I have a ’92 F150 2wd with manual transmission. If I’m driving at around 35mph the coolant temp runs normal in about the middle of the range. If I get up to highway speeds the temp will climb up to the upper limits and, depending on ambient temps, would probably go all the way to the red if I let it. If I then slow back down to around 35mph the temp will drop back down (even if I stop and let the truck idle it will drop). As I mentioned, if the ambient temp is high and/or I am running the ac, the higher the temp wants to run.

Two years ago the truck was running hot when driving slow so I replaced the radiator, water pump and fan clutch (I believe the fan clutch was the main problem). That fixed the problem then and the truck has been running fine up until this summer.

For my current problem I have flushed the radiator, installed a new thermostat and cleaned all the bugs and stuff out of the radiator and ac condenser fins.

The next item to check I plan to check is the gage to make sure it is reading accurately.

It has also been suggested that a leaking head gasket could be the problem. The only thing is that it doesn’t seem to be using any coolant and it isn’t pressurizing the coolant system. After driving with above normal temps for maybe 20 minutes the fluid in the reservoir is only maybe ¾ of an inch higher than when the engine is cold.

I appreciate any ideas or suggestions. Thanks

 

It sounds very much like the Gauge is the main issue here. If it were going to run hot, while under load, it would do it at low speeds, and be even worse as load/rpm increased if the radiator could not flow enough.

I'd suggest taking a radiator hose and spraying thru the grille into the radiator, and see if water comes thru the radiator into the engine compartment. Do this across the entire radiator to make sure it doesn't have any gunk or anything else blocking the radiator.

Also another good idea might be to pull your thermostat and check or replace it just to eliminate that also.

Good luck, hope ya find it.

 

Thanks for the fast reply. Yes hopefully it is the gage. Actually I did just replace the thermostat (twice) so that shouldn't be the issue. This past weekend I removed the radiator and washed all the bugs and debris out of it and I also rinsed out the ac condensor to make sure I was getting sufficient airflow through both.

When the fan clutch goes bad they usually just cause overheating at slow speeds corrrect? Is it possible for the fan not to be free wheeling at highway speeds and not allowing enough air to flow through the radiator. This was just another idea I had.

 

Dead fan clutch would make you overheat at low speed. At highway speed the fan is just along for the ride so clutch operation is not an issue unless it was stuck "closed" and the fan was spinning all the time. That wouldn't cause overheating, just performance/ MPG loss. Symptoms say thermostat but you've changed that. If it's not the gauge then this is a stumper. Might try running temporarily without a thermostat just to see what happens.

Good luck and let us know what you find.

 

Have checked your timing lately, incorrect timing can cause overheating.

 

I would have to agree. In my newbee days I used to set the timing up without disconecting the advance(computer or vaccum) My truck did that for over a year. Read somewhere on that too retarded of advance will cause the same thing. Lo and behold I set my timing right and she has ran cool as a cucumber. -Dave

 

You mentioned doing some radiator work in the past. Did you replace the hoses. If so did the lower rad-hose have a wire spring in it. This hose is on the suction side of the w/pump. Without the spring, the hose can collapse at high RPM. This will stop water flow through the engine. Just a thought. Good luck.

 

No, I have yet to replace the hoses. I do have new ones that I am planning to install this weekend. Neither the old hose or the new one has a spring in it. Maybe I'll try and find a hose with a spring in it. At least that way I know 100% that isn't the problem.

 

Have your rad cleaned. Had the same problem, and had to drive 500 miles at 45 mph because of a 90% blocked radiator. 15 hours to get home from southern Missouri! This was after we discovered a dead fan clutch and a bad thermostat, and blew the lower hose.

Word from the newly wise: NEVER shut off your truck when it is running hot, unless you want to spill your coolant, and spend the night in Western Auto's parking lot. Ask me, I know. Learned the hard way.

 

I have the same exact problem with my truck currently. Please let me know what you find out.

 

could it be the thermostat.......?

 

We have tried that and other things. Read previous threads.

 

have you rtried replacing the temperature gauge sending unit? it is located on the back of the block kinda above the starter. the part is less than $10 and is easy to change, only pulling one wire and one wrench are required. its a cheap try. with the unit out you could also try flushing the system and letting it drain through the bottom of the block throught the sending unit hole. maybe theres some gunk trapped down there?

 

I am willing to try that. My only question is if the gauge moves up and down under different circumstances, what makes it possibly wrong. I not doubting you just trying to learn something.

 

i dunno. just trying to start with the cheap stuff. sounds like thermostat, but you said you've already replaced that. how long ago? I've had my truck for 6 years, and I've put 4 thermostats in it. I don't know what the deal is but my truck eats thermostats. Doesn't matter if I use the cheap or expesive ones, or if I put it in or an ASE certified mechanic puts it in.