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Discussion Starter · #1 ·
My 96, which I've had for a number of years, seems to be running hotter than normal. I was used to readings down around the O and R of NORMAL, even when towing. Now towing 11,000 pounds of trailer, pulling a grade I'll run up to just about the right line that says the top of the "normal range." When on the flat, it drops down a smidge, but still runs above L. I thought maybe the tow load was creating issues, but I dropped the trailer, and running upgrade, I'm still almost as high, and on the flat, slightly less than when towing.

I've changed out my fan clutch, changed coolant, and when I had the top radiator hose off, took a peek inside the top radiator connection--looked as clean as a whistle. Doesn't appear to be losing coolant. Not sure if maybe the coolant temp sensor is haywire, and unsure of how to test, save for installing an aftermarket temp gauge. Is it possible to get some sort of adapter that would allow me to run both the stock temp gauge and an aftermarket at the same time?

My son-in-law (bless his heart) thinks I'm just being an old lady. "Gee, pops, what part of NORMAL don't you understand?" Well, I'd like to have a little more headroom, thank you very much.

And if anyone has any suggestions on where to look next at this cooling issue, I'd appreciate it.
 

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It's a 25-year-old gauge; it's gonna have some drift. Have you ever cleaned any of the terminals in the circuit? There aren't many - at the sender, at the engine connector & firewall (which are probably Gold-plated and don't likely need cleaning), and at the instrument cluster (the most-likely location for corrosion & high resistance). But until you know the actual temperature is going up, there's no reason to disagree with your son-in-law. An aftermarket gauge is LESS-likely to be useful than the original - get a non-contact IR thermometer, which is likely to be more accurate than anything else. Check near the temperature sender, and near the thermostat (on the engine side of it, if possible). Check the back of your throat to confirm its accuracy - it should show near 98°F.

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Another useful test is to see how far the water pump can shoot a stream out of a heater hose when the engine is revved cold. If it doesn't go far, the impeller may be degraded...

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I for one do not take any vehicle I own running hotter than what I consider "normal" lightly. I have daily driven and raced many Rotary's over the past 25 years and any overheating is almost always a death sentence for those motors, so I always watch my temp gauge on anything I am driving.
On the line on high end of normal is too hot for these trucks to be running with their well built cooling systems. I have a 97 250, SC, flat bed,4x4, auto, with 4:10's, 203 T-stat, 3 inch downpipe to 4 inch open pipe, and Tymar intake. Pulling my old 29 foot fifth wheel 10-12k up back roads in the mountains of SWVA working the truck hard trying to run with my buddy's built truck, I never got close to that hot. The truck was more stock back then too. Now running my race trailer only around 7,000lbs at 70-75 mph for hours on end in summer heat I never even get close to the temps you say you have.
There is a test of the temp sending sensor buy grounding something out to make sure the gauge is working. It has been I while since I have done it so you will have to search for it on this site or another place. I used the test to see if my sending unit was sending a reading to the gauge. In my case the gauge was always reading below cold. ( There are also two different sensors Green and Red if I remember right)
How many miles are on your truck? Does you truck come up to temperature quicker than normal? Any modifications since this started happening? Chips Or tunes?
 

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Discussion Starter · #5 ·
Just a smidge over 200,000 miles. No, it takes 10, 15 minutes for temp to come to normal, then it generally takes a while for it to rise from there. When towing, yes, it rises quicker. And towing or solo, then I back off the fuel, lower speed, or get of the grade, it drops--and quickly. On start-up, when engine cold, the gauge runs flat to the bottom.

Yes, has a tuner. However, this overheating business started long after the tuner was installed. And because I had a transmission issue--threw a code and went into "protect mode," I have deliberately NOT been using the tuner--just set it for stock mode. That has made zip difference in engine temp.

I had wondered if maybe the thermostat might be stuck part-way open, and not opening fully. But various folks have told me that they felt that couldn't be the case--that if the t-stat was stuck open, I'd never get the engine to warm in the first place. Don't know that I can necessarily agree with that thinking. Would seem that it would take longer to warm up to me. I have always been a fanatic about changing coolant regularly, and using only distilled water for the "other half." I'm second owner, and the first owner was probably more of a nut than I am about maintenance. But what, maybe radiator needs to be changed out? As I said, looking down on the top plates, everything "looks" clean, but of course, I can't see any of those fine tubes running top to bottom.
 

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I would be looking more at the thermostat or water pump before the radiator. I replaced my thermostat and housing many years ago with the 203 degree one. I do not like how the temperature gets up under load pulling grades but nothing close to what you say you are seeing. And as I said earlier I am paranode about heat. My water pump was done at around 226k because the bearing was going out and it was leaking out of the weep hole, the impeller looked fine.
Can you tell a major difference in temp between the top radiator hose and the bottom one? I would loosen the belt and see if you have any water pump play also.
 

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Discussion Starter · #9 ·
If there was a large difference then I would suspect a pour flow of coolant or a blockage somewhere. Did you flush the system when you changed coolant? And what condition was the old coolant in?
When I changed coolant last, it was in good shape, no appearance of anything but coolant, no muck, or anything else. I had fully intended to flush the system, but my "helper" son-in-law peered inside the top of the radiator through the hose fitting and declared he could see absolutely nothing on top of the radiator, and recommended I not flush. Maybe I should have been a good father-in-law and ignored him.
 

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Before you start all kinds of other diagnostic work, Steve is onto something about the connections. I have not experienced anything on my one truck I have been referring to. However, my other truck (97 PSD as well with 303K) always reads cold unless you are working it really hard then the gauge barley comes up and I changed the sending unit out thinking that would fix the issue. Rereading his post got me thinking about checking mine out this weekend. IR all around the cooling system is a great idea too, should show if anything is hotter or colder than it should be.
Also I would still check for play in the water pump shaft.
 

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Test the water pump. If it is bad, then change both the water pump and the thermostat. If the water pump is good, then just change the thermostat.

As you said, if the thermostat was sticking open, then that wouldn't seem to explain the problem. But if the thermostat was not opening when it should, that could be the problem.
 

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Discussion Starter · #12 ·
Test the water pump. If it is bad, then change both the water pump and the thermostat. If the water pump is good, then just change the thermostat.

As you said, if the thermostat was sticking open, then that wouldn't seem to explain the problem. But if the thermostat was not opening when it should, that could be the problem.
Other than wiggling the shaft, how do you test a water pump?
 

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Discussion Starter · #13 ·
And since I posted last about the gauge readings, I had an event that seems to verify the gauge works. I had to pull a 20 mile grade, mostly 3-4% yanking my 11,000 pound trailer. It took no time at all before the needle was "in the attic," but still "inside" the normal limit. After holding our own for much of the grade, I suddenly got a ENGINE TEMP warning light--first time I have EVER had that go on. It stayed on until I got to the downgrade, and when the analog gauge showed about 3/4 of full swing, the lamp went out.
 

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Since you may replace it anyway if you do the water pump, just replace the thermostat and see what happens.

You can also take the old thermostat and place it into some hot water that you know the temperature of and see if it is opening and closing.

I don't know how old you are but I remember automotive parts stores 40+ years ago having a porcelain cup with a probe that they would plug into 110 that would heat up the water to check the operation of the thermostat.
 

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Other than wiggling the shaft, how do you test a water pump?
There were two methods mentioned in the posts above in this thread. Wiggle the shaft, and the heater hose method.
 
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Flow should be proportional to engine RPM, and even near idle, it should shoot a stream out - not dribble.
 
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