What would cause a truck to overheat at speed only?

[QwikKota]

A friend has a 2014 Longhorn with 100k miles that is overheating when driving but cools off when idling. I have never heard of that. Thoughts?

 

[Bigskyroadglide]

From google

A car overheating at speed but cooling at idle usually indicates a problem with the cooling fans or a lack of airflow through the radiator, such as a malfunctioning fan, a blocked or dirty radiator, or insufficient coolant levels. Other potential issues include a failing water pump, a clogged thermostat, or an airlock in the cooling system.

 

[Racer9]

First place I would look is around the radiator. If it has the shutters, are they opening?
Is the radiator or its cooling fins plugged?
Is the grille clear? Some aftermarket grills look cool but don't allow much airflow.
Are the fans working properly?

 

[QwikKota]

But why would it cool off at idle, less load? I mentioned to check the active grille shutters.

 

[Bigskyroadglide]

Absolutely less load. It could be as simple as a fan clutch.

Turn the air conditioner on. The secondary electrical fan is commanded to ON with the AC on, see if it stays cooler.

 

[QwikKota]

Texas in September, the A/C is definitely in use. They took it to the dealer, doh.

 

[mikeru]

I’ve only ever seen that happen when the ignition timing was too advanced. I don’t see cooling fans causing it, as speeds above 35 mph push more air through the radiator than the fans pull through.

 

[A child of God]

My mother's 92 RS Camaro did this waaay back when. It was just missing a simple plastic air dam under the car. With these trucks and all the criss-cross electronics, I'd consider changing the batteries in your home remotes. ‍

 

[Ken226]

My mother's 92 RS Camaro did this waaay back when. It was just missing a simple plastic air dam under the car. With these trucks and all the criss-cross electronics, I'd consider changing the batteries in your home remotes. ‍

I had 92 Camaro that had the same problem, and the same solution.

Those stupid things had literally no grill whatsoever for airflow, and the radiator was right behind the non-grill.

There was an air dam that hung down below the radiator and all the air was scooped up from beneath the car.

I still remember the sound that air dam makes when it scrapes it's way over, and onto the other side of a parking curb. Both directions.

 

[mdc1990zr1]

I had 92 Camaro that had the same problem, and the same solution.

Those stupid things had literally no grill whatsoever for airflow, and the radiator was right behind the non-grill.

There was an air dam that hung down below the radiator and all the air was scooped up from beneath the car.

I still remember the sound that air dam makes when it scrapes it's way over, and onto the other side of a parking curb. Both directions.

View attachment 572743
View attachment 572744

You think that was bad, I had a ZR-1 Corvette. That was a real bottom feeder. If it was't moving, the oil and coolant temps would skyrocket, literally. Watching those temps creep into a very uncomfortable zone. Once you started moving, the temps would come down fairly quickly, but watching those needles rise made you buttocks pucker.

 

[hunterdan]

So what temp are we considering as overheating? Any truck with the 8 speed and active shutters will reach 220-223 degrees while cruising when the shutters are closed, but when it reaches those temps, the shutters will open and the temps will cool down to about 199-203, then repeat the cycle. At lower speeds the shutters stay open and temps don't reach that high.
As was said, above 35-40 mph, the fans are basically useless. If getting above the temps mentioned above, and have active shutters, I'd look at those first, then check the thermostat.

 

[crash68]

A friend has a 2014 Longhorn with 100k miles that is overheating when driving but cools off when idling. I have never heard of that. Thoughts?

Diesel engines do this naturally, the winter front you see on diesel vehicles in the winter are not only to help warm the engine up but help keep the heat in while stopped.

 

[QwikKota]

I believe an overtemp warning light came on but not sure at what temperature. I am getting second hand info.

 

[Jeepwalker]

Could be a weak lower cooling hose to the water pump that maybe doesn't have a spring ....and it gets sucked-in under highway speeds (restricting coolant flow). A low engine speeds the hose expands to it's original shape and cooling flow resumes.

Squeeze it. If there's a spring in there and it's in the middle (or the hose is good and stiff), that probably ISN'T the issue. But check it out in case someone put on a hose that can flex in on itself.

 

[Jeepwalker]

My brother had a water pump which the impeller would expand when the coolant got hot and would spin (slip) under load. But when idling there must have been enough friction to hold and it would cool. We tested it off the car and yeah... it was definitely slipping when warm/hot. But at ambient temp it was nice and snug. I think that's pretty rare but worth considering.

I've replaced radiators on a number of vehicles and it astonishes me some days how much crud can get trapped between the condenser and the radiator. Years of bugs and sometimes leaves. And you can't tell until you have either the radiator or the condenser out. One recent car was pretty much plugged in the center of the radiator. Dead bugs, leaves, grass. It was easy to back-wash with a hose once I had the condenser moved out a bit.

You may see if you can look between them, or shine a strong flashlight from the rear of the radiator at night... see if you can get a sense how much light is shining through and how much might be blocked. Even that's probably not a good test, but a good place to start.

 

[Jeepwalker]

Thermostat is usually the first place to start. The *Pin* ....or sliding shaft where the valve slides up/down on can become scored after many years and heat/cool cycles. Not always...but 'sometimes' they can. I've seen it. I've even seem them get bent!

In some cases it can cause the sliding valve to 'bind' and not fully open. Sometimes they can open ok in a pan of boiling water, but act differently under water flow conditions. If there is pressure sideways, it might bind. Sometimes if ya slide the valve back you can see scoring on the Pin/shaft.

There are a few things which can fail on a thermostat. If it cannot open enough, that would restrict coolant flow which could cause the truck to overheat at highway speeds. If you bother to take it out, I would replace it with a new Mopar T-stat anyway (notice I didn't say cheap parts store t-stat). Even if they 'look' good, I've still had it (after wasting time looking at other parts of the cooling system) where the thermostat was the ultimate issue.

Understanding a cooling system thermostat - Farm Machinery Digest

The purpose of a thermostat is to reduce engine warm-up time, maintain optimum running temperature and help with heater output,…

farmmachinerydigest.com

 

[EdGs]

I believe an overtemp warning light came on but not sure at what temperature. I am getting second hand info.

I think that happens at 240°F.

 

[QwikKota]

Thanks for the suggestions. I don't have the truck in front of me to check anything unfortunately. I told them to take it to a local shop but they didn't listen.

I was thinking 140° as well.

 

[Dean2]

You will have to let us know what the dealer did to fix it.

 

[QwikKota]

For sure.