretty fast or heat/ac is on but the temp drops almost immediately after me turning the heat or ac on. It also tends to happen after at least 15 20 minutes driving sometimes even an hour or so. Sometimes it doesnt even over heat but its becoming more frequent.Coolant levels appear fine definitely isn't low.

Thanks in advance everyone! Really appreciate any advice.

 

Do the radiator fans turn on by themselves? They should when the temperature gets too high. Your car is old enough to have two sensors, one for the dash gauge and one for the fans. If you have a bad fan sensor (or switch, since it turns the fans on and off) I strongly recommend a Honda brand switch. As for a water pump you will know it is failing because it is designed to have a small leak when the shaft seals begin to fail.

 

1. Leave the A/C off in this test. Unplug the harness connector to Fan Switch A and insert a small paper clip or wire into the connector's cavity terminals. Then turn the ignition to ON, car doesn't have to be running. Does both fans turn on?

Fan Switch A is the switch that suppose to trigger the two cooling fans to run, if the water temperature at that switch reaches ~ 199 degree F.

2. Check coolant level in radiator when it is cool, as precautionary step. Also top off reservoir.