Engine runs hot only on highway when AC is on

Dino Dino Bambino · Jul 15, 2007

Since the summer temps. over here reached 110+*F, I've been finding that the engine runs hotter (the gauge needle on the L of NORMAL or ~222*F) when I'm driving on the highway with the AC switched on. If I switch the AC off (uncomfortable in this heat), the temp. gauge drops back to the M or NORMAL (an acceptable 208*F). In slow traffic, the gauge doesn't go higher than 208*F and the low speed fan works fine.
The cooling system is stock except for the t'stat being removed 3 months ago (it was stuck part-closed). The radiator cap is a new 16psi unit and it holds pressure fine. There's no coolant loss that I can detect. I flushed the system a while back and even left it for a couple of days with a 50/50 mixture of water/vinegar to clear it out. I then refilled it with a 30/70 mixture of coolant/water. The drivebelt is brand new and is properly tensioned.
I'm trying to put my finger on where the problem might lie. The symptoms were present even before I installed the ASP crank pulley and this made no difference. The AC blows cool air and works very nicely so I hate to switch it off to cool the engine down, but I'd rather sweat like a hog than damage the engine.
There are two likely possibilities that I can think of for the engine running hot. The most likely, I think, is that the stock radiator isn't man enough for the job. The other possibility may be the high speed fan not working but surely that wouldn't matter because there should be sufficient airflow into the radiator at highway speeds.
Can anyone offer any ideas? I hope HISSIN50 chimes in 'cause I know he's the cooling/electrical system expert here.

SeventyMach1 · Jul 15, 2007

Check that the radiator air dam thing is still present on your car. I had similar problems before. Someone said that to me, so I went out and checked. Sure enough, there wasn't one.

It does make a HUGE difference in cooling.

cjones · Jul 15, 2007

first off you should have a thermostat in the car...the water pump/cooling system needs the resistance to fill the entire block and heads with coolant and to get proper contact the them. 180* is what most of us run, at least in AZ.

have you tried Water Wetter??

you're correct that at highway speeds you should not need the cooling fan.

Blackened302 · Jul 15, 2007

+1 for both the thermostat and air dam.

the air dam is about $13 from 50resto, and a good Mr. Gasket 180* thermo is about $10--both are well worth the little cost and will help w/ your cooling issues.

reddy351 · Jul 15, 2007

cjones said:
first off you should have a thermostat in the car...the water pump/cooling system needs the resistance to fill the entire block and heads with coolant and to get proper contact the them. 180* is what most of us run, at least in AZ.

This is correct.....kind of. When a thermostat closes, it keeps the water that is in the radiator so that it can cool down. When there is no thermostat, the water just rushes through, not having a chance to properly cool. When I read the title of your thread, I thought "T-stat locked open." Then in the post, you said that the thermostat was removed. Same net result.

Get a thermostat in there! Quick like! Use a Ford one. Not a "discount parts store" one. Use the temp that Ford recommends (192?), or the ones that the guys in your area are "having no problems" with. (180, from the sound of it).

Post here, the result. Good luck!

mustangsammym · Jul 15, 2007

ditto on needing the thermostat the backpressure is needed for proper cooling

i had the same problem with my engine overheating with the ac on at highway speeds
my problem was my cooling fan plug was not making good contact because it was getting hot and burning up the terminals in the plug.
just open the hood when it is running hot and wiggle the plug on the cooling fan
careful its hot!!!!
if the fan comes on then that is your problem.

Dino Dino Bambino · Jul 15, 2007

I have an OEM 192 t'stat in my spare parts bin so I'll drop it in and see what's the result. The theory about the coolant rushing through the radiator not having the chance to exchange heat does sound plausible.
As far as I can see, the front bumper is intact but I don't see any airdam. Where exactly should it be? Does anyone have a photo of it as it's mounted on the vehicle? It would be a great help so I'll know what to look for.
Mustangsammym, my cooling fan works fine (at least in low speed mode anyway). I'll check the high speed relay and see if that's functional.

HISSIN50 · Jul 15, 2007

Good thoughts on here.

I totally agree about the air dam. Once you get one, you should see highway temps drop a bit (I dont notice the presence of the air dam until we hit about 100*F. Then it, or a lack of it, makes a tangible difference).

Once you have that and know the rest of the system is up to par, if you still run hot at speed, it sure sounds like you're simply overloading the system. Turning off the AC reduces the thermal stress on the system and it isn't quite so overrun. A nice aluminum radiator will REALLY help in those rediculous temps you see. There are not many folks on here whom see a climate hotter than mine, but I've read UAE gets beyond rediculously hot.

I dont have a picture of the air dam but it resides just under the radiator. There should be two trim pieces that flank the center air dam which you sound to be missing (you fill the gap between those two trim pieces). Mine was AWOL when I got the car so I just used machine screws and nyloks to hold the new unit in place.

Good luck bud.

EDIT: Honestly, unless there's a part's sourcing issue, grab a new 180* Mr Gasket (or Robert Shaw, et al) performance t-stat. The 180 rating will give you a head start with keeping it cool, and the performance stats are totally worth it. I did my own testing of one versus a new 180* parts store stat and the Mr G dropped my average operating temp ~8*F. The perf stat opens fully sooner and doesn't suffer from hysteresis like the cheapies.

SeventyMach1 · Jul 15, 2007

This is it ...

http://www.latemodelrestoration.com/iwwida.pvx?;item?item_no=LRS-8349C 1&comp=LRS

Stand in front of the car, then look under the car. Beneath the radiator you should see it. There are 2 small deflectors on the left and right side. This one goes in between them.

EDIT: JT beat me to it.

igotyofire · Jul 15, 2007

reddy351 said:
This is correct.....kind of. When a thermostat closes, it keeps the water that is in the radiator so that it can cool down. When there is no thermostat, the water just rushes through, not having a chance to properly cool. When I read the title of your thread, I thought "T-stat locked open." Then in the post, you said that the thermostat was removed. Same net result.

Get a thermostat in there! Quick like! Use a Ford one. Not a "discount parts store" one. Use the temp that Ford recommends (192?), or the ones that the guys in your area are "having no problems" with. (180, from the sound of it).

Post here, the result. Good luck!

huh.....i thought the coolant cycles the block until the coolant temp reaches a certain temp(IE 180degrees) then the t-stat allows it into the radiator to cool before continuing back into the block again... the way u stated it as if it needs to cycle coolant to radiator and sit to cool then go back to the block.....which i think is wrong because radiators should be more efficient then that.....

Intercoolers off boosted cars dont work like that...it is a continous flow. Infact i believe the thermostat helps coolant work more efficiently by allowing the coolant to carry as more heat out before returning back to the radiator.

Also i think their is a chance their is another problem. my car will do the same thing once the AC is on my guage instantly jumps up, altho not to that part of the guage i believe their is somthing going on with the surge of power that causes the temp guage to read incorrectly. as soon as i shut off the AC the temp guage begins to settle lower. Ill hafta see if other sources affect it such as my power windows....which i know dim the headlights. But maybe someone else can chime in on what i just mentioned

Dino Dino Bambino · Jul 15, 2007

SeventyMach1 said:
This is it ...

http://www.latemodelrestoration.com/iwwida.pvx?;item?item_no=LRS-8349C 1&comp=LRS

Stand in front of the car, then look under the car. Beneath the radiator you should see it. There are 2 small deflectors on the left and right side. This one goes in between them.

I just looked underneath and it seems that I don't have one. The trim pieces on either side are there though. There are two small electric pusher fans in front of the AC condenser that a previous owner must have installed, and there's another small cooler underneath the fans in front of the AC condenser that has lines going to the PS pump, so I would assume that's a PS pump cooler. Are these standard on '95 Mustangs? There's not much room between that cooler and the underside of the front bumper for an air deflector.

HISSIN50 · Jul 15, 2007

The PS cooler (it's just a metal tube) is standard. I'd have thought you might have a trans cooler (an aftermarket item) up there too but it doesnt sound like it.

It sounds like the PO might have moved the PS cooler downward to accomodate the added pusher fans.....?? As I recall, the PS cooler is up pretty high [in stock form].

Fopar · Jul 15, 2007

anyone have pics of these air dams on the car?

igotyofire · Jul 15, 2007

Fopar said:
anyone have pics of these air dams on the car?

it helps, air hits it and it directs it up to radiator altho i dont know if its gonna help in ur situation but it is a general good idea to have it regardless in my opinon, and cellphone pics per ur request, kinda dark but u get the idea i think

View attachment 377102

bmac50 · Jul 15, 2007

Ford says 50/50 when it comes to coolant/water and use distilled water rather than tap. Also put in a bottle of water wetter (about 6 bucks). Air dams are very important for anyone in the south. Look under the front bumper and see if it's been broken off, the tabs may still be there, if not there will be some holes where it attaches or at least the holders will be there. The air dams are plastic like and they get broken off by parking curbs.

There are various year pictures here so you see what your replacing.
http://mustangunlimited.com/SubCat.asp?CatKey=74-01&SubCategory=Air+Dams&category=Body&KeyWords=LMC1

reddy351 · Jul 15, 2007

igotyofire said:
huh.....i thought the coolant cycles the block until the coolant temp reaches a certain temp(IE 180degrees) then the t-stat allows it into the radiator to cool before continuing back into the block again... the way u stated it as if it needs to cycle coolant to radiator and sit to cool then go back to the block.....which i think is wrong because radiators should be more efficient then that.....

He said it was well over 100 degrees. The radiator losses efficiency, as the ambient temps rise. If the coolant in the block isn't moving, neither is the coolant in the radiator. Therefore, the coolant in the rad is allowed to cool more efficiently. (Water DOES NOT like to shed/pick-up heat.)

igotyofire said:
Intercoolers off boosted cars dont work like that...it is a continous flow. Infact i believe the thermostat helps coolant work more efficiently by allowing the coolant to carry as more heat out before returning back to the radiator.

Intercoolers typically have a much larger volume of coolant compared to the amount of heat that they are designed to dissipate. An engine cooling system does not. There are also restrictions (using smaller connection hoses or lower volume pumps, for example) designed into most of those systems, to increase efficiency. Coolant can only hold so much heat, then it stops absorbing it. That's why a car with a stuck closed radiator will overheat.

igotyofire said:
Also i think their is a chance their is another problem. my car will do the same thing once the AC is on my guage instantly jumps up, altho not to that part of the guage i believe their is somthing going on with the surge of power that causes the temp guage to read incorrectly. as soon as i shut off the AC the temp guage begins to settle lower. Ill hafta see if other sources affect it such as my power windows....which i know dim the headlights. But maybe someone else can chime in on what i just mentioned

I would put an aftermarket temp gauge from a reputable manufacturer, to make sure that the temps that your factory guage is showing are not erroneous. A bad sending unit can cause all kinds of "issues". Although, I don't think that's what's going on here.

Dino Dino Bambino · Jul 16, 2007

My temp. gauge works fine. The low speed fan kicks in when the gauge needle is on M and I've determined that to be 208*F. The only time the gauge goes higher than that is when I'm driving on the highway with the AC on (and even then only when it's over 100*F) or if I'm climbing a steep hill.
I'm very happy with the response that I'm getting in this thread and your ideas have been very useful. I have the following to work on:
1. Install OEM t'stat. I don't think it'll help but I'll need it in the cooler months to speed up the warm-up phase.
2. Check if high speed fan is working.
3. Find an airdam and install it. Mine's definitely missing.
4. My tranny may be slipping and producing too much heat. I'm having it overhauled soon and plan to have a Transgo shift kit installed.
5. If all else fails, I'll buy a Mishimoto aluminium radiator. I've seen the pics and for the bling factor alone it's worth it.

final5-0 · Jul 16, 2007

Lots of good info given by all

To me it all boils down ... or ... maybe I should say ... boils up ... to this

You say all is OK until you turn on the AC

It would seem to me ... with the AC off ... you are right at the limit of
how effective your cooling system is currently working.

You turn on the AC ... which is just one more request to remove heat
and
That request is what pushes you over the limit.

You most likely got some build up in the oem radiator.

Heck Fire ... when the oem system is 100% totally efficient ..........
It might or might not adequately cool a h/c/i combo

Sooo ... It ain't no powerhouse of cooling kinda thing

Grady

Dino Dino Bambino · Jul 16, 2007

I'm a long way from doing the HCI combo but I'll definitely upgrade the radiator before I do.
I'll start working on the first 3 items and see how that goes. The tranny overhaul/shift kit will probably be next month as soon as I get the parts. By then the weather will start to cool off again so I might be able to delay the radiator replacement a while longer until my wallet recovers.

Dino Dino Bambino · Jul 17, 2007

I had a look at two other SN95 GT's today and they didn't have the air deflector either, and they also have the PS pump cooler mounted low in front of the AC condenser like mine.
It just occurred to me that there might be another possible cause of my problem. The plastic trim that fits in front of the radiator support under the hood and covers the openings above the AC condenser has been off since I bought the car because the retaining clips are missing. Is it possible that I'm losing some of the ram airflow effect into the radiator at speed because some of the air is "leaking" out from above and going under the hood? I'll try to refit the trim piece and find out.
I also have the t'stat to install and need to check for the high speed fan operation, so that's three little jobs for me to do this weekend. The fan sounds the same even when I turn the AC to max so I suspect the high speed isn't working.

Engine runs hot only on highway when AC is on

Dino Dino Bambino

SeventyMach1

Keep it lubed .... keep it straight .... and keep

cjones

Blackened302

Active Member

reddy351

mustangsammym

New Member

Dino Dino Bambino

HISSIN50

"How long does it take to get help in here?

SeventyMach1

Keep it lubed .... keep it straight .... and keep

igotyofire

Member

Dino Dino Bambino

HISSIN50

"How long does it take to get help in here?

Fopar

Member

igotyofire

Member

bmac50

New Member

reddy351

Dino Dino Bambino

final5-0

Mustang Master

Dino Dino Bambino

Dino Dino Bambino

Similar threads

Engine runs hot only on highway when AC is on

Keep it lubed .... keep it straight .... and keep

Active Member

New Member

"How long does it take to get help in here?

Keep it lubed .... keep it straight .... and keep

Member

"How long does it take to get help in here?

Member

Member

New Member

Mustang Master

Similar threads

Privacy & Transparency

Privacy & Transparency