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'03 Civic - coolant not returning from overflow container

I’ve owned this Civic since new, now at 153K miles. This is a recurring problem that I’ve been having for over a year. First symptom was there would be no heat coming from the heater at idle with the motor well warmed up. Add some rpm’s and the heat returned. The reason was not enough coolant in the system, filled up radiator and all OK. Then it happened again and there is coolant in the overflow tank, but pull the radiator cap and no fluid under cap. Fill radiator and all OK, but it kept happening. I replaced the OEM radiator cap with a new “Slant” brand for an '03 Civic. No better, no worse.

Last week I had a significant overheating event while on a highway. I pulled into a “Dollar Store” parking lot, went into the store and bought 50/50 coolant and shop rags and let the car cool off for 30 min. When I pulled the cap, it was a steamy mess and I let it cool some more. Eventually I put in the coolant and finished up the drive with no more overheating. The overflow coolant container was at the “full” mark all during the overheating event.

At this point I took it to my trusted mechanic. My mechanic determined that the overflow tank was clogged and he blew out the line. So, I hoped all was fixed. But I don’t think so. With the car cool this morning I pulled the radiator cap and got a swish noise indicating some pressure release. The level of coolant in the overflow contain hasn’t moved. It still seems coolant is not being pulled from the overflow container back in to the radiator when the car cools off. I blew into the radiator top (where the cap goes) and that did produce lots of bubbles in the overflow tank. It seems it is no longer plugged - but why isn’t the coolant behaving properly? I just ordered a genuine Honda brand radiator cap and I will try that next. Any other ideas?

The hose going from the radiator neck could be collapsing under vacuum from the radiator. Or maybe could be getting pinched when the hood is shut.

There could also be a leak in the hose to the reservoir, causing the system to suck in air instead of fluid from the reservoir when the car cools.

You probably have a small leak in your cooling system that allows air to be drawn into the system when the engine and coolant cools down. Have you had a pressure check on the system? How long did the pressure hold? You could also have a pressure test with the engine cold and the system drained i.e. just holding pressureized air. That would be a more thorough way to find a leak. You might even hear where the leak is located.

I suspect a leaking radiator tank, heater core, or water pump shaft seal. In any case the coolant is going somewhere when it is low in the radiator. A good running system should show a low reservoir level until the engine is at thermostat openning temperature and then the reservoir should be at Hot Max Have the mechanic check for combustion gases over the coolant at the radiator fill neck. Hope you don’t have a leaking head gasket.

Hope this helps.

Replace the radiator cap. It’s only a few dollars. It might not be the problem, but it can’t hurt to replace it anyway.

A new genuine Honda radiator cap is ordered and it will be a few days before I can see if it solves the problem. The hose to the expansion (or overflow) tank isn’t pinched or kinked anywhere, but it might be collapsing, and/or it could have an internal breakdown. The expansion tank is shaped and fitted into a relatively tight space (pretty much standard Honda stuff). To remove the tank and hose looks like I’d have to pull the battery for sure and perhaps some other stuff just to get the tank out to inspect it and clean it properly. If I get that far I’ll replace the old hose with a new one.

The suspicion of a head gasket leak is lurking in my head. No evidence of any coolant in the oil. The coolant in the expansion tank is clean, no fumes or bubbling in the expansion tank when the car is running is evident.

What I’ve been doing is using a siphon to take excess coolant out of the expansion tank and putting it back into the radiator. When I didn’t bother with doing this for a few months the overheating event took place. I wasn’t doing the drain and refill process because I saw fluid in the expansion tank and thought my problems were resolved, but that wasn’t the case.

I had a similar problem with my Ford truck. The overflow hose felt really loose at the radiator connection. I added a small hose clamp to the overflow hose at the radiator and that fixed it.

@UncleTurbo, although it’s true a Honda OEM radiator cap will last longer than an aftermarket radiator cap, you’re really supposed to replace the radiator cap every time you flush or change the coolant, so I’ve decided to save a little money by using aftermarket radiator caps. If you don’t want to wait, pick up a cheap aftermarket radiator cap and try it out. You can always save the OEM cap for your next coolant change when it arrives.

Classic symptoms of a blown head gasket…

The very first thing to do is to remove the overflow tank and flush it out. If the tank is the type with the tube that goes to the bottom of the tank hanging from the tanks cap, replace this hose. It may have cracks at the top that will allow high pressure air to blow through to the bottom, but will break the vacuum when the return draw is very small as in normal operation.

If it has the tube molded into the tank, then there could be a piece of crud in to bottom acting like a one way valve. You can only get rid of this by flushing out the tank upside down. The molded tube could also have a tiny crack near the top that would act the same as a bad tube hanging from the cap. The overflow tank would have to be replaced if that is the case, but I have not seen this actually happen, but it could in theory anyway.

You may need to replace the hose that goes from the radiator to the overflow too if it has any micro cracks in it.

Update - appreciate the helpful comments. I did get a genuine Honda radiator cap. Before installing it I removed the overflow tank, I found it was “clean” inside with no obstructions, and I replaced the tank with a new hose from the radiator to the bottom of the overflow tank. I put clamps on the ends of the hose to be certain of a tight seal.

Next day I ran the car for approx. 100 miles. It pushed fluid from the radiator into the overflow tank - raised the level in the tank about 1". Next morning, the level of the fluid in the tank was unchanged after sitting and cooling overnight. The fluid was not pulled back into the radiator as it should be.

Figuring that was about all I could do, I took the car to my mechanic for an evaluation and told him I suspected a head gasket breach. Hoping I’d hear something else, like a small leak in the water pump or something. This evening the mechanic confirmed a bad head gasket, there is exhaust gases getting into the coolant. He feels the 15 lbs of pressure isn’t enough to force coolant into the cylinders, but 150 lbs of pressure and combustion is forcing gases from the cylinders into the coolant somewhere.

Repair estimate to replace the head gasket is circa $1,000. The Edmunds TMV is $1,380 (trade-in) for average condition and $901 for rough. The body and interior are OK, but the motor issue puts it into rough condition IMO.

Now weighing options.

A $1000 is not a bad price. Don’t just look at the given value of the car, but also consider the cost of replacing it. Can you find a car that would be in as good a shape once the BHG is replaced for that much? Or is your confidence gone with the head gasket? If you feel it will not be reliable again, maybe cut your losses now.

The car runs fine at least for now. To keep using it as is means taking the fluid that collects in the overflow tank and returning it back into the radiator, perhaps once a month. I can rig up a turkey baster with a long tube on the end and that should make the process pretty easy and not messy.

I have relatively new summer tires, and also just as new winter tires for the car. If it were to get really bad I have other cars that I can use, just nothing that gets near the 35 mpg of the Civic. One option is to simply drive it as is to use up some of the money spent on tires, brakes, etc.

I have a friend who used to wrench for a Honda dealer back in the ''80’s. He looked at the motor and said it is a bit rusty which will add some extra work to the job. Problem is he lives about 5 hours away. A DIY job is an option, but I feel I’d need an experienced helper. I do have a shop manual, which I plan to look in a few minutes.

The $1,000 isn’t a firm quote, but my mechanic did a similar job for $1,300 but he also did the timing belt and water pump as part of that job.

Yep, keep driving it. But you might want to check it every week. It will get slowly worse.

Roll the dice:

good video, thanks.

Before you give up, remove the radiator cap and the overflow hose from the port under it. Then use something like a small nylon wire tie or a piece of wire and poke it through the port. This is just in case there is a piece of debris inside that port that is acting like a one-way valve.

as extra insurance, drop in a can of $30 miracle gasket fixer. it could seal the leak and allow you to drive for a couple more years without the cost. keep in mind you need to drain after a couple hundred miles, and you should put a new thermostat in when throwing the new fluid in. just a cheap fix to buy time/not recommended for permanent fix.

Thanks all. I decided to go ahead and have the head gasket repaired properly. Use of a stop leak agent didn’t look like it would work. This particular problem was not coolant leaking into the cylinders, rather exhaust gases being pushing into the coolant. It seemed the pressure was working against the stop leak agents and they weren’t likely to plug the leak and then create a bond to stop the leak for long if at all.

I looked at new cars, and just didn’t see any manual cars that I liked enough to deal with monthly payments. I’d hoped to put about 250K miles on this Civic and at 155K now I decided to make the repair and stick with the plan to run it for another 5 to 10 years.

Is that a good decision? Only time will tell.

Ouch…You could dope it up with some canned head gasket repair and drive it until it will drive no more…Also, pull the valve cover and re-torque the head…Can’t hurt anything…