2001 Honda overheats when at a sustained speed of 50 or more

According to the temperature gauge on the dash, my 2001 Honda Civic is running hot when I am on a highway going 50 miles or better. As soon as I get back into traffic, the needle drops back to normal immediately. If I’m running the A/C, it will stop blowing cool air. Turning off the A/C has no effect, and the air blows cool again when the needle begins to drop. No lights on the dash come on to indicate a problem. Nothing showed up either when a diagnostic test was done. I’ve replaced the thermostat and gasket, and had the radiator flushed and coolant replaced. I’m trying to avoid the dealer. Any suggestions?

Your ECU does not monitor your engine for overheating and will not store a fault code for same.

The possible things that can cause an engine to run hot on the highway include:
a radiator getting coated with crud in it’s core tubes (common cause)
low coolant (common…but the cause needs to be determined
worn water pump (typically impellar erosion)
sticking thermostat (VERY common cause)
collapsed hose inner liner (rare, especially on an 11 year old car)
radiator clogged outside with leaves or a plastic bag or something of that nature
failed radiator cap (not as common IMHO as many people think)

Occasionally the root cause will be a blown headgasket, but you have a ways to go in the troubleshooting process before determining that.

Start by checking your coolant level. And its condition. If it’s cloudy, gumped up, or anything other than translucent and clean, let us know. That too can be a clue. Post your results and we’ll walk you through the troubleshooting process from there.

I slightly disagree with TSMB, which is kind of rare, but I would replace teh radiator cap first and see if the problem goes away. IMNSHO, it is more common than many people think.

I’m good with that.

While I agree, I would have a look at the front of your radiator for a build up of bugs. That can cause a diminished cooling capacity of the radiator after ten years. Its hard to see and harder to spray out because of the air conditioning coil in front of it, but it can make a big difference. Also after ten years, your radiator could be nearing the point of needing replacement due to gunk on the inside not allowing a good transfer of heat.

This seems to be an unlikely cause given the symptoms you report, but since it is simple to check, be sure that the cooling fan is coming on and working correctly. The cooling fan will usually come on with the car at idle (and not moving) for 5-10 minutes. Monitor the temp gauge as you do this.

One more thing. Did this start to happen after work was done previously involving the cooling system? If so, it is possible you have an air pocket somewhere. You might just need to have the cooling system air-bled.

Silly question maybe but the radiator fans do come on, right?

It can’t be your fan, you don.t need your fan at 50 mph or over. I once saw a car overheat at 50 to55 because the fan was running backwards after a collision repair but that is easy to check, when the fan comes on at idle the hot air blows forward thru the radiator. Same for a stuck thermostat- it wouldn’t overheat only at speed.
I think a new radiator is going to be your only cure.

Check to see if the air dam is missing. It is usually located directly under the radiator. The purpose of an air dam is to keep airflow under the car to a minimum. Car makers use it for the dual purpose of directing air into the radiator. They like to keep grille openings to a minimum, also for that better fuel economy.

If the radiator is mising some of the fins between the tubes, overheating can happen. Check the radiator from the rear. If you see a lot of white or green, don’t bother checking for fin condition, just replace the radiator.

Good one pleasedodgevan2, I forgot about the air dam. Its worth a check. Also, I forgot about the bleed screw on the thermostat housing, it the system isn’t bled of all air, it will overheat at speed too.

I a similar issue on a 95 Civic. They did a flow test on radiator and it was not flowing enough hence the near overheating at upper speeds. A replacement radiator solved it.

Modern radiators use aluminum cores and road-salt spray just eats them up from the outside…Inspect the outside of the radiator for damage…

raj suggests a flow test for the radiator. A simple substitute test would be to get the engine good and warm. Shut it down and remove the fan relay located in the underhood relay and fuse box (for safety), then feel the backside of the radiator for cool spots. It should be hot at the top and cooler near the bottom, but the transition should be gradual and even. If a section goes from hot to cool suddenly, then you have a restriction. Also expect it to be a little cooler just behind the fan blades, but just a little.

Modern radiators use aluminum cores and road-salt spray just eats them up from the outside…Inspect the outside of the radiator for damage…The thin foil fins must be bonded to the coolant filled tubes… Once they start to detach from the tubes, the radiator is junk…