I've had my '94 since it was new and put over 140,000 miles on it (5 speed manual transmission (M5OD)). For the first ten years or so the temp. gauge always stayed between 5 - 10% of the "NORMAL" range - actually below the N in NORMAL. The ambient temperature never mattered and the A/C on or off seemed to have no effect. During the past two years, or so, the gauge has crept up and now usually the needle sits over the O or R - about 40% of the range.

Over the years I have replaced the radiator (due to a leak at the aluminum/plastic interface), upper and lower rad. hoses (as a precaution, not a failure), coolant (water) pump (due to a leak at the shaft seal), thermostat (when the water pump was replaced - not due to a failure) and the drive (fan) belt (every 40k miles or so, never due to a failure). I've replaced the coolant six or seven times (approx. every two years) with 50/50 to 60/40 Prestone and distilled water but I've never used any of those "flush" products. It always comes out looking just like it went in - clean and translucent green. When I replaced the water pump I could see into the engine to the first cyclinder (I could put my fist in there) and the cast iron looked brand new - no scale, corrosion, etc.

What are your thoughts on the temp. increase? Real or instrument error? If corrosion on an electrical connection increases resistance in the circuit would that give a higher or lower reading. If real, what should I check or replace.

Great forum. Any thoughts appreciated.

 

Are you still using the same type of fuel? If you were running a different octane, that might make the truck run hotter (not sure which direction that'd work in) - are you using hotter plugs maybe?

Your temps are still well within range though, I have an older 92 I bought cheap and so am working my way through its issues and it has gone over halfway at times uphill on a hot day with the ac running - I'm going to switch over to an electric fan for better mileage/power etc...

As an engine wears, it may run differently, but personally I like your theory of the gauge or the sender unit altering with age, I'm not sure how sender units work, but as you observed, corrosion, age etc can result in increased resistance which might account for it.

You didn't mention the health of your heater core but everything sounds good, I suppose you could try a full-on flush and see what that does. personally I'd try to read the temps with a high accuracy thermometer and compare it with what's accepted as factory standard temp (anyone?) and continue to keep an eye on it...being as vigilant as you are, you're bound to catch any issues early.

 

Trouble is the gauge is nothing more than an idiot light with a needle. Heck even the oil pressure gauge is designed not to move. Add an actual water temp gauge if your wondering about it.

Adrianspeeder

 

Try replacing the sending unit.

 

Are you sure about that? The '92-up trucks I've seen had functioning water temp gauges. I know my '95 does.



Coolant actually retains more heat than water does. Running a 60/40 mix might have something to do with it. Regardless, the only way to know for sure is to verify the actual temps, either with an infrared temp gun or an aftermarket gauge.

 

My Ford Telstar factory manual had a procedure for verifying the performance of the gauge, basically, a resistance substitution wheel so you could vary the current input and see how it deflected the needle.

My temp gauge always stuck to the same spot regardless, it would shift slightly if I turned on the heater. I suspected the gauge wasn't working properly and tested it, and it passed. Then some time later, my thermostat jammed shut, and suddenly the temp gauge shot up impressively.

Ok, it's a bit more than an idiot light, but it seems far from accurate. Are we surprised?

 

True. That's why I don't rely on mine.

 

I had a 92 4.9L and had the same problem. Turned out to be the sending unit. I replaced it and temp gauge started reading correctly again.

 

A good point - a temp gauge is only going to be as accurate as the sending unit.