Have 1988 F-150 . Two wheel drive four speed stick, 300 straight six. (with 1970 carb and manifolds) Dual tanks long bed regular cab. Engine is rebuilt wit has about 80,000 miles. Truck has total of about 255000 on it with 31 10.50 tires. I had a leaking freeze plug and a non functioning heater. Last weekend I drained the coolant, and replaced both heater and freeze plug. Because it was running hot, would get about 215 or 220 when cruising at 65 mph on highway with or with out cargo. When I pulled the old heater core out, it was plugged up. I had fluid draining out of one port but no matter how I held it or shook it I couldn't get fluid to drain out the other side. So I assumed this was cause running over temp. Put new heater core in, tapped in new freeze plug. I put new coolant in it and drove it about 8 miles to gas station, this morning. Well heater still doesn't work, and the engine is still running at the same temperature. The previous owner had removed the thermostat to solve this problem and the owner before him had a set of electric fans added onto the radiator in addiction to the normal belt driven fan. I had a water pump go out about a year ago and put a new thermostat in when i replaced the water pump. . So what is wrong with it? why has it been running hot for so long I don't want to use any of these "stop gap" solutions to keep her cool. What else may be wrong? I'm running out of ideas, heater still doesn't work and still runs hot...

 

I've seen the impeller come off on a water pump so belt is turning pump but no water is moving in engine. Can you see water movement in the radiator neck with engine running?

Also on some engines, if you remove the thermostat the engine will not cool properly because the coolant is moving too fast through the heads (engine). The thermostat actually slows the movement of the coolant (even at full open it provides some restriction to flow). If the water moves too fast through heads it doesn't have enough "contact time" to pull the heat out of the engine. If all other checks/tests fail, install a 165 degree thermostat. Someone may have removed it trying to fix a different problem (which you then fixed) and crated a new problem in the process.

EDIT - you said you installed the previously removed thermostat - so never mind...

Does the cooling fan have a fan-clutch? If so when the engine is hot (not running of course) the fan should not turn easily. https://www.yourmechanic.com/article...ing-fan-clutch

Lastly don't trust the factory dash temp gauge. Could be a bad gauge or sensor. Be sure the engine is running too hot (unless it's really obvious) by testing with a known good mechanical gauge.

 

The in dash water temp gauge went bad before i got it. previous owner added after market guage. i know that no thermostat is bad. Thats why i put one back in. Im going to check the hoses to the heater core next make sure theyre not clogged too. Iwill try and get a lower temp thermostat in there. impeller breaking off is exactly what happened to previous water pump. Has new pump. made awful noise i thought it was a rod nock and went an spent 10k on a new car. only to find it was just a water pump...

 

If it gets hot while running down the highway, but cools off putt putt ing around town, I would suspect you do not have a spring in the lower radiator hose, they can suck shut at highway speeds. And new hoses sometimes don't come with a spring, you have to pull the one out of the old hose and some people don't know this.

Another cause would be the radiator clogged. There is a test for that also.

Check for a spring in the lower radiator hose.

 

Im not sure, but I dont think it has a spring. ill check that too.

 

Mike & Dave F both bring up good points.
I would check that lower hose for a spring first.

With everything cold pop the radiator cap and make sure you have a level in there.
Start the motor and let it run to get up to temp.
Keeping an eye on the gauge start feeling the heater hoses, 1 should get hot before the other but in time both should get hot as the motor reaches normal temp.

Also keep checking the radiator and as the stat opens you should see a flow.
The upper hose will also get hot as the stat opens.
When the stat opens a few times or the temp gauge is close or at normal temp feel the lower hose is it hot and do you see a flow in the radiator?
If you can get to the front side of the radiator feel the core if you have flow it should be the same temp across the full face.
If you have no flow up top, lower hose does not get hot and the face of the radiator is not evenly hot you could have a plugged radiator and even more so if the heater core was plugged. That is if the pump is still working as it should.

You sure the motor driven fan is installed right? It is not pushing air forward as you drive, then you have no air flow.

Now there is also something else I am thinking but I don't think you can answer it as I am guessing it all happened before you got the truck.
You said the motor was rebuilt was the head gasket installed right or maybe the wrong one?

Was it board over size and if so to what? When you start going on the large size you could start running into issues of running hot.

Why did the 1 freeze plug leak did it rust out? If so others may not be far behind.
Also when it was out you should have checked if there was mud in the bottom of the block. I have seen (and I have one also) a block that flushing ran clean but had mud in the bottom and ran hot till the motor was hot tanked (not mine yet) then was ok.

Just some things off the top of my head.
Dave ----

ps: are you sure that after market gauge is right?

 

I dont know if the aftermatket gauge is acurate or not. I have no idea about bore or head gasket. I had nothing to do with rebuild.

 

I also have an 1983 with same engine and trans ( but shortbed and 4x4) been debating about selling one or both just because of how much time i have to spend under the hood. butbi cant makebup my mind.

 

A plugged heater core won't cause overheating. Some older vehicles didn't even have a heater.

With a new heater core, and the coolant possibly warmer than normal, you should have plenty of heat. Can you confirm the heater core is getting hot? Maybe the lack of heat is caused by an airflow problem in the HVAC ducting, but the core itself should be plenty hot. Is there a coolant shutoff valve in the lines to the heater core?

Note I mentioned the coolant is possibly warmer than normal. Save yourself a lot of grief and beg, borrow, or steal an infrared thermometer. Cheap ones only run around $20 and work just fine. Make sure you're dealing with an actual overheat condition, not an indication problem.

Dave had a lot of good advice. One thing he mentioned was airflow through the radiator. With the engine at idle, a quick test is to hold a sheet of paper on the front of the grill. Good airflow through the radiator core will hold the paper against the grill.

What kind of belt-driven fan is installed? Does it have a fan clutch, or is it direct drive? Is the fan blade stock, or some high-flow aftermarket version? Some aftermarket fans, especially if directly driven (no clutch) can cause a weird situation of too MUCH airflow. Seems kinda strange, but hear me out. If the blades are trying to take too big of a bite, or the fan is turning too fast, you end up with a spinning mass of turbulence that actually reduces airflow through the core, where it really matters. Rather, you want nice, clean, non-turbulent airflow for maximum effectiveness. Sadly, some aftermarket fans cause just the opposite.

Also, you mentioned the truck runs hot at 65mph. Does this also happen at lower speeds? What about at idle when parked?

Lots for you to think about so far. I can bore you to tears with additional troubleshooting, but first we (meaning you) should really confirm what is happening temperature-wise.

 

Heat still does not work. heater lines are not plugged, lower radiator hose does have a spring has facotry fan with a clutch that does function properly. i think the radiator is wrong size...

 

Not trying to give you a hard time, but what makes you say that?

What about the vehicle speed? Only around 65? Lower speeds? At idle, truck stationary?

 

Dave, I'm not sure I agree 100% with this bit. Playing around with an infrared thermometer, I've found in a properly functioning system, the radiator outlet should be about 30 degrees cooler than the inlet. On the face of the radiator core, you should see a proportional temperature drop from the inlet tank to the outlet. If checking by hand, yes, both will feel hot. However, I strongly urge getting an infrared thermometer for troubleshooting. It will show a temperature drop your hand just can't discern.

 

its fun up to anout 50. over 50 it starts getting to hot. i aay the radiatormight be to small cause it looks same size and the one the a/c system.

 

I was talking by hand and more to see if it had cold areas ie: no flow.

Can you post up a picture?

On the heater you are going to have to pull a hose to see if you get flow to it.
Dave - - - -

 

I don't know if you have the right size radiator, but a old time radiator guy showed me how to check to see if it's partially plugged up(usually with lime deposits from the water).

Drain the radiator and take the lower radiator hose off the radiator. Get a garden hose with a sprayer, and use one hand with the sprayer to fill the radiator with water while the palm of your other hand is over the lower radiator outlet plugging it up. It would be easier with two people to do this.

Once the radiator is full to the top, quickly take your palm off the lower outlet. The water should come out full flow the full capacity of the lower outlet. If it's less than full and struggling to come out(gulping and gurgling) then your radiator is partially clogged up.