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Discussion Starter #1
I don't think a day goes by when page 1 on the forum doesn't have a thread about overheating. I read them all, trying to learn what to do about my issue. I really value the experiences described and inights, particularly from @GypsyR , @22GT and @Woodchuck among others. My stang runs hot. The gauge in my previous 66 seldom ever climbed higher than the E in T-E-M-P Here's my gauge in my current ride when going down the interstate:

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To try to bring down the temp, here's what I've done:
Replaced t-stat (added burp hole)
Replaced water pump
Removed factory brass radiator and replaced with two-core aluminum
(Still use factory shroud)
Replaced thermal clutch
(Still use factory fan)
Two Evapo-Rust Thermo Cure treatments
Changed timing from 6* to 15* (total timing is now 30*)
But nothing changed on the gauge. I got an infra-red temp gauge to compare readings at various temps and places trying to match up something, but with no luck.

This past week, I bought an AutoMeter 2333 mechanical temp gauge. I compared results with the bulb in a pan of hot water on the stove with a digital temp gauge I use in my smoker. Both could be out-of-calibration, but if they are, they are both out-of-cal by 2* from 100 to 200 on the stove, and 220 to 250 in the oven. The likelihood is near 0 so I accept the AutoMeter gauge is fairly accurate. I installed the gauge in the ash tray slot allowing me to view it and the factory electrical (cluster) gauge at the same time.

With the mechanical gauge measuring at the intake heater hose and the electrical (cluster) gauge measuring nearby on the intake, I took a 30 mile drive on the interstate at 70mph on a hot day. Here is the comparison:
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I couldn't get the temp to 220 even though the factory gauge was reading at the P on T-E-M-P, what I considered "hot". From the advice of the smart guys, I don't have a temp problem, I have a gauge problem. In fact, my engine is actually happy when the needle points to the P.

So, my point to those who have hung with me this far on this long post, is to question everything. If you think you have a temp problem, get a second opinion or at least an ACCURATE opinion.

(Looking back, why on earth would I believe my temp gauge? My gas gauge shows empty when I still have 1/4+ tank, my oil pressure is >45psi when the needle points to O in O-I-L.)

Thanks for users weighing in. Your recommendations pushed me to work harder at figuring out my root cause and saved me from my next step, pulling the engine, knocking out freeze plugs and pressure washing the lower end!
 

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Great summary of an issue many people have. Honestly I had the same experience with my car. Now I run an autometer in parallel too. One at the thermostat housing and the other on the intake manifold.


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I had a similar issue with my '70 convertible. I ended up pulling the heads and the block water jackets were caked with this mudlike material. I'm not sure if previous owner had tried a leak stop treatment or what. After a good pressure washing of the block (still in the car), it was good to go. That was 3 years ago and hasn't overheated since.
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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A really good approach. I do the same with oil pressure gauges. I've found many Ford oil pressure gauges work pretty well, once you're able to match what they display with a number.
 

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Congrats, Trust but verify is a great motto to live by. In the world of Sunbeam Tigers there is a saying, If it ain't puking, it ain't overheating. I carried that attitude over to the mustang. You think the Mustang world obsesses over overheating, you got nothing on the tiger. Small block Ford, incredibly tight engine compartment, a radiator smaller than a 6 cylinder mustang and a bunch of side holes in the radiator support that allows cool air to bypass the radiator AND hot air to recirc under the radiator and back through the radiator.
 

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Looking at your photos, I'd say the OEM gauge matches your aftermarket gauge surprisingly well.

BTW, advancing the timing causes engines to run hotter, not cooler.
 

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I agree with 22GT. The OEM gauge reading in the middle corresponds with 200*F exactly. See the attached chart.
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Discussion Starter #8
Sadly, I was foolish enough to compare the gauge reading of one mustang (66#2) with another (66#3.) I was so proud of 66#2 never having heating problems, and I used its gauge readings as a benchmark for #3. I never compared #2's to a mechanical, so perhaps it was reading low all along.

@Woodchuck - That table is great. It confirms that my temp gauge actually provides good/accurate info unlike my gas and oil gauges. Now I know that as long as I'm below P, I have nothing to fear.

For future readers, like @GypsyR says, the important thing is to match up a gauge's needle position with "actual". Don't guess. Spend a few bucks (in my case $50) and equilibrate your gauge to known values. I spent a lot of time and mental energy on solving a problem that didn't exist.

(@22GT I'm going to disregard your comment about advancing the timing and running hotter. I'm basking in afterglow right now. :) )
 

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Looking at your photos, I'd say the OEM gauge matches your aftermarket gauge surprisingly well.

BTW, advancing the timing causes engines to run hotter, not cooler.
Advancing the ignition timing reduces combustion temperatures provided you're not on the threshold of pre-ignition. Engine heat load increases as combustion becomes less efficient as more heat is wasted instead of turning into motion. When spark advance is retarded, such as when ported vacuum is used versus manifold vacuum, the combustion process begins late and the fuel/air mixture continues to burn after the exhaust valve opens, sending excess heat into the exhaust port. This is the whole theory behind Thermactor and Air Injection Reaction (GM) systems.... to increase the heat in the exhaust port to the point where unburned hydrocarbons will spontaneously combust when they hit oxygen-rich air.
 

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That's a great chart, is there a similar one for the OEM oil pressure gauge?
For fuel and oil pressure just ignore the temperature readings and calibrate to the resistance values. Once you have verified that your sending units are more or less accurate you can "calibrate" your actual gauge to match those readings.

Here's a couple articles on that....

Mump 1005 05 O+mustang Instruments Tech+inside Temp Oil And Fuel Gauge - Photo 28806175 - Ford Mustang Instrument Panel Troubleshooting

Ford gauge calibration
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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I seem to recall a temperature sensitive vacuum port setup where the timing could be advanced a bit to make the engine idle faster. Thus increasing water pump flow a smidge and airflow from the mechanical fan quite a bit. Thus in such very particular set of circumstances, advancing the timing would indeed make the engine run a little cooler. Also. :) A bit of engineering thinking I've always admired as a simpler and more reliable alternative to a an electric solenoid "idle kicker" for AC.
 

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Been putting this off forever, finally got it done. The car has been mostly sitting for years. A few weeks ago I put it back into daily driver duties. It does over 100 miles a day right now (over 1500 miles in the last month). It has always run hot and overheated in traffic. It has a new waterpump, thermostat, and has the block flushed multiple times. I finally pulled the old 2 core stock radiator, and installed a Champion (~$200 on ebay) 3-core aluminum radiator. I have a 14" electric fan on the front of the radiator, and a 6 bladed flex fan with no shroud. The car now idles at a max of 170 degrees! Before I was hitting 220 degrees idling at a stop light.
 

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A 170 temp on the stock engine is too low to bring the oil to temp. The stock SBF is designed to run in the 195-210 range. This is a common misconception where lower temp is always better. What’s best is running at the temp the build is designed for. Using it as a daily with the temp that low will likely sludge the oil over time.
 

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I seem to recall a temperature sensitive vacuum port setup where the timing could be advanced a bit to make the engine idle faster. Thus increasing water pump flow a smidge and airflow from the mechanical fan quite a bit. Thus in such very particular set of circumstances, advancing the timing would indeed make the engine run a little cooler. Also. :) A bit of engineering thinking I've always admired as a simpler and more reliable alternative to a an electric solenoid "idle kicker" for AC.
Are you referring to the Distributor Vacuum Control Valve which switches the distributor's vacuum advance from ported to manifold at a certain temperature?
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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Probably. There was an implementation that functioned as an AC kicker that I have in mind but I can't for the life of me remember what it was on. Could have been a Mustang, a Buick, a Volvo, I don't even know. I stick my head under a lot of hoods and some things blur together sometimes. So it may not be strictly relevant to vintage Mustangs.
 

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I thought advancing the motor helped with cooling as well.
The Distributor Vacuum Control Valve (DVCV) does indeed advance the timing in an engine that has exceeded 220°F, but the point is, and this is in the Ford Shop Manual, that it has the effect of increasing the idle speed. Increasing the idle speed increases the speed of the fan and water pump, cooling the engine. Obviously Ford was anticipating motorists who were stuck in traffic, not overheating on the freeway.
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