# Mechanics: Coolant Leak Only During AC Operation **URL:** \ **Category:** In My Humble Opinion\ **Created:** [September 30, 2019, 8:30pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062 "2019-09-30T20:30:02Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [September 30, 2019, 8:30pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/1 "2019-09-30T20:30:02Z")
Sorry if the title was at all cryptic, but as we all know title space is limited. I have a 1999 Ford Ranger. She’s been a mostly reliable steed but given her age she is starting to have some difficulties. Mostly recently, I have noticed a tendency to run hot when the AC is operating and to drip not just water when using the AC but also some radiator coolant. Over the past week I have kept careful observation both before and after operating, and with and without AC while monitoring the coolant reservoir. Now, don’t get the idea anything is _gushing_ or anything, this is not a fast or super obvious leak. I noticed it largely because I pay attention to my vehicles (which might be one reason they run as well as they do at their ages). I am very, very, very sure it is NOT leaking coolant when the AC is not used. I do not find evidence of leaking after it has sat overnight (I have left it for three days in the same spot, no evidence of leaking of anything underneath). It does not leak after driving when the AC is NOT used. When the AC is used, the coolant drip is roughly equal to the water drip, so it’s not a LOT of leak, but very definitely I have one spot of coolant and one spot of water under the truck when using the AC. Well, _obviously_ the first thing to do here is _stop using the AC_, which I have done. Then I topped off the coolant fluid - probably didn’t _actually_ need it, but the fluid had gone down slightly and I figured, what the hell, I keep reserve fluids for the vehicles for just such an occasion. No sign of coolant in the motor oil (which seems to be a common question when I describe the above). So, with the disclaimer that you are not my mechanic and you may be reluctant to [del]diagnose the patient[/del] commit too strongly over the internet, could the Doper mechanics throw out a few guesses as to what might be going on here? Loose hose? Gasket leak? Impending zombie apocalypse? Yes, I will take it to a meat-space mechanic in the near future, I just like to do a little research before doing that on general principles. I’m assuming I can continue to use it for short trips, but might just park it and use my car until I can get the problem looked at and taken care of, again on general principles. The only downside being that the car is going to need rear bearings soon. (I can cover the cost to do that immediately, but I was hoping to use the truck for a month or two to save up a bit more more as September had some unplanned for bills and I’d like my savings account to rebound a bit before tapping it again, if I can. Safety first, though) I had hoped to drive mostly the truck for month or two before getting those bearings done. If it matters, most days I’m driving less than 15 miles total and 40 mph or less. Anyhow, input appreciated. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [September 30, 2019, 8:37pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/2 "2019-09-30T20:37:05Z")
First question, maybe an obvious one…is it possible it’s only leaking coolant when you run the AC because you’re only running the AC on hot days and/or longer drives? I’m gonna WAG, that it’s a small enough amount that you simply don’t see it dripping on days that it’s not flowing out with the condensation from the AC. A couple of drips of coolant from the heater core and they might just hang around inside the box where the core is mounted or drip away while you’re still driving. But those same few drips, mixed with all that water and suddenly you can see that coolant colored sheen under the car. Next time you have the car up to temp, turn on the heater and see if you smell it and/or notice more of it dripping when you park. (ETA, I’m actually a bit surprised you don’t smell it when you use the AC or even just the vent, unless you do smell it and didn’t mention it) Also, it’s entirely possible the extra strain on your engine from the AC compressor creates just enough extra heat to start pushing coolant out of whatever small hole it found. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [September 30, 2019, 8:42pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/3 "2019-09-30T20:42:10Z")
> [@Broomstick](#): > > No sign of coolant in the motor oil (which seems to be a common question when I describe the above). I’m not sure why that would be a common question given your situation. That’s usually a common question if you have billowing white smoke or you’re losing coolant but can’t find the leak. But it should be noted that if the leak is small enough and you drive it for long enough at a time, you won’t see signs of coolant in the oil since it’ll burn off. There are ways to detect it, but since you have a visible leak, it doesn’t seem necessary to worry about it. ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [September 30, 2019, 8:59pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/4 "2019-09-30T20:59:57Z")
First be sure of the integrity of the hoses. A very small leak like that could be a crack in a hose or a leaky fitting. A quick and easy check for even a non-mechanic would be to operate the truck under the conditions where it usually have the small leak and then turn it off. Then take a small rag or paper towel and run it around the bottom of the hoses, the fitting/clamps on the hose ends, around and under the air conditioning pump, etc, and see if you pick up any moisture on the towel. Probably a small hose leak or a gasket leak on the A/C pump area. ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [September 30, 2019, 9:09pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/5 "2019-09-30T21:09:22Z")
> [@Joey\_P](#): > > I’m not sure why that would be a common question given your situation. That’s usually a common question if you have billowing white smoke or you’re losing coolant but can’t find the leak. > But it should be noted that if the leak is small enough and you drive it for long enough at a time, you won’t see signs of coolant in the oil since it’ll burn off. There are ways to detect it, but since you have a visible leak, it doesn’t seem necessary to worry about it. I am not a Ford guy, but there is a Ford engineer on this board, hopefully he will see this thread. Some vehicles have the oil cooler, or power steering, or transmission cooler inside the radiator. My LS1 GM engine has the transmission cooler inside the radiator, which can be a problem. That is why the question is commonly asked. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [September 30, 2019, 9:20pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/6 "2019-09-30T21:20:32Z")
I just reread the OP. Broomstick mentions that she notices water and coolant and I’m reading that as coolant coming from the condensate drain, but maybe I’m wrong. Broomstick, are you seeing coolant coming from the condensate drain or two separate drips, one condensate from the AC and one coolant. If it’s two distinct puddles then it’s not the heater core like I was thinking (which would also explain why you didn’t mention the smell). In that case, ignore what I said above but now I’d want to know where the coolant puddle is under the car. But again, if it’s leaking on the ground, it’s not going to be mixing with the oil. Or, I guess it could be mixing with the oil, but that would be a different issue. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 30, 2019, 9:28pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/7 "2019-09-30T21:28:50Z")
IANA mechanic, and I may have missed something in reading. Where is it leaking from? I think most cars have a tank to collect overflow from the radiator when the coolant heats up and expands. I think it is recycled from there, back into the radiator. Is the tank leaking? Is the hose to the tank leaking? As to why it leaks when the a/c is running, it might be that that’s when the engine is running hot enough to cause coolant to overflow. How is your thermostat? Is it functioning correctly? Should you install a cooler one? ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [September 30, 2019, 9:49pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/8 "2019-09-30T21:49:46Z")
> [@Joey\_P](#): > > First question, maybe an obvious one…is it possible it’s only leaking coolant when you run the AC because you’re only running the AC on hot days and/or longer drives? As I said, most days I drive 15 miles or less. I haven’t had a drive longer than that in several weeks, and I did not notice either leaking or dropping coolant levels or rising temperatures on/after that drive. (Airplane pilot habits - I frequently do a walk around prior to using my vehicles and also frequently afterwards) So it’s not longer drives doing it. Hotter days, yes - it was hotter than it has been for awhile so I was running the AC today after not doing so for a week or two. > [@Joey\_P](#): > > I’m gonna WAG, that it’s a small enough amount that you simply don’t see it dripping on days that it’s not flowing out with the condensation from the AC. A couple of drips of coolant from the heater core and they might just hang around inside the box where the core is mounted or drip away while you’re still driving. But those same few drips, mixed with all that water and suddenly you can see that coolant colored sheen under the car. No, it’s two separate drips, not a combined flow. Two distinct and separate wet spots, one clear and one green. > [@Joey\_P](#): > > Next time you have the car up to temp, turn on the heater and see if you smell it and/or notice more of it dripping when you park. (ETA, I’m actually a bit surprised you don’t smell it when you use the AC or even just the vent, unless you do smell it and didn’t mention it) Nope, don’t smell it. On the other hand, the stench of hot asphalt during part of the drive while passing a construction area so of drowned out everything else during part of my trip, and it’s allergy season so my sniffer isn’t working that great right now. > [@Joey\_P](#): > > Also, it’s entirely possible the extra strain on your engine from the AC compressor creates just enough extra heat to start pushing coolant out of whatever small hole it found. I was wondering about that. Clearly, running the AC does something to generate more heat under the hood as the radiator temp always rises slightly when I use it. > [@Dallas\_Jones](#): > > First be sure of the integrity of the hoses. I’ll check that out. > [@Dallas\_Jones](#): > > Probably a small hose leak or a gasket leak on the A/C pump area. I’m hoping that’s a relatively inexpensive repair? That does make sense. > [@Joey\_P](#): > > I just reread the OP. Broomstick mentions that she notices water and coolant and I’m reading that as coolant coming from the condensate drain, but maybe I’m wrong. Broomstick, are you seeing coolant coming from the condensate drain or two separate drips, one condensate from the AC and one coolant. If it’s two distinct puddles then it’s not the heater core like I was thinking (which would also explain why you didn’t mention the smell). In that case, ignore what I said above but now I’d want to know where the coolant puddle is under the car. Two separate drips. OK, imagine you’re sitting inside the truck, in the driver’s seat, and all directions will be in reference to that position. The place that AC condensate drips onto the ground is inboard from right front (passenger side) tire. That is as it has been for 20 years. The coolant drip is a separate and distinct drip from inboard of _that_ maybe 2-3 inches closer to the center of the truck. > [@Joey\_P](#): > > But again, if it’s leaking on the ground, it’s not going to be mixing with the oil. Or, I guess it could be mixing with the oil, but that would be a different issue. It’s possible to have more than one leak or problem at the same time, right? > [@Johnny\_L.A](#): > > Where is it leaking from? I think most cars have a tank to collect overflow from the radiator when the coolant heats up and expands. I think it is recycled from there, back into the radiator. Is the tank leaking? No. The coolant reservoir tank is not leaking. It is, very helpfully, marked with lines to make it easier to monitor how much coolant is in it. That, along with a lack of drips, is one of the reasons I know it is not leaking when it’s not running. > [@Johnny\_L.A](#): > > Is the hose to the tank leaking? Admittedly, I have not crawled under my truck to check that out in detail (yet). But it’s possible. > [@Johnny\_L.A](#): > > As to why it leaks when the a/c is running, it might be that that’s when the engine is running hot enough to cause coolant to overflow. How is your thermostat? Is it functioning correctly? Good question! How would I know? I’m not a mechanic, either. In twenty years only one time has it ever overheated. Other than that, all _seems_ well. Note that when I say the temperature rises it is getting _nowhere near_ the top end of the idiot gauge. It’s not even getting a quarter of the way up the scale. Due to long years of observation I know where that needle usually sits and only because of that knowledge do I know that it’s running hotter than it used to, and only slightly so. But yes, it is possible the instrumentation and gauges on the truck are getting old/less accurate/etc. That is one reason I pop the hood once a week and double check everything I have the knowledge to check, and do so a bit more thoroughly around the first of the month (oh… look at that… tomorrow is October 1…). Which, again, is probably why I noticed this before my truck started vomiting rather then slightly drooling coolant. If we have a Ford guy on the forum I, too, am hoping he checks in. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 30, 2019, 9:56pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/9 "2019-09-30T21:56:56Z")
> [@Broomstick](#): > > > [@Johnny\_L.A](#): > > > > As to why it leaks when the a/c is running, it might be that that’s when the engine is running hot enough to cause coolant to overflow. How is your thermostat? Is it functioning correctly? Should you install a cooler one? > > Good question! How would I know? I’m not a mechanic, either. I found this: [How to Check your Car’s Thermostat](https://www.desjardinsgeneralinsurance.com/blog/-/how-to-check-your-car-s-thermostat). ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [September 30, 2019, 10:02pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/10 "2019-09-30T22:02:39Z")
I’m going to put my guess in as a heater hose. Either a bad hose or bad connection, but a few inches towards the center from the AC drain would have heater hoses right over it. You’re taking it, so it doesn’t really matter. If that’s what it is (or any hose for that matter) it’ll take them just a few minutes to do it. They’ll hook up a pressure tester (easy to use and you can rent them for free at most auto parts stores), put a few pounds of pressure into the system and look for where the water is squirting out. It’s a lot easier than warming up the engine and trying to find the source of steam. As for why it only happens when the AC is on, I’m going to guess it’s a coincidence. That is, there’s nothing mechanically connecting the two systems that using one could cause the other to leak. The AC makes the engine work harder and hotter, the AC condenser coil sits in front of the radiator so it can add heat to the radiator so from that POV, using the AC could add heat to the coolant, but it’s not like turning the AC on opens a relief valve for the coolant. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 30, 2019, 10:17pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/11 "2019-09-30T22:17:13Z")
My bet: water pump. ---
**Author:** ![Iggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iggy/32/3364_2.png) [@Iggy](https://boards.straightdope.com/u/Iggy)\ **Post date:** [October 1, 2019, 2:03am UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/12 "2019-10-01T02:03:41Z")
IANAM but I had a 1999 Ford Ranger that developed a coolant leak. IIRC, activating the A/C closes some sort of valve that lets a loop of coolant go back to the heater corein the other side of the firewall to provide heat to the passenger compartment. If the A/C is not on I _think_ that valve opens allowing coolant flow through that loop. My guess is the OP’s leak might be from a fault in that loop of hose of the valve itself somehow? Only thing I can think of that directly connects the A/C and coolant system. ---
**Author:** ![DorkVader](https://avatars.discourse-cdn.com/v4/letter/d/73ab20/32.png) [@DorkVader](https://boards.straightdope.com/u/DorkVader)\ **Post date:** [October 1, 2019, 10:41am UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/13 "2019-10-01T10:41:09Z")
One thing I discovered, and this is not make/model specific, is that after time, where the radiator hoses connect, between the hose and the component itself, there is a formation and buildup of “gunk” that can cause intermittent leakage such as described. This can happen even if everything else is good. It probably wouldn’t hurt broomstick, if your mechanic, in the course of diagnosing and repairing the leak, serviced the system with a drain and removing the hoses to check and clean with a light emory paper the places the hoses connect to. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 1, 2019, 11:32am UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/14 "2019-10-01T11:32:40Z")
> [@Broomstick](#): > > Mostly recently, **I have noticed a tendency to run hot when the AC is operating** and to drip not just water when using the AC but also some radiator coolant. (bolding mine) that’s key right there. Your problem is overheating. The A/C condenser sits right in front of the radiator, and the condenser heating the air before it passes through the radiator means the radiator can’t get rid of engine heat as effectively. the coolant dripping may be because when it runs hot excess coolant is pushed out of the radiator and there’s a small leak somewhere in the overflow/recovery system. So, knowing that here are a couple of possibilities: - the radiator tubes are getting clogged with gunk. since your truck has the older green silicate-based coolant this is a possibility; the corrosion-inhibiting silicates can “fall out” of solution with age and build up on surfaces. With the A/C off there’s still enough flow through the tubes to keep the engine cool, but the additional heat load from the condenser may be too much for it to handle. - the fan clutch may be bad. the cooling fan driven by the engine has a thermostatic clutch on it; it’s supposed to engage when it heats up and the fan should pull more air through the condenser and radiator. if the fan clutch is not engaging properly it won’t pull as much air, and again it’ll be worse with the A/C condenser adding heat. note- this would only be a problem at low road speed, since once you get up to about 35-40 mph there’s enough “ram air” being forced through the radiator to make the fan less of an issue. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 1, 2019, 11:40am UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/15 "2019-10-01T11:40:20Z")
missed edit window: I misspoke above; it’s not “excess” coolant, it’s that as it gets hotter the rise in pressure means the radiator cap has to open to regulate the pressure, directing some coolant back to the overflow tank. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [October 1, 2019, 6:01pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/16 "2019-10-01T18:01:34Z")
The only difference to the engine cooling system when the A/C is operating is that hotter air is being blown across the radiator. This is from the A/C condenser which is positioned directly in front of the radiator, getting rid of the heat the A/C has removed from the passenger compartment. Presumably this additional heat is enough to cause some marginal joint to change from not-quite-leaking to leaking-just-a-bit. At the top of the suspect list are coolant hose connections, possibly a hose clamp that’s not quite tight enough. There are other possibilities, and a cooling system pressure test combined with visual inspection is the standard method to find the leak. If the temp gauge is showing noticeably hotter with the A/C on, there is the possibility that the coolant level is low (again not the only possibility). To check for this it’s necessary to remove the radiator cap (ENGINE COLD ONLY) to see if the liquid is up to the top of the radiator. The level in the overflow jar is **NOT** a reliable indicator of the level in the system as a whole. If the system level is low, and topping it up results in it not running hot with the A/C on, the only problem is the leak. If it still runs hot, there’s a further problem to find. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [October 2, 2019, 12:32pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/17 "2019-10-02T12:32:28Z")
Why not find the source of the coolant leak and just fix it? An $8 dose of UV dye and a $6 UV flashlight will quickly locate it. Perhaps when you find the leak, the mysterious connection to your AC will become self evident. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 2, 2019, 1:52pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/18 "2019-10-02T13:52:28Z")
> [@beowulff](#): > > My bet: water pump. I’ve had the water pump replaced in the Jeep and the Prius. I didn’t notice any leaking or sounds from the Prius (the dealership – the one I trust – said it was failing), but the Jeep’s water pump was very noisy as it began to fail (no leakage). ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [October 2, 2019, 7:37pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/19 "2019-10-02T19:37:48Z")
> [@ZurBob](#): > > Why not find the source of the coolant leak and just fix it? Well, sure, I even stated at the beginning it was my intention to get things fixed, I just like to do a little research first. The responses here have been helpful, so thanks everyone. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [October 2, 2019, 9:07pm UTC](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062/20 "2019-10-02T21:07:56Z")
I would check to make sure the radiator fan runs with the A/C on. Next thing I would consider is the radiator cap and also cleaning the radiator. [Next page](https://boards.straightdope.com/t/mechanics-coolant-leak-only-during-ac-operation/841062.md?page=2)