1994 F-250 5.8 GAS 205K miles 2WD

• New Stant radiator cap, 13lbs.
• New Temp Guage sending unit (pic shows old one). I know it's the correct sending unit, because I disconnected it on the way to parts store, and temp guage stopped working.
• Much new coolant just added after draining radiator
• Temp guage shows overheating (pic below) at idle for 15 minutes, probably longer, but radiator cap doesn't get HOT. Water temp at this point is only 120°F (Removed cap and inserted meat thermometer). Is this normal? Shouldn't it reach thermostat-opening temp/195°F and then show water circulation movement inside radiator with cap off? Too big an engine for this? Need to run it to get thermostat open?

• Maybe an obvious temp guage problem?
• Water pump problem? Pump is only 3 years/4 months old.
• No leak from pump or anywhere else.
• No milky oil.
• No change in engine performance
• Gave the cooling fan (supposed to be a thermal type, but haven't looked for the bi-metal spring on the front of it yet. However, just gave the fan a strong push and it stopped before one complete revolution.

As I need to hit the highway this weekend for about 30 minutes, going to do that now (hit the highway for a few minutes ), then pull over and loosen cap to get some coolant spewing into a container so I can check its temp. Have rubber gloves, 5-gallon bucket water in case I need it to get back . I assume anything under 195° would be normal.

Maybe this is an unnecessary, stupid step, but will make me feel a lot better until I get some input from you guys. Be back after this run if anyone's around. Have no smartphone, only desktop PC.

Thx much in advance!

 

I went about 7 miles down the highway, and just before I got to the exit, stopped to attempt checking the coolant temp after loosening radiator cap, but cap wasn't even hot. Opened the pressure release lever on the cap and that didn't do anything either. Didn't even put any pressure into the overflow reservoir tank. Same thing on the way back home. Guage stayed pretty much the same and it was earlier when I first started it up and let it idle for 15 minutes (same as pic below with very little variation).

The one thing I don't understand is why when I removed the radiator cap near the end of my trip out (where I stopped to attempt checking the coolant temp), I expected to look into the filler neck and see coolant being moved by the water pump. The coolant level was down just below the reach of my meat thermometer probe.

Could air have been trapped in the engine (after draining radiator and adding new coolant), which could explain why my coolant level was down a bit. Could that also contribute to erratic operation of the temp guage?

 

I'm thinking your thermostat is closed/broke.

 

I know you can get a bad one, but I just installed a new one yesterday when this started.

 

If it is not circulating water in the radiator then it could only be a few things.
Bad thermostat - bad water pump - or something has clogged the thermostat.
These are mechanical and if the radiator cap is not getting hot or the top radiator hose is not hot. Then a mechanical issue is involved.

But, there are far more savvy people on this forum then me.

 

If it were either one of those things (defective/closed thermostat or water pump), would the radiator cap remain cool enough like it was when I stopped to check it, because of air rushing over the engine at 50 mph? In other words, if there were no circulation, wouldn't there be some other indication of the engine overheating?

 

remember that the flow of water comes from the top of the engine out of the thermostat - into the radiator and back down into the lower engine block.
So if the upper radiator hose is not hot then the thermostat is not opening.
Hope that make since

 

I'm letting the engine idle with radiator cap off. Coolant level at first was up inside filler neck, until it pushed out some antifreeze. Then I suctioned out a little more antifreeze so I could watch to see if thermostat would open and circulate coolant. Don't see any circulation, but coolant temp only goes to a maximum of 120°F.

Intermittently while doing this, the coolant level rises up just about to top of filler neck, and brings bubbles with it, then immediately goes back down. I have air in the system from draining and refilling radiator with new coolant? Is this rising and falling intermittently because thermostat is starting to open, then closing almost as fast as it opens? Just checked it again, and coolant intermittently rises to top of filler neck (with air bubbles), then immediately goes back down.

Have had it idling now for about twenty minutes and guage is still in the same position as pic below.


Mbizzal>>>>>Remember that the flow of water comes from the top of the engine out of the thermostat - into the radiator and back down into the lower engine block.
So if the upper radiator hose is not hot then the thermostat is not opening.
Hope that make since.

Oh yes, makes sense.

 

I'm letting the engine idle with radiator cap off. Coolant level at first was up inside filler neck, until it pushed out some antifreeze. Then I suctioned out a little more antifreeze so I could watch to see if thermostat would open and circulate coolant. Don't see any circulation, but coolant temp only goes to a maximum of 120°F.

Intermittently while doing this, the coolant level rises up just about to top of filler neck, and brings bubbles with it, then immediately goes back down. I have air in the system from draining and refilling radiator with new coolant?

Temp guage remains at same place as in pic below, and coolant temp is only about 120°F, while idling for about twenty minutes now, while watching the coolant bubble up intermittenly, then immediately go back down. This from thermostat opening, then closing as fast as it opens... pushing the coolant level to top of filler neck, then going back down immediately? I take that back... thermostat opening temp is 195°F - couldn't be that.

 

I believe the movement you are seeing in the radiator is from the water pump turning.


What about the heat in your truck. Will it blow hot? What type of coolant did you put in?




















Is the upper radiator hose hot at all?