Overheating problem

COramprat said:

You may still have air in the system. Did you fill it at the thermostat housing when you replaced the coolant? That allows you to completely fill the system and eliminate air.

Also...my car always ran hot no matter what I did. The thing that changed that considerably was a new fan when the original seized. From what I've been able to gather is the design of the old fan wasn't getting airflow as it should. I think the design changed in 98 and the issues went away.

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Just curious CO, how do you fill the coolant at the thermostat housing? I'm going to change all of my hoses out this winter. I also believe there still may be some air in the system. I also think I would replace that 3.8 fan with a GT one. I would think that the fan from the 3.8 would be enough to cool it down but maybe not. Pearl02.

You need a 4.6 GT fan. V6 and 5.0 fans are one speed and the 4.6 is a 2 speed fan. The computer turns on the low speed fan if the temp exceeds 221 degrees and turns it back off again at 200 and it turns on high speed if the fan is already running and the A/C is turned on or if the temp gets to some really high number which I don't remember. With the one speed fan you may have lost one or the other of those functions.

When you said the coolant was boiling out of the cap, that was with the cap still on, right? You didn't open the cap and then water started boiling out, I hope?

Which light are you talking about - the red light on the gauge or the amber coolant level light on the bottom with the other indicator lights?

I agree that you may still have some air trapped in there somewhere. Filling at the t-stat worked very well for me when I flushed mine the other day. Turn the heater on to make sure water is circulating through there too. Rev it up a little and hold it at maybe 2K RPMs for a bit to really move some water.

Did you completely drain the block or flush the old coolant out when you replaced the intake or water pump? How many water pumps have you put on and in what ways did they fail?

Do you have access to a OBDII scanner that can read the temp data from the computer? Does the reported temp track with to what you see happening on the gauge?

If you've had ongoing overheating problems with this car, maybe you should get the radiator checked out.

You might put the stock water pump pulley back on to get some more coolant moving through the system while you figure out what the real problem is.



My '96 idled for 20 or 30 minutes the other day and I could never get the temp above 190 degrees and then driving it hard it wouldn't go over 198. You've got that thin mountain air up there in Colorado though so it may not be as effective in removing heat and you may run a little higher on average but you shouldn't be having continual overheating problems.

Pearl02 said:

Just curious CO, how do you fill the coolant at the thermostat housing? I'm going to change all of my hoses out this winter. I also believe there still may be some air in the system. I also think I would replace that 3.8 fan with a GT one. I would think that the fan from the 3.8 would be enough to cool it down but maybe not. Pearl02.

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I don't know how he does it, but just take the t-stat housing off, pop out the t-stat and start pouring until it's full. Spin the water pump around by hand and squeeze the lower radiator hose a few times to help work some bubbles out - don't start the engine though or water will come shooting out of there like a geyser.

Be aware that without the clamping force of the t-stat housing, water may leak under the intake a little while you're doing this.

Flush radiator, pressure test

What is the anti-freeze percentage? Should be between 40% - 60%. Below 40% there is not enough anti-freeze to prevent corrosion. However, as the concentration of anti-freeze goes up, its cooling ability goes down. That is why anti-freeze concentration above 60% does not remove heat as well.

Recommend an anti-freeze tester. Test while the coolant is at room temperature.

Check the coolant level. If too high there is not enough room for expansion. This will slow the air migration to the degas bottle. If too low, the pump will starve and cavatate.

To work well the system must be pressurized. When the system boiled over was the top radiator hose hard? If the radiator cap does not hold pressure the coolant will boil at too low of temperature.

It has already been mentioned about air in the system. The 4.6 SOHC will collect air in the manifold crossover. Any air in the crossover will cause the temperature gauge to read low because it is not immersed in coolant. The temperature gauge may read normal when in fact the car is over heating.

Normally the SOHC 4.6 the air will migrate to the degas bottle in a couple of heat/cool cycles (the 3.8 and the DOHC will NOT clear the trapped air without help). Feel the top radiator hose it should be full of coolant.

If air still remains in the system then it is NOT pressure tight. Gas is entering via the head gasket or other source. Suggest getting a coolant system pressure test. Some auto parts stores will rent out the testing equipment.

It is also possible the t-stat is bad. IMO, this is not as likely in your case.

Is the radiator clean on the outside? What about on the inside? If clogged on the inside, the ability of the radiator to remove heat will be reduced. Suggest flushing the system with a good commercial product. Prestone is a good choice.

Note, a classic symptom of a clogged radiator is it works fine at idle but is unable to keep up at higher loads.

Has there been any other mods done to the car? For example, rear end gears? If the PCM is not aware of the car’s true speed, this could cause the fan to not cycle correctly as the car is driven. The PCM does make fan decisions based upon the car’s speed.

In summary, there are many parts of the cooling system that have to work together. Each part will need to be checked to be sure it is in good working order. Perform the tests and let us know the results. Don’t’ cut corners as you post indicates this has been a chronic problem for you. To me this indicates the solution has been over looked or something thought to be OK is not.

Update/additional information: the 1999-2004 uses a two speed fan for the the V8. The V6 uses a single speed fan.

The wiring diagram indicates the 1996 Mustang uses a single speed fan for both the V6 and V8. I don't know for sure about the 1997-1998 model year.

If someone has the detailed wiring diagrams from other model years, please chime in.

I know from personal experience that a SOHC modular will self purge any air caught in the cross over. It DOES take several heat/cool cycles. Further, this assumes the system is tight and is working correctly.

The DOHC and 3.8 V6 will not self purge air caught in the cross over and t-stat housing.

One one of the easier ways to refill is to disconnect the heater hose. Add coolant until it begins to run out. Then re-connect the hoses and complete the filling process.

I don't have detailed wiring diagrams but I just looked and my '96 GT has the 3 wire fan connection as described in the repair manual for 2 speed operation.

94-96gt's have a small style fan & the 97-04gt's have a larger fan. The speed is the same on all of them. I swapped out one on my old 96gt to one of the 97-04 fans. No wiring changes just some rad support mount mods. As far a the v6 fans, I believe it's just the fan speed module control that is different, not 100%.

Big Show 96 said:

94-96gt's have a small style fan & the 97-04gt's have a larger fan. The speed is the same on all of them. I swapped out one on my old 96gt to one of the 97-04 fans. No wiring changes just some rad support mount mods. As far a the v6 fans, I believe it's just the fan speed module control that is different, not 100%.

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You will also need the 97-04 radiator to make the newer fan mount properly.

use caution interchanging rad fans

The question about fan interchange may be more complicated than 1st thought. I did some searching just looking at interchange with the fan motor. Here is an example.

1994-1996 V8/V6 Murray 7504
1998-2000 V8 Murray 75762
2001-2004 V8 Siemens VDO PM9157
1999-2003 V6 Murray 35123
1997-1998 V6 Siemens VDO PM9061

What is difficult for me to tell is WHY the changes were made and exactly what the changes are.

The Mustang underwent a large re-design in 1999. The electrical system of the 1999+ is much different than previous Mustangs. There are many more fuses. The CCRM and GEM modules are in wide use. The electronic cluster/speedo with internal module communication via a small network (SCP).

From the wiring diagrams, I am fairly confident 1999-2004 V8's use a 2 speed fan and the 1999-2004 V6 use a single speed fan. That means that a V6 fan will not interchange with the V8.

From the parts interchange, it appears the 1998 V8 has a two speed fan.

From there things get fuzzy. It seams there are model year differences in the fan shroud and radiator arrangement that makes interchange even more challenging. I do not have access to wiring diagrams from 1997-1999. So I am unable to look it up for myself.

IMO, when replacing the radiator fan, it would be a good idea to research your specific model year and engine combination to be sure it will work. Use caution interchanging V8/V6 applications as well as model year interchanges.

Note, installing a 2 speed fan in a car using a single speed fan WILL work at some level. However, installing a single speed fan in a 2 speed car will NOT work. The fan will run on low speed only and will shut off when high speed is called for.

Get a stock replacement. There best for everyday use.

ahhjaws said:

I'll try filling up the system from the t-stat and see if that helps

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Please don't make that the first thing tried. If the problem was as simple as just air caught in the cross over tube after system fill, it would have resolved itself after a few days of driving (1-2 full heat/cool cycles).

The first thing to check should be the FAN. In fact, it would explain EVERYTHING if your car needs a two speed fan and you have installed a single speed from from the V6.

As stated previously, everything will appear to work normally at idle and short trips. Depending upon weather and AC settings, there may not be any problems at all. The problems would start during slow/medium speed stop/go driving. Remember that above 45 MPH, the fan doesn't run anyway.

Image driving around on a warm day. When the speed get above 45 MPH, the temperature stays in check. Slow down below 45 and the fan only runs when the PCM calls for slow speed. Just as soon as high speed is called for, the fan quits running alltogether. The fan is not running at all when it is needed the most. This begins an overheating cycle.

As mentioned before, this sort of delayed/load based overheating could be a clogged radiator. It could also be a weak/bad pressure cap.

The 2 speed fan will have 3 wires. The single speed with have 2 wires. Compare the number of wires/terminals between the car's harness and the fan's connector.