Hello folks,

I have recently acquired a '91 f250 7.3 idi. It has been mostly sitting for a LONG time and only has 121K miles on it. Previous owner used it maybe twice a year over the last 11 years to haul his car to the track.

It ran fine when i picked it up and for the next day or two, with the temp gauge at "N", but then the coolant temp gauge started doing the funky chicken. Suspecting it's corrosion at contacts (lots of stuff is corroded, i had to replace all brake lines and some wire connectors), I have taken the lead wires off both sensors, cleaned the contacts and put them back. Now the gauge sits at "A" or "L" and one or twice went higher and caused the check engine light to go on on the freeway at about 60mph with ambient around 39F.

Here's what the rest of the truck is doing:
-Top end tank of the rad is too hot to place the hand on.
-Bottom hose and end tank are cool to touch
-Waterpump is new as of last year, the bolts and housing still shiny
-Thermostat unknown
-Coolant flushed with silicate-free ELC at time of waterpump, looks brand new, i have tested for and added SCAs as req'd
-Heater work like a charm and blows nice and hot
-Heater setting does not seem to impact the running temp of the engine (i know that my dodge 318 could be "helped" by running the heater core at max with fans blasting, don't know if it's the case with these)
-Hoses feel pressurized
-Fan clutch seems to feel good at normal temperature -- resistant but fluid
-Fan does not kick in at any point
-Rad looks good from the top of the cap

So far i am not sure if this might be a thermostat, or a sending unit, or perhaps a plugged rad wehre i can't see it? Could one of you kind folks suggest a way to troubleshoot from here without buying a bunch of parts unnecessarily? The truck is really in very good shape, everything works and it drives great. Just a little concerned that it is actually overheating.

Thanks a bunch,

Tom

-'91 F250 Lariat Reg Cab 7.3IDI / E40D / 3.55 Open diff/ Power tailgate

 

Top hot bottom cold, sounds like classic plugged radiator, or a poor water pump, but sounds like it should be in good shape Sounds like you may need a fan clutch while youre at it. You can tell when the fan kicks in thats for sure. As far as why i think its the radiator, i can drive summer and winter without a fan at all( sitting in the shop lol has a loose blade) as long as i dont work real hard. IE long grades, towing a car and trailer etc.

 

Thanks for the info, that is definitely not the scenario i was hoping for though ... What are the odds of both the fan and the rad being bad on this truck? If it was just the rad that was bad, the fan would kick in, no? Or is it common for those clutch-fans to go? Is there a way i can check the rad before pulling it off?

 

i have never in 25 years and 496,000 miles heard my fan kick in.
first thing i would try is putting an aftermarket mechanical temp gauge in it to see what is actually going on.

 

Tom, do you not tow with your IDI? I can see not hearing the fan while cruising, not sure if I ever have or not. But towing a trailer up a steep grade in the heat I absolutely hear the fan kick in! And out! I got to where when I was towing quite a bit over the 299 I could tell by watching the temp gauge just when that fan would kick in, then it'd cool off enough and the fan would kick out. There was no mistaking it whatsoever!

 

well, i will also admit to having open exhaust up until about 2 years ago, so there is probably 450,000 of those miles with the pipes ending 2 ft from the driver side window.
i think that is most of the reason i am almost totally deaf in my left ear.
and up until 5 years ago the truck was a combination tow plow rig. usual load was combined weight 29,000 lbs. 17k on the mini excavator, 3k on the trailer and 9k on the truck.

 

Wow, that would do it, it'd be hard to hear anything like that.

 

yea. between the pickup, D-8H dozer with no muffler, and close to 25 years in large diesel trucks with no AC it has made it very easy to ignore people i don't want to hear.

 

Do i need to drill/tap for a mechanical temperature gauge installation, or is there a place to screw it into?

 

remove the temp sender on the front side of the driver side head and put the mechanical sender there.

the sending unit on behind the thermostat housing id for the cold idle/timing advance in the injector pump.

 

Does that mean that the dashboard gauge will not work then? I don't know if i care if it doesn't

 

correct, it will not work. but you could put a "T" in the block and run both, or just use the aftermarket gauge to see if the in dash one is accurate or screwed up and then remove it once you have it figured out.

 

I would not put a T in for a coolant gauge. T's work fine for pressure applications as the pressure will equalize throughout the T. However, temperature gauges require the coolant to circulate past the gauge and that doesn't work with a T.

There is a larger 1/2" pipe plug (square head) on the drivers side head towards the rear. There were a couple good pictures of that in another thread about coolant gauges a few weeks back. Thats where I put my aftermarket gauge with a bushing to reduce the hole size. The stock gauge and idiot light will still work fine. Its just a little harder to get to that plug, and some creativity to remove it. If you have a 9/16" square socket (or 8 pt) that will work.

 

there are two temp sensors on the front drivers side, one screws into the block and is for the stock gage, the other screws into the head and is for the temp idiot light. we normally remove the idiot light sensor to add a real gage.

or if you want, there is another 1/2NPT port further back on that head, which should just have a plug in it. it'll work just as well for you

 

The fan clutch kicks in when it senses hot air coming through the radiator. If the radiator is plugged up, the fan clutch may not get enough heat to kick in...