7.3 temperature gauge dropping under load

[92gold]

Truck is a 1992 7.3 idi with ats 093 turbo kit. Truck runs great besides its little air intrusion issue through the selector valve on cold starts. Put a new motorcraft thermostat in in November along with Napa anti freeze with SCA already formulated in. Has a champion aluminum radiator. Truck always runs with the temperature gauge right below the N. Recently when going up the grade or under heavy acceleration the temperature gauge will drop down almost all the way to cold, then when I let off the gas a little to go down the guage will come back but up to the o, and will sometimes fluctuate between colder than normal and warmer than normal. After it does this if I look under the hood and squeeze the upper rad hose it is soft and I can pull the radiator cap off and there isn’t hardly any pressure if any. No oil in coolant, no coolant in oil, doesn’t seem to be leaking or loosing any coolant either. I was thinking maybe it’s the cap but I’m skeptical as it only does this when it’s under a load with around 3/4 throttle. Holds pressure for hours with the truck off if it doesn’t do this before I put it away.

 

[Nero]

I wouldn't trust the stock gauge too much, if you're worried about coolant temp, get an aftermarket gauge.

 

[92gold]

That’s what I was thinking before that it was just the gauge, but I found it kind of weird that the temp would drop then when I let off would go up higher than normal. And the fact that the upper radiator hose is usually hard and when it does this it will be soft after and won’t have pressure in the system. I usually drive on the freeway for a good 30 minutes before I even get the the grade

 

[Cubey]

equus water temp mechanical gauges are about $35 at O'Reilly and are typically in stock. they come with the fitting to adapt it to either port (dummy light port or stock gauge port)

 

[92gold]

Why would the system be loosing pressure though after it does this?

 

[Nero]

It might be the cap isnt maintaining pressure well. Are you using the stock 7lb cap or a 13lb?

 

[92gold]

It had the cap what came with the radiator, not sure what pressure it was. I just bought a 13lb pressure release cap gonna try it tomorrow

 

[KansasIDI]

I once managed to blow up a radiator with a 13lb cap. I’ve since switched to a 9lb

 

[IDIBOBS]

I’d for sure start with an aftermarket gauge. Stock ones are not accurate at all.

 

[Cubey]

I once managed to blow up a radiator with a 13lb cap. I’ve since switched to a 9lb

I've always used 7lb with good luck, including the RV in the mountains. i personally like the red lever safety caps. It's less messy to check the fluid, since it sends the pressure to the reservoir before you remove it.

 

[Clb]

+1 on factory idiot gauge( it's not a gauge) see the tech 101 for how to make it work as a gauge...

Eta
I can't find the thread to alter the gauge to make it read normal

 

[mexicanjoe]

Have you checked your fan clutch to make sure it s not locked up like mine. Cant get mine past 165 with a trailer load of cattle and a/c going on a hot june day. Man it makes a horrendous noise like aB-52 on takeoff.

 

[92gold]

Did the same thing again today but the gauge wasn’t acting up as much. Still dropped down again though when under heavy throttle going up the grade, and the top hose was soft but not as soft as with the old cap when I stopped 10 miles later. Doesn’t make sense to me why the system would be pressurized and the hose would be hard until it’s under heavy throttle then will lose pressure. How do I test the fan clutch? Doesn’t sound like the fan is very loud but it’s worth a try

 

[Clb]

Longshot
Any chance of air intrusion in the rad?
Huge l o n g s h o t

I wonder, could the sender have a dead spot?

 

[92gold]

Not sure if there’s air intrusion in the rad or not, I don’t see any bubbles. I know the thing doesn’t get close to over heating so I’m fine with the gauge acting up, I’m more worried about it losing pressure like that