Overheating...but not?

Hey, had a slight problem this morning. 2011 F150 6.2L motor. just over 100,000 miles on it, no modifications other than a cold air intake. No repairs or anything in the last few months aside from a few oil changes and a set of new tires.

Drove around for about 15 minutes and the temp gauge spiked and the warning told me the engine was overheating. I reacted by switching it to heat and cranking up the fan. Took a few seconds to park it in a safe place to figure out what was going on. Before I even tried to park it, literally within a few seconds the temperature started dropping rapidly and returned to normal at about the same time I put the truck in park. Opened the hood to assess what's going on. Coolant in the reservoir, radiator cap is cold to the touch but the radiator fan is in high gear. Turned off the truck and released the pressure in the radiator by turning the cap just a bit, pressure release didn't take any longer than normal. No fountain of coolant or anything. Came to the conclusion that the truck wasn't actually overheating, just had a false reading on the temp gauge that caused a chain reaction of the temp gauge *******, notification on the dash, and the radiator fan going at a higher setting than usual. Jumped back in it and started it again, temperature was normal and remained that way with no problems for the rest of the drive, approximately 20 mins. The only thing prior that I can think of that may be related is a slight bubbling of coolant on start up occasionally that goes away as soon as I start driving.

So to recap for the tl;dr:

• 15 mins into a drive the temp gauge pegged.
• Turned on the heat full blast, returned to normal temperature within a few seconds.
• Truck on, radiator cap cool to the touch, coolant in overflow reservoir, radiator fan on ludicrous speed almost plaid.
• Truck off, no bubbling, normal radiator pressure, no fountain of coolant with cap off within 2 or 3 minutes of shutting the truck off.
• Turned truck back on within 5 minutes and back on my way with normal temperature and operation for the remainder of the drive, approx. 20 mins of mostly back road driving, ~30-50 MPH.
• Other possibly related symptoms: bubbling upon starting the truck cold once in a while which goes away as soon as I start driving.

Where's a good place to start and test? My initial thought is that the temperature sensor is starting to go bad. Anyone know what the resistance is supposed to be across the terminals if I put an ohmmeter on it?

It's my understanding that air in the system can do this. Do you have one of those radiator funnel things (no idea what they are called) that you can use to remove the air?

Dane said:

It's my understanding that air in the system can do this. Do you have one of those radiator funnel things (no idea what they are called) that you can use to remove the air?

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I don't. The ol' raised front with the truck on and the radiator cap off won't do? Also, I was thinking it might be the thermostat sticking causing the coolant in the motor to overheat and then, in this case, coincidentally opening as I was parking causing the temperature to drop suddenly? Thoughts? I don't like coincidences though.

I was thinking the Thermostat also

Another interesting thing to note is that the ambient space around the motor didn't feel hot when I opened the hood to inspect it, which also leads me to think that the motor isn't actually overheating and the coolant temperature sensor is malfunctioning.

With over 100k on it I would do the thermostat any ways. This may also be the culprit... Could kill two birds with one thermostat!

Gatica said:

I was thinking the Thermostat also

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ICLOSEM said:

With over 100k on it I would do the thermostat any ways. This may also be the culprit... Could kill two birds with one thermostat!

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Yeah, I'll replace it this evening and hope I don't have another episode. Thanks for the input, gents.

Not the same vehicle, but bear with me.
I had an ’06 Outback before the Rap. Out of the blue, about 3 or 4 years into ownership I get a CEL, temp gauge pegs, cruise disabled, but car drives normally - not in limp home mode. First time, i did like you, max heat, max fan, pull off etc. The code was ( I think ) p0500 or p0600 - generic bus fault

I looked it up, it’s a fault for a catch all code. I got it reset and it happened again about 9 months later. I did some research and found a TSB, the dealer swears they followed it but didn’t find any issues. Strangely enough, I never had the fault again. hmm...

based on your description, I’d be inclined to start with the temp sensor, but I wouldn’t rule out pulling codes too.

bubbling in the coolant doesn’t sound good though. T-stat is cheap and should be pretty easy to try as well.

smurfslayer said:

Not the same vehicle, but bear with me.
I had an ’06 Outback before the Rap. Out of the blue, about 3 or 4 years into ownership I get a CEL, temp gauge pegs, cruise disabled, but car drives normally - not in limp home mode. First time, i did like you, max heat, max fan, pull off etc. The code was ( I think ) p0500 or p0600 - generic bus fault

I looked it up, it’s a fault for a catch all code. I got it reset and it happened again about 9 months later. I did some research and found a TSB, the dealer swears they followed it but didn’t find any issues. Strangely enough, I never had the fault again. hmm...

based on your description, I’d be inclined to start with the temp sensor, but I wouldn’t rule out pulling codes too.

bubbling in the coolant doesn’t sound good though. T-stat is cheap and should be pretty easy to try as well.

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Smurf, I haven't tried to pull codes yet as the CEL hasn't come on at all. I pulled codes not too long ago though, however, after one of the transmission solenoids went out on me and threw a few codes but nothing since. Also, I should clarify that the bubbling isn't really a bubbling sound, more of a whooshing sound and it's only on cold start ups for a few seconds.

Also, I changed the thermostat last Tuesday, tried to break it for 20 mins to see if it would overheat again. All good. Then I really tried to break it this weekend. Drove ~640 miles round trip (out of the mountains, to the beach, and back into the mountains) towing a camper with everything weighs about 7000 lbs. No overheating.

However, one small problem remains, no heat at idle. If I rev up to about 1500 RPM I have heat. Thoughts? Wore out water pump impeller?

Have a mechanic test your coolant for carbon... If you have a head gasket going bad, the exhaust can leak into the cooling system and create an air pocket that will trigger an intermittent guage spike like you describe.
Have you ever flushed the cooling system? Aluminum heads on an iron block causes electrolysis over time, turning coolant acidic, which will then eat away at gaskets.