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Temperature gauge dropping to almost cold at highway speeds

Tiny
CSOSA123456
  • MEMBER
  • 2003 FORD RANGER
  • 2.3L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 90,000 MILES
-Replaced the thermostat (twice) along with the water pump and the overflow tank cap.
-Heater core is not clogged.
-I have burped the lines countless times and made sure the coolant is at the appropriate level.
-Reasonably sure there is no coolant leak, definitely not at the t-start or water pump.
-Followed the procedure to bleed the system, 5 mins at 4k RPM three times.
-Has not overheated yet. Think that it could be due to an air bubble in the cooling system. Any ideas? Feel like I’m chasing a gremlin.
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Monday, April 5th, 2021 AT 3:27 PM

43 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,070 POSTS
I have seen a bad connection at the engine coolant sensor cause this issue. If the connection is good I would replace the sensor. Here is at the location so you can check it out. Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.
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Wednesday, April 7th, 2021 AT 11:11 AM
Tiny
CSOSA123456
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  • 31 POSTS
I neglected to mention I also installed a heater control valve, since the previous owner bypassed it.

Checked all of the connection points in the coolant flow and noticed there was a small but steady drip from the joint in the heater control valve unit, which could be the cause of this. Unfortunately the POS doesn’t have a warranty from AutoZone, so I also bypassed it :-)

will update to see if the coolant temperature drop goes away.
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Monday, April 19th, 2021 AT 4:03 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,320 POSTS
Hi,

We haven't heard from you for a couple of days. I was wondering if any progress has been made. We're interested in knowing.

Take care,

Joe
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Wednesday, April 21st, 2021 AT 8:11 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Hi Joe,

I haven’t had a chance to take a long highway drive, but will report back once I do.
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Friday, April 23rd, 2021 AT 11:24 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,320 POSTS
Hi,

No problem whatsoever. We just like to make sure you are taken care of. When you have a chance, let us know.

Take care,

Joe
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Friday, April 23rd, 2021 AT 6:23 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Hi all,

i’ve attached some images from my long drive today. Observations:

1. Truck did not overheat.
2. The temperature gauge only read higher than center when coming off the highway, or immediately after going from high speed to slow speed. After that it stayed in the middle or dipped below center.
3. This time the temperature gauge was below center at various speeds; however while idling it was dead center.
4. I don’t think it’s the coolant temperature sensor because I switched the temperature to full hot and while the gauge was below center the temperature at the air vents was not that hot.
5. The last photo of the engine bay: car is off after my drive, no coolant spilling from the overflow tank and the system is fully pressurised; the coolant hoses are all hard.

Should I suspect the thermostat at this point?
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Monday, April 26th, 2021 AT 1:50 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,320 POSTS
Hi,

It appears that the thermostat isn't restricting coolant flow based on temperature. You already replaced it. The only thing you could do is remove it and test it.

Basically, place the thermostat in boiling water and see if it fully opens. Then, remove the heat and allow the water to begin cooling. It should start going shut right around 195°F.

Removal and replacement directions are in the attachments below.

Let me know.

Joe

See pics below.
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Monday, April 26th, 2021 AT 7:51 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Replaced the thermostat a third time yesterday with a part from 1A auto at fleabay, will report back.
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Monday, May 3rd, 2021 AT 10:02 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Sounds like a good plan. Let me know the results if you get the chance.

Also, make sure all the air is out of the cooling system. I placed the directions for refilling and bleeding the system below.

I hope this helps.

Take care,

Joe

See pics below.
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Monday, May 3rd, 2021 AT 5:54 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Replaced the thermostat again, still having the overcooling issue so at this point i’m ruling out the possibility of the thermostat causing the issue.

I know the truck is:

A. Circulating coolant
B. Maintaining coolant system pressure, although I don’t know how much pressure. I’m sure the coolant cap has a bypass when it passes over 16 PSI, so doubt it could be over-pressurising.

Any logical next steps? Should I do a full deep coolant system flush and refill/bleed?
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Saturday, May 15th, 2021 AT 7:49 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Is the cooling fan staying fully engaged all the time? The fan clutch may not be released when the underhood temperature is below a certain point.

See pic below. When driving, does it sound like there is excessive noise from the fan?

Let me know.

Joe
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Saturday, May 15th, 2021 AT 8:15 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
I spun the fan around with the truck completely cold and:

1. It’s not stuck.
2. It’s not consistent in resistance around 360 degrees; i.E. From 0 to 270 it’s harder to spin than from 270 back to 0.
3. As far as making excessive whooshing sound when driving, I don’t have a good base for it, but as I mentioned above, it’s not completely stuck.
4. The overcooling isn’t consistent, it started happening yesterday towards the tail end of my 2 hour drive and it started when on the freeway driving at around 70+ mph and 3k+ rpm.
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Sunday, May 16th, 2021 AT 7:05 AM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Update: bit the bullet and picked up/installed a new fan clutch from autozone. The old fan clutch wouldn’t really spin at all when the truck was cold.

Off the bat there is much less startup noise. Will report back after the next time I take a long drive.
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Sunday, May 16th, 2021 AT 3:50 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

If there was a lot of fan noise at startup, then the fan clutch was engaged when it shouldn't have been unless the underhood temperature was really high.

Let me know how things turn out for you. I'm interested in knowing.

Take care,

Joe
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Sunday, May 16th, 2021 AT 6:57 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
The mystery saga continues. Had some overcooling today after heavy city driving even with the new fan clutch.

Something is causing the thermostat to stay open for too long but I literally have no idea what it could be. Should I power flush the whole cooling system?
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Wednesday, May 26th, 2021 AT 10:17 AM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Joe, do you have any diagram of the cooling system flow? From the radiator to the t-stat to the block; there is also the coolant bypass system which lets coolant flow from the expansion bottle and heater core to the t-stat etc.

Just trying to figure out if it’s a thermostat problem or something else that’s letting too much coolant into the block.
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Wednesday, May 26th, 2021 AT 12:33 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I'm sorry but that isn't something that is posted in the manuals. Tell me. When you were in traffic, how hot did it get? Then, when you started driving again, how cold did it get?

It honestly sounds like the thermostat is sticking open. Are you certain (and I don't like asking this) the thermostat is turned in the correct direction?

Let me know.

Joe
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Wednesday, May 26th, 2021 AT 6:01 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
Went for another long drive today and the truck finally overheated. The expansion tank was completely full and overflowing with coolant. I let the truck cool down, refilled the expansion tank, burped the lines, and it didn’t do it again for the rest of the drive but it did stay below the middle of the gauge the entire time. I’m truly and completely stumped.

The thermostat I purchased came as a unit with the thermostat housing, so I just swapped it and followed the procedure to bleed the system.

I don’t think there’s any exhaust gases bleeding into the cooling system, but I could be wrong. The oil isn’t muddy and I don’t have any misfire issues or white smoke coming out of the tailpipe while starting the car.

I don’t know what else to do at this point besides completely brain the cooling system and flush it.
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Thursday, May 27th, 2021 AT 5:54 PM
Tiny
CSOSA123456
  • MEMBER
  • 31 POSTS
This is what the temperature gauge read and what my scanned read:
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Thursday, May 27th, 2021 AT 5:55 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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The temperature your scanner show is actually fine. The thermostat is designed to fully open at 195°F. You averaged 196°. At highway speeds, 188° is fine as well.

It is possible that you are getting compressed air/exhaust gasses into the cooling system. Are there any bubbles in the coolant? Take a look through this link:

https://www.2carpros.com/articles/head-gasket-blown-test

Let me know. You could also do a compression test to see if any cylinders are somewhat lower than the others.

https://www.2carpros.com/articles/how-to-test-engine-compression

Let me know.

Joe
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Thursday, May 27th, 2021 AT 8:16 PM

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