1998 Honda Civic Overheating...then back to normal!


I am looking for some advice on my 1998 Honda Civic EX. This has been a great car and have got a ton of good miles on it. The problem that I am having now is with the temperature gauge. The measurement is completely all over the place while driving. While moving around town, the gauge shows the temperature VERY hot. As soon as I reach a consistent speed, the temperature drops back to normal (moves very erratically!). The gauge seems to get even hotter while idling.

Another issue (may or may not be related) is the heat coming out of the vents is only working while the car is in motion. While stopped, it pumps out cold air. The temperature immediately changes when I get going again.

Both problems seem to be getting worse.

The car has coolant, and all other levels of fluids are normal.

Any ideas??

Thanks for the time!

Eric T

This sounds like you either have a low coolant level or air in the cooling system. I would see a good independent mechanic and have this checked out. Simply checking the coolant level at the overflow container is not sufficient.

Your temp gauge might be accurate. If so, you don’t want to overheat the engine. This means you need to figure out what is going on.

One possible is the electric cooling fan isn’t coming on. This would result in overheating at idle and at slow speeds in town. At “consistent” speeds there would be air moving by the radiator to provide adequate cooling.

Another possible is a weak and failing water pump. Again not enough circulation at lower rpm’s but ok at highway speeds.

As another reply said your coolant level could be low, meaning there might be a coolant leak somewhere. Or, if someone had worked on the car recently perhaps they didn’t get all the air out of the cooling system.

Is there coolant in the overflow tank? Have you added any coolant?

Don’t keep driving the car without checking the coolant level and adding coolant if needed. Again, overheating this motor can result in expensive damage to the engine.

TEXTBOOK…AIR IN YOUR COOLING SYSTEM. Look for your top rad hose…follow it to your engine. It connects to the thermostat housing…on that housing will be a bleed nipple…or should be the bleed nipple. Get vehicle warm and crack open the bleeder and burp the air out. This will fix it…BUT how did you get air in the system? Did you recently gave to add coolant? Let us know… Your answer will dictate where I/we go from here

Wow…thanks for the help! I have not had any recent service and there is coolant in the overflow. I have not recently added any because there is already some in the reservoir.

Thanks for all of the advice!

Yes, there is coolant in the overflow. No one has worked on the car in ages (except an oil change 3000 miles ago). Maybe the fan?? Without the fan, would you be able to physically watch the gauge go up? It happens that quick as you slow down.

Thanks for all the help!

The resevoir is not an indicator of an air bubble in your system…You ureally need to bleed that out.

Are you sure there is coolant in the overflow tank? The reason I’m asking is that in my Civic (an '03) the overflow tank is hard to see. Also the tank can be dirty and it might look like there is fluid in it because it has a stain or dirt coating the bottom of the tank. It is like a “bathtub ring” making a darker area that you see even if there is no fluid in there.

If you can pop the top off the overflow tank and see the level of the fluid through the fill hole, then disregard.

I would go one step further and find out why there’s air in the cooling system.

I know you know this Blackbird, but for the benefit of the OP I’ll explain.

Thye coolingg system works thusly. When the coolant becomes hot to the pressure that the radiator cap is rated, typically 15-16 psi, the cap allows the coolant to flow past the cap and into the reservoir, regulating the pressure of the coolant in the engine. When the engine cools again, the coolant contracts and draws coolant back in from the reservoir, again through a mechanism in the radiator cap, preventing air from enetering the system.

If you have a leak in the cooling system, the system when it contracts canb craw air in through the leak rather than coolant from the reservoir. That leaves fluid in the reservoir but creates an air pocket in the engine where the hot coolant had leaked out.

A second way air (enhaust gasses actually) can get into the water jacket is by being blown in by the combustion process…via a headgasket breech, common called a “blown headgasket”.

Start by doing a pressure check on the cooling system. If you’re unable to find a leak, do a pressure leakdown test on the cylinders. The air got into your engine somehow. You need to figur eout how. Post back with the results.

But don’t rely on the overflow tank. When cold, also open the rad cap. Is coolant all the way to the top?

There may or may not be a bleed nipple in the cooling system, and it may be inaccessible. Another way to bleed air out is to raise the front on ramps, set the heater temp on Hot, open the rad cap, and start the engine and let it idle until quite warm. Catch any overflow coolant and dispose of properly (or put back in as the level goes down.) Do not leave it out for animals to sip! Once level at open cap has stailized, shut off engine and top up rad.

Head gasket leak seems like a real possibility. There are various ways to diagnose that. It is serious and can ruin the engine.


Don’t hit the panic button here, the solution could be inexpensive and simple. Start with a new radiator cap, its due, its not expensive and a leak down test does not include the cap as part of the test. Having said that, I not sure its the complete solution. It can be the source of the air bubble if you have one. One more indicator of a bad cap is a temperature rise while going up long grades or while doing high speed driving.

Over heating at idle is usually either the water pump or the fan relay or the fan motor. If you can, idle the motor until the temp gauge goes above normal and open the hood and check the fan. There are two fans under the hood if you have AC. If you test as above, only one fan will come on. Another test is to turn on the AC, then both fans should come on.

If both fans come on with the AC test, but neither come on with the idle test, then the fan switch is at fault.

I’m afraid to say that I think your fans are going to pass the tests though because the heater stops working at idle. Now the air bubble could be the cause of this if some of the air is trapped in the water pump. If you replace the radiator cap and bleed all the air out of the system and the fans pass their tests, then the water pump would be next on the list.

I’m afraid that replacing the water pump in this engine is a big and expensive deal as I believe it is driven by the timing belt. Now your timing belt has a 7 year, 105k mile life on it. If you haven’t done the second change on it due to miles, 210k miles, then you should be about due based on time alone.

The next step would be a pressure test to insure the integrity of the head gasket, probably good, and if so, schedule a timing belt service with a Honda mechanic or a dealership and have the water pump and oil seals all changed at once. This would solve your problem if the simple thing above don’t, and you should be good, with other proper maintenance, for another 7 years or 105k miles.

BTW, many dealers have a 14 year/210k mile package deal for all of this at a very competitive price. In this case, don’t assume they will be the most expensive. This job does require a special tool and some Honda specific knowledge. It ain’t like other engines.

WOW!! Thank you all for the great knowledge! It turned out to be an air bubble in the cooling system. Although I haven’t recently done any work on it, some how it got in there! Once leaking it out, the problem went completely away. Thanks to everyone who helped!!

Before you celebrate however…You need a firm answer on HOW did you get air in the system. The best answer is that your thermostat stuck a little bit when it was cold out and forced some coolant out the overflow…creating an air bubble…and there may not have been enuf coolant in the overflow to suck back into the rad. This and a simple leak would be best case.

The answer you dont want to give is that the air bubble just “appeared” That would indicate a leaking head gasket due to a faulty head gasket. Then it is either new head gasket time or time to do the Blue Devil Treatment. Find out why this happened or you may be back to square one in a hurry. Glad we could help

I think I may just wait to see if the problem comes back…last time it came on very slowly. Thanks again for all the feedback!!