Why is it overheating?

Sam8950

Founding Member
Mar 13, 2001
85
0
7
Haslet, Texas
Daily driver ’92 GT, DSS crate 306 motor, 303 cam, AFR heads, Vortech intake, 24# injectors, oversized aluminum radiator, stock type water pump, 5 1/4 in. drive pulley, new stock fan clutch, fan shroud installed. Coolant is fresh. When I bought this car late last summer, it had a dual electric fan unit installed, but I thought it still ran on the hot side. When the electric fan unit failed, I went back to the stock motor driven fan and the temps got a little cooler. But now that the daily temps are back up in the upper 90’s, the near-overheating is back.

So, when I’m moving at greater than 35-40 mph, the temp stays right in mid-range where it should be, even with the A/C going full blast. But, if I get in stop & go traffic for just a couple of minutes, or stop at one traffic light, the temp starts to rise rapidly, even if I turn the A/C off. Any more than a couple of minutes at those conditions, and the temp gets up toward the 270 reading, fast. Steadily holding a few extra revs on the engine doesn’t seem to have any effect one way or the other. Once I get back up to speed again, the temp drops rapidly back to mid-range. I know the stock gage is not the best, but I got coolant overflow once just as the gage hit the top, so I know it’s close.

Doesn’t seem to act like a sticky t-stat since the temp drops right back down when I get moving. Maybe a failing water pump? Should I go to a bigger size pulley? Or is there just not enough airflow through the radiator? It still has the stock-type GT front fascia, so maybe I need to install a Cobra-type front grill for better airflow? It also has a power steering fluid cooler installed in front of the A/C heat exchanger & radiator, which further reduces airflow. Should I remove this (don’t do enough rally type driving to warrant it, in my opinion)? Any ideas, anyone . . . anyone? :shrug:
 
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Check your fan clutch and make sure it isn't worn out and spinning on it's own. If that's not it (should be), sounds like a good electric fan is needed. Try a Taurus fan. I think 1994 model. Also, did you bleed your system of air?

A bored out motor car can be bad when it comes to overheating. My old 306 did that.
 
water pump? NO Should I go to a bigger size pulley? NO Or is there just not enough airflow through the radiator? YES, and the reason is that your fan isn't pulling enough air through the radiator when you're at a stop. It still has the stock-type GT front fascia, so maybe I need to install a Cobra-type front grill for better airflow? NO It also has a power steering fluid cooler installed in front of the A/C heat exchanger & radiator, which further reduces airflow. Should I remove this NO Any ideas, anyone . . . anyone? :shrug: You need a large enough electric fan with a shroud that covers at least 80% of the radiator.

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Things I've learned from this board and experience.

1. Thermostat installed backwards.

2. When running AC fan on high amp draw affects factory temp and gas gage and both move 1/4 distance higher showing hot engine and more gas in tank. This happens on both 84 and 86 cars I own. Turn fan off needle sits dead center even in 95F. The number one comment I received on this board is to replace the temp gage with a "good one". So long as one knows what is causing a false reading does it matter? I don't run the fan on high anymore:) This may not affect a newer car such as yours?

3. Fan clutch covered in dirt on front side indicating leak and also if you drive in a lower gear with higher rpm car will cool down indicating slipping fan.

4. Heavy mineral build up in engine from using tap water needed a good chemical flushing and refilled using distilled water or premix antifreeze. Water wetter can also help but costs $10.

5. Ford recommends burping your radiator twice after emptying it. Heater temp control full on hot. They say to leave radiator cap on first notch after refilling and feel hose off water pump for warmth to determine thermostat is open. Shut off car. Slowly remove cap and top off with water. Repeat one more time. In addition to that I checked the following two mornings and also when the engine was fully hot topped off the overflow tank to full hot mark. Getting all the air out of there makes a really big difference. You may even have to remove the temp sensor with engine cold and off as a bubble of air can get trapped under that giving a false reading.

Those are the main issues. Good luck.
 
Take the thermostat out and see if it is working correctly by putting it on the oven and using a cooking thermometer and see where it opens up at. Also what gauge are you reading this off of? I'd be very worried about the heads on the motor if your getting them up to 270*
 
Another vote for not enough air flow at no/slow movement.

I'm guessing it's hot where you're at in Texas. The fan situation needs to be top-notch for triple digit temps.
 
Assuming the rad is in good shape, and the tubes are large enough inside (big radiator won't make much different if the tubes are tiny inside...

I'd suggest a mark viii electric fan...that thing can suck cement through a straw it blows so hard.

FYI, I had purchased an autzone radiator. It was a 67 radiator that I installed into my 66...the 67 radiator is 4 inches wider and should have colled pretty damned well.

it didn't. The temps instantly crept up and up and up, even in 50 degree weather at a stop light.

I took it back, and got the below radiator. The rad on the left is a 2 row generic ebay radiator, but it kicks ass. In 90 degree weather with NO fan shroud it won't go past 205 with a 195 tstat



radiator%20tubes%202.JPG
 
I'm guessing it's hot where you're at in Texas.


Yup, 98/99 forecast for the next seven days. Pavement just makes it hotter . . . My truck's outside air temp gage routinely reads 103/104 at these levels. Guess I need to get a couple of those for the 'Stangs so I can tell what's getting sucked in.

Here's an update: Fan clutch checks good, t-stat checks good. I'm going to reinstall the dual electric fan unit this weekend with a new controller. The fan motors work fine, it was the control module that failed. I'll let you all know the result.
 
UPDATE: :D Dual electric fan unit re-installed with new controller. A week of driving in triple digit temps with the A/C going full blast . . . did not even come close to overheating. Hard to believe the stock fan could be that inefficient.
 
:nice:

It's not inefficient... Unless you have thin cylinder walls ;)

Wasn't sure about this, so I ran some example thermal calculations (0.040 overbore on a stock block, etc.) and came up with about a 14% increase in the thermal transfer rate with the thinner cylinder wall. Sure, there are other factors, but that doesn't indicate very much margin in the stock cooling configuration. Guess I'm used to airplanes where we overdesign by 50%.