Engine Fox body doesn’t run hot until I start driving

Rose5oh · Mar 17, 2025

Hello all, in need of some help here.

I have a foxbody with a 5.0 in it. The only mods the engine has is an Explore upper intake and an E303 cam. I’ve replaced the thermostat 3 times. A water pump was installed a few years ago. It has an aluminum radiator with dual electric fans. I can idle the car after starting it and the temperature is about 185 degrees, no issues there. The problem is when I leave to drive. It will slowly creep up. After about 15 mins it’s running at about 250, in which case I’m back at home with the car shut off. And this isn’t even in summer whether. This has also been happening while cold. Anyone know what the problem is here?

AeroCoupe · Mar 17, 2025

Air locked?

What lower intake is on it?

Rose5oh · Mar 17, 2025

I never checked this. Good point. It’s a motor that I received already put together and just changed the cam. But I always assumed explore upper and lower. Assuming that they match. Have you ever seen anything like this or a list of things that could cause this?

Noobz347 · Mar 17, 2025

Are the fans pushing in the wrong direction?

AeroCoupe · Mar 17, 2025

I only asked as its not likely that the stock lower is on the car with an Explorer upper. You can see here the difference which makes is physically impossible. Top is a stock 1987-1993 Mustang 5.0 lower and the bottom is a GT40, Cobra, Explorer 5.0 lower.

Some things that can make the car overheat:

1) Air locked - issues with refilling with coolant
2) Having the thermostat in backwards
3) Electric fans pushing air through radiator rather than pulling (will work at idle but fight the airflow when moving).
4) Installing a standard rotation water pump (our cars require a reverse rotation due to belt routing)
5) Routing the belt so the pump turns clockwise (standard rotation) - from what I know it cannot be done so just follow the standard way of routing it.
6) Head gasket issues.

I am sure the guys can add more.

Rose5oh · Mar 17, 2025

Noobz347 said:
Are the fans pushing in the wrong direction?

I did check this. Fans are pushing towards the motor

Rose5oh · Mar 17, 2025

AeroCoupe said:
I only asked as its not likely that the stock lower is on the car with an Explorer upper. You can see here the difference which makes is physically impossible. Top is a stock 1987-1993 Mustang 5.0 lower and the bottom is a GT40, Cobra, Explorer 5.0 lower.

Some things that can make the car overheat:

1) Air locked - issues with refilling with coolant
2) Having the thermostat in backwards
3) Electric fans pushing air through radiator rather than pulling (will work at idle but fight the airflow when moving).
4) Installing a standard rotation water pump (our cars require a reverse rotation due to belt routing)
5) Routing the belt so the pump turns clockwise (standard rotation) - from what I know it cannot be done so just follow the standard way of routing it.
6) Head gasket issues.

I am sure the guys can add more.

I have 2-6 that you’ve listed. So I’ll get to work and try to narrow all of these down. Just hope it isn’t the head gasket. But it hasn’t really shown the typical symptoms of a blowed head gasket. I will try to keep you all updated. Thank you

rednotch · Mar 17, 2025

Does the dual fan set up have a metal shroud blocking 2/3's of the air flow threw the radiator like all this cheap China made one's.?

Rose5oh · Mar 17, 2025

rednotch said:
Does the dual fan set up have a metal shroud blocking 2/3's of the air flow threw the radiator like all this cheap China made one's.?

So yes, as a matter of fact. It came from eBay. However, I have a foxbody of my own with essentially the same radiator except it has more mods and I haven’t had these problems . The one with the problems I speak of is my brother’s car just so that there isn’t any confusion.

My foxbody has a 306, dss flat tops, tfs top end, but a nitrous comp cam instead of the tfs stage 1, sct chip. Temp stays at 165 when traveling and the highest it’s probably been is 225-230 in traffic, if that and that was in 95 degree weather with the AC on.

But I know it can sometimes be an issue. So thanks for mentioning that

FastDriver · Mar 18, 2025

1. do you trust your gauge? Maybe use an IR thermometer
2. Make sure you have an air deflector below the front bumper, especially if it's a GT
3. Remove the shroud and see if it frees up enough air to keep temps down. Obviously, you'll need to go highway speeds and avoid trafic for this test.

I agree with others, I've seen terrible shrouds limit air flow.

Rose5oh · Mar 18, 2025

The air dam deflector i completely forgot about that! That’s one thing my car has that my brother’s doesn’t.

But yes I do trust the gauges. I’m going to buy an IR thermometer anyway and Ill try the suggested things if I have to. Hope it works!

AeroCoupe · Mar 18, 2025

There are also pieces on the sides of the radiator that redirect the airflow to go through the radiator.

Mustang Radiator Side Air Deflector Pair (79-93) E4ZZ-8310

Replace those rotted out radiator side air deflectors on your 1979-1993 Mustang with these direct replacements from 5.0 Resto!

lmr.com

85GTStangGuy · Mar 18, 2025

There are also deflectors on the bottom of the frame rails behind the radiator. They are part of the whole system to help with control of the airflow through the engine compartment.

https://lmr.com/item/LRS-8310B-K/5-0resto-mustang-frame-rail-air-deflectors-79-93

And fyi....

Mustang Radiator Air Deflector Kit (87-93) E7ZZ-8349

Replace that broken, or missing 1987-1993 Mustang radiator air deflector with this direct replacement from DCR!

lmr.com

or if you'd prefer an original Ford part, rather than a repop....

Original Radiator Air Deflector for (87-93) E7ZZ-8349-A - #M-8349-2 - National Parts Depot

Buy part #M-8349-2 Original Radiator Air Deflector for (87-93) E7ZZ-8349-A for your classic 1987-1993 Mustang from National Parts Depot. Free shipping on orders over $300, fast delivery & everyday low pricing! Order now!

www.npdlink.com

Rose5oh · Mar 18, 2025

85GTStangGuy said:
There are also deflectors on the bottom of the frame rails behind the radiator. They are part of the whole system to help with control of the airflow through the engine compartment.

https://lmr.com/item/LRS-8310B-K/5-0resto-mustang-frame-rail-air-deflectors-79-93

And fyi....

Mustang Radiator Air Deflector Kit (87-93) E7ZZ-8349

Replace that broken, or missing 1987-1993 Mustang radiator air deflector with this direct replacement from DCR!

lmr.com

or if you'd prefer an original Ford part, rather than a repop....

Original Radiator Air Deflector for (87-93) E7ZZ-8349-A - #M-8349-2 - National Parts Depot

Buy part #M-8349-2 Original Radiator Air Deflector for (87-93) E7ZZ-8349-A for your classic 1987-1993 Mustang from National Parts Depot. Free shipping on orders over $300, fast delivery & everyday low pricing! Order now!

www.npdlink.com

Thanks man

limp · Mar 22, 2025

Is your water pump spinning in the correct direction? Not sure if ford does a reverse flow pump?? Did you install the belt the correct way??
EDIT.. I see Aerocoupe already asked these questions....

limp · Mar 22, 2025

I have seen add on electric fans block air flow when the car is moving...

Rose5oh · Apr 1, 2025

Hello all. I’ve got good news. I fixed the car. All it needed was an air dam deflector. I took the air deflector from my car and attached it to my brothers to see if it would work and it did! Now I don’t have one but I will in a few days. Thanks for all of the input from you guys. It’s much appreciated. Here’s a pic of our cars. My brother’s is on the left

FastDriver · Apr 1, 2025

Good deal. Glad that worked. I'd still be concerned, given that an LX has an open grille. My experience has led me to believe that they aren't strictly necessary on LXs, which means you might still have some kind of constriction to airflow that is not normal. This could be, and likely is, due to the shroud, and I get that you/your brother are resistant to changing it, but it may also be due to clogging in the radiator, A/C condenser, etc... Since it's free, it definitely merits radiator & condenser instpection/cleaning/straightening fins as needed, etc..,

Noobz347 · Apr 2, 2025

This thread sucks.

Get it?

Negative pressure area behind the radiator?

Get it?

Sucks?

Mindseye007 · Jun 7, 2025

AeroCoupe said:
There are also pieces on the sides of the radiator that redirect the airflow to go through the radiator.

Mustang Radiator Side Air Deflector Pair (79-93) E4ZZ-8310

Replace those rotted out radiator side air deflectors on your 1979-1993 Mustang with these direct replacements from 5.0 Resto!

lmr.com

I replaced those and they made a world of difference. I also replaced the

Mustang Frame Rail Air Deflectors

https://lmr.com/item/LRS-8310B-K/5-0resto-mustang-frame-rail-air-deflectors-79-93

Engine Fox body doesn’t run hot until I start driving

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