Hot day + air conditioning = coolant leak at new overflow cap

98 Ford Taurus…

  • Car overheats and coolant leaks from overflow cap in hot weather, when air conditioning is turned on.
  • Overflow tank and cap is new.
  • Coolant leaks if I’ve added it up to the fill line. Leaks occur soon after parking.
  • Coolant always looks fine, no changes in color or consistency.
  • Radiator fans DO turn on when A/C is turned on.

Last year I noticed that my coolant was leaking somewhere around the overflow tank. The level would stay low even after adding and the car would overheat in 90 degree weather. I found that there was a large crack in the tank, so I got a new (used) one. The problem persisted and I determined that the coolant was purging from the cap, so I got a new one. The new cap did not seem to fit snugly, so I got yet another new one. By this time, it was fall and even though the coolant level would stay well below the refill line in the overflow tank, the car would not overheat.

Now it’s summer again and the car overheats in hot weather when the air conditioning is on. Also, if I add more coolant, it will again purge from the cap until there is only about an inch in the reservoir.

It’s almost as if the pressure is too great for any cap (rated 15psi). The car is not exhibiting any other symptoms.

Any suggestions for a novice shade-tree mechanic that does his best to solve problems on his own?

Thank you in advance!

Try cleaning your radiator and A/C coils. You may have a bunch of stuff blocking/partially blocking the airflow.

This has electric fans, too, right? They should be on when the A/C is on.


They are in good shape because my “car bra” has a fine mesh over those parts.

There are loud fans that you can hear kick on when the AC is turned on.

Try taking the bra off and see if that helps. Are you sure you are filling the radiator and not just the coolant tank? If neither of these help have the rad. pressure tested and if that is ok have someone use a temp sensor gun to see if the radiator has cold spots (plugged).

I had the bra on in Florida for years, so that’s probably not it, but I’ll try it anyways.

I am filling the (plastic) overflow coolant reservoir, and I’ve only ever used the premixed 50/50 coolant.

Going off of oldtimers hint, I’m going to try bleeding the coolant via this tutorial:

Air in the system is not a problem I have heard about before, but it makes sense…

After setting over night, removed coolant overflow tank cap, filled up tank to cool level, turned on car, turned on heater with fan on lowest setting. Waited 10 min. No bubbles in tank. Turned on A/C, waited a few minutes, saw bubbles in tank. Let A/C run for about 15 minutes, at which point I no longer saw bubbles and switched back to heat for a while; still no more bubbles. Replaced cap, turned off car. Watched coolant level rise a bit. It was not too hot here today, so I could not stress test it, but I will throughout the week.

Something else I should note: when the A/C is switched on, the RPMs go from about 500 to 750 and constantly fluctuate between the two. I also noticed a “clicking” sound when I was under the hood, coinciding the with RPM changes. This clicking stopped after the bubbles ceased, but I’ll check for it again throughout the week. The radiator fans do come on when the A/C is on.

Also, my car does not have a dedicated radiator cap - at least one that is readily accessible. The manual describes adding coolant via the overflow tank.

So, I have no idea if I’m out of the woods yet.

Every year I back flush my cooling system (I do not think this could hurt) and spray green clean in all cooling radiators then hose them out of course the engine and all needs to be cold?

"I had the bra on in Florida for years"
Have you ever cleaned the radiator and condensor coils, then? Those lovebugs get everywhere. I spent quite a few hours after driving back right after a hurricane went through, and days after that cleaning...once we had water again.

The clicking is your A/C compressor turning on, and the engine RPM’s will slow a bit, then kick up because of the extra load. When the compressor shuts off, the idle settles back down. It’s pretty common at idle. Most don’t notice it, because they aren’t sitting with their head under the hood. Probably why you never really noticed it before. (In general) the newer the vehicle, the less obvious that is.

The cap on your overflow tank is your radiator cap. It’s just a remoted one. Most vehicles are going that way now (most, except trucks it seems). The radiator itself is sometimes sealed, except for the hose connections (yes, and fan switch, etc). Nothing there for you to open unless you’re doing real maintenance, not routine stuff.

Thank you chaissos. I’ll figure out how to get the front off and hose down the coils this weekend.

Today, in the 95o sun, the engine began to overheat again - until I turned off the A/C…

I don’t think I’ve lost any coolant - I’ll check in the morning.

Today (friday), after running the AC in the 95o heat, a lot of coolant purged out from under the cap soon after parking.


Yesterday I installed a coolant flush-valve thing to my heater inlet hose so that I could attach a garden hose to flush the radiator. So I flushed everything a couple times - the liquid coming out of the coolant drain port went from yellow to clear, no signs of rust or anything. I then filled it up with a coolant cleaning solution (Peak brand). The directions said to drive around for 3-6 hours, so today I took it out.

It was in the 90s again today, and I tried the air conditioning for a while. At one of the stop lights I could smell coolant, and sure enough, a lot of it had purged out. I left the AC on (shouldn’t have), but after a few minutes the temp gauge began to climb and my obd2 scanner went from 195 (normal) to, I believe, over 270. I pulled the car over. While jostling some hoses under the hood, one broke. Actually, a large 4-way adapter broke. Got a new one and replaced it. Flushed the coolant again using the garden hose and this time the liquid was brown (rust?). Continued flushing until clear. Filled it up with regular coolant and tested it in the driveway (everything normal).

Even though I cannot find any mention one one in my car manual or in the Chiltons manual, it looks like my car has two air-bleed valves for coolant. One is on a tall, rigid hose next to the overflow tank. When I took the cap off of it air started coming out. It would alternate between air and (cold) coolant-steam while the car was running (with the heater on). I figured that once all the air was purged it would start spraying coolant, but after maybe 5 minutes, it still just kept alternating between air and cold steam… Near the firewall is another valve that has coolant stains around it, but even though I could see some coolant when I took the cap off, it did not do anything.

Another thing, and this is a question, is that I noticed that only one of the radiator fans was on. I turned the car off, spun the fan manually (it spun freely) and turned the car back on. This time, the fan did come on, but was not turning as quickly as the other fan. Could this the source of all the problems?

So, I believe that that there are three possible culprits for my problem, either there is air in the coolant lines, maybe near the pump, in which case… what do I do, leave the cap off of the purge valve for even longer? Or the blades in my water pump are in terrible shape (150k miles); or one of my radiator fans isn’t working as well as it should.

What do you good people think?

I read this in another forum:

"the bigger fan is the coolant fan(passenger side), the smaller is the A/C (driver side) "

In my case, it was the driver side fan was not on (they are the same size, I think). However, I will test later today to make doubly sure that it does not come on when the AC is on. Maybe it’s fine, maybe it’s the relay (cheap fix!), who knows.

I have the same exact issue and have been trouble shooting just as you have. I tend to not use the A/C because of this issue. Did you ever find the cause of this issue?

Since it’s been 9 years on this thread, you may have better luck getting suggestions if you click on “+New Topic” and tell us your car’s make/model/year/miles.