I have a 1996 F250 Super Cab with a 5.8l engine. The truck started running hot (never overheated), so I put a thermostat in the truck. The temp gauge showed the truck started running more normal. The needle would run right around the "o" in normal, then when it was under a load like climbing a hill and having to downshift to keep speed, it would range up to the "r or m" then when you came to the crest of the hill would go back down to the "o". Not feeling this was right, I had the truck's cooling system tested. It was determined that the clutch fan was weak. So I had the clutch fan replaced. The temp gauge is still doing the same thing but a little more frequently without the load issue. The other issue I am having is that now with the colder weather, I have turned the heat on, and have had the temp setting full hot, the fan motor blowing full blast, and it cab temperature is barely getting warm. It blows luke warm air at best. I know the thermostat could be faulty even though it is new, but it doesn't appear to be bad. I need this truck to blow hot air as it is a plow truck and I need to be able to keep the windshield clear. Does anyone have any other ideas what I should be checking. The mechanic who checked the cooling system said the water temp was good, and seemed to feel the water pump was working correctly. But I just don't know any more. I don't want to dump a lot of money into the truck as it probably only has 1 or 2 more years of plowing. That is all this truck is really used for.
Thanks in advance.

 

Hi rcnease... before you spend another dime, let me ask you a few questions.

Did your heater blow hot before you replaced the thermostat? (last season if not recently used?) And, how did your mechanic check the water pump?

 

trip2way: I really don't know how he checked the water pump, he just said that he checked the temp, and the flow and it was good. He said the fan clutch was weak and should be replaced. I wish I could tell you exactly what he did as it sounds like you have an idea or have been through this before.

As for blowing hot last season, it was at the beginning of the season, it would run you out of the cab. At the end of the season, it was not blowing hot, and it was actually on the way home from the last storm that it started running hot. I believe the heater stopped blowing hot on the last storm as well as that is when the driver mentioned it to me (I drive another truck). I don't know if it was getting weaker each storm or what. He never said anything.

Thanks

 

What is the manufacture month and date of your F250?

You'll need to check some basics in the heating system...especially since the truck is not overheating and the thermostat is operating normally.

You may have an issue with the blend door or a stopped up heater core. Check those out and let us know what you find.

 

I will have to get a hold of the other driver to check manufacture date. I am not adversered at checking out the things you mentioned. Is there an easy way to check it. I have read a little and most of them require taking hoses off and putting stuff in between that I don't have. Any help is greatly appreciated. Thanks

 

Oh, as for the manufacture date, I can tell you it is the old body style. It looks like the 92 that I also have.

 

Okay. Find out the date, let me know. The blend door may be cable driven or electronic (gear driven )...this will determine how to check it properly.

In the meantime, get the truck up to normal operating temperature. Locate the heater hoses going into the firewall. Check that both are hot. If one is hot and one is not, then you have a stopped up heater core.

 

96 F250 has a cable operated blend door, just like any other OBS truck.

Weak heater and that erratic temperature gauge sounds to me like you have air trapped in the cooling system. Did you bleed it after you changed the t-stat? Park uphill in your driveway or jack the front of the truck up slightly, then run it with the radiator cap off til the stat opens, then add as needed while watching the temp gauge and feeling the heater vents. If the gauge acts the same and/or you get no heat, shut the engine off and add more coolant, then restart and check again.

When the level holds steady while running, cap it and go for a ride. Be sure to eyeball the level once more the next morning after the engine cools overnight.

 

Didn't they switch that in late 96?

 

Your cab heating problem is a separate issue from the engine temp issue. To diagnose the cab heat simply touch both heater core lines with the engine at operating temp, if both are hot your heater core is OK, if one is cold then the core is plugged. Sometimes you can clean out the core by simply forcing water through it with a garden hose. If both heater hoses are hot then you have a blend door obstruction, this is common in vehicles where the driver has a habit of throwing stuff up on the dash. To clear the obstruction remove the glove box and then the rear cover of the heater box up under the dash and inspect for debris and proper door operation.

 

Not necessarily a separate issue in my experience. I've had air trapped in the cooling system on a Windsor motor... no heat and the temp gauge behaved exactly the same way that the OP describes.

BUT...

In my case the air burped itself once I shut the engine down with the rad cap off. It's not likely (only happened to me once) but it wouldn't hurt to check...

 

I agree with the statements above. The only other tidbit of info I will add is what brand of t-stat. Cheap t-stats like to fail right out of the box, either sticking open or not opening correctly. I've seen 3 fail out of the box, mostly the ones made in China.

 

Trip2way, the manufacture date is Nov 1995. The heater hoses are getting hot. But we checked the overflow bottle again, and it is dry. So there is a very good chance that some of this is an air in the system issue. We will try to re-burp the system again and see what happens. I am really hoping that takes care of it.

Thanks everyone else for your conversation. There is a lot of good things to look at with this.

 

Thanks for the update. You would have cable driven blend door. If you can pull the glove box, you should be able to see were the cable connects...verify it moves when you adjust the temp control from cold to hot.
Also, check for a vacuum leak under the hood near the blower. Should be a white vacuum line coming out near the shroud around the evaporator. I'm not sure what it connects to on a 96 f250...some have a small metal can the size of a can of viennas. This could also prevent the damper from moving when you switch to defrost or heat to floor.