I’m having a strange issue and hoping the community here can lend some assistance.

93 F250 w/ EFI 460. Bone stock, excepting the previous owner removed the air pump.

I noticed the truck wasn’t getting up to temp when the mornings started getting cooler. I thought maybe the thermostat was stuck, so I bought a new thermostat and decided to flush the cooling system when I installed the new T-stat.

Under the housing I found a gutted thermostat and replaced it with a 195-degree replacement from NAPA.

Initially everything seemed fine, the truck gets up to temp, gauge climbs and levels off, heat works… everything is good.

Then yesterday I notice the temp gauge is reading a lot higher than it has been… not into the red, but high enough to be worrisome. I smell coolant, so I pull off into an AutoZone parking lot. Under the hood the overflow is overflowing, but the radiator cap is barely hot to the touch.

The level in the radiator is down, I can see water, but it’s a few inches below the neck. I grab another gallon of coolant, fill the radiator and drain the excess from the overflow tank into the bottle.

At this point my guess is that I didn’t get all the air out when I flushed the cooling system. A steam pocket occurred and forced the coolant into the overflow.

I kept a close eye on the coolant gauge on the ride home and nothing doing. The needle moved around a bit, but stayed between the “O” and “M” of normal.

Then today on my drive home from work the gauge starts creeping up again… I noticed that the idle went up with it. As the gauge passes the “L” of normal the idle was up to nearly 1500 rpm.

This time under the hood the overflow tank is level at the cold fill line and the radiator cap is barely hot to the touch. I put a rag over the cap and get ready to jump back. At the first notch…

Nothing.

No jet of scalding coolant, no hiss of steam, nothing.

The coolant level is steady, right at the radiator filler-neck. Fire the engine back up and I can see that coolant is circulating. The gauge still reads high, but it’s dropping rapidly and the idle is back at the 850 rpm lope I like to hear.



Now I’m in a spot where I no longer trust the gauge, but know I HAVE to stop driving if I see the gauge start reading hot.

Would a bad coolant sensor cause the high-idle?

What else should I look at?




be safe




Jeremy

 

Yes, a bad ECT will cause the ECM to just assume that it's cold outside, so it's going to go into the cold-start enrichment strategy and high idle.

That being said, I don't know for certain on a '93 460, if the gauge pulls it's signal from the same sensor as the ECM. I know on my '87 with a 4.9, the gauge has a different sender that is in the head.

Get a non-contact thermometer (the thing with the laser in it.) Point it at various spots while the engine is running, to see if it's actually overheating or not.

 

I had a similar issue with a '51 ******, nearly 50 years ago. Brand new thermostat would randomly not open. Even back then, you could get a bad part.
I tested the next thermo through several cycles in a pot on the stove.
Might wanna try a different thermostat

 

If you popped your cap and there was no pressure on the system, but yet still full. Sounds like your cap is not holding pressure. The coolant system needs to be under pressure when hot to work correct. Try a new cap and see if your problems go away.

 

92 460 has 2 coolant temp senders - one for ECM and another for guage. Assume 93 is same.

 

It is my understanding that there are two sensors… not likely they both fail at the same time.

I’m gonna take the advice of using an infrared thermometer and verifying actual temperature and going from there.




be safe



Jeremy

 

Yep, my first thing I would either check or replace is the radiator cap. Should hold 13 psi before releasing into overflow bottle.
Like mentioned above, check temp with an actual thermometer.

Start with the simple, easy and obvious stuff before replacing parts at random.

 

When I replace a thermostat, I usually always replace the radiator cap.

You say previous thermostat was gutted?, meaning the inside is gone? Where did it go? still in the engine plugging something?

 

Years ago, I bought a vehicle were the PO gutted the thermostat. I guess their thought process is that it helps to seal and in some instances, there is an oring to seal the thermostat to its housing. Sort of like the 5.4 in my Expedition.