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Discussion Starter · #1 ·
Truck is stock w/ ~175,000 miles on the odometer. Original owner.
It started with a leaky water pump. Water pump went quickly. Dumped about half of the coolant on a 10 mile round trip. For my initial diagnosis, I added a gallon of distilled H2O to the degas bottle. (Engine not running.) I could hear water hitting the ground before I got the full gallon in there. Water pump was dumping it. Easiest leak detection ever.

All testing below has been done with ambient temp between 50° and 65°F.
Replaced water pump, (new water pump has the curved connector to lower rad hose, original had straight - replacement lower hose is correct for the curved connector) t'stat and gasket, upper an lower rad hoses, and coolant temp sensor. Refilled. Checked for leaks. All good. Fire up engine and idle for about 10 minutes before temp guage starts climbing. Eventually showing max hot.

Shut it down. Rad hoses cold. Heater not blowing hot. Just hot on the guage.

Continued investigating. Pulled t'stat and bench tested (stovetop). It would open ~3/8" between 190° and 210°F. (192°F spec t'stat).
Replaced t'stat with another new one.
Refill system. Run again. Same results.
Decided I must have air trapped in system. Tried filling the system through the t'stat housing before installing t'stat and upper rad hose. Same results.
Tried vacuum filling the system (at a stable 26 inches of vacuum - indicating no leaks).
Same result.
Now getting suspicious that the new water pump is a complete dud. Hot on the sensor but cold hoses suggest no flow to the radiator.

Did a "modified-Gooch" coolant flush. Flushed radiator separately with garden hose followed by distiled water. Pulled block plugs and drained block. Installed a gutted t'stat. (Essentially NOT a t'stat. just a shim to hold the t'stat gasket in place to prevent leaks.) Refilled system with distilled water (gravity fill not vacuum) to continue the flushing. Crank it up and let it idle with intermittent revving to ~2000 rpm and heater set on high.
All was good for about 15 minutes. Temp guage stayed right in the middle of the scale. Heater air was warm (but not hot). Rad hoses were warm (not hot or very firm). I was feeling good.

Still with the dummy t'stat installed, I took off to drive it a few miles, and within 1/2 mile the temp guage starts to creep up. By the time I can get turned around and back home, the guage is on full hot and the Check Guage light is on. NOT Good.
Then it got weird. Heat was still warm. (Warmer than at idle - but not as warm as I would want on a cold morning). Rad hoses still warm. I drained the radiator, and coolant was warm, but not uncomfortably hot.
I immediately pulled the block plugs and again coolant was warm but not uncomfortably hot. Certainly not hotter than the 192°F setpoint or whatever the top of the guage should be. (250°+ F ???) If that water had been at 250°F, I would have been headed to the hospital. Warm coolant in the radiator is telling me that the pump is pumping. That finally seems like some good news.

So... I'm thinking that I have a bad temp sensor - or I have a bad connection to a good temp sensor. I say that because - if the system was truly hot, I should have been burned while draining the block. It's possble that the sensor is good but the connection is bad. Maybe the connection at idle was good but the vibration of driving it 1/2 mile moved it a little and caused a faulty reading. I did put a little dielectric grease on the temp sensor connector when I installed it. Is that causing the faulty reading?

I have it all put back together with a real t'stat now, but I haven't refilled it yet. Is there something else that I could be overlooking here before I really attack the sensor. I'm hesitant to remove the sensor because I'll probably just do more damge to the connector in that process. I'll probably cut the wires to the sensor and remove with the connector attached and then splice the wires for reinstall. The access / visibility to the locking tabs on the connector is pretty limited. I guess that's my fault for not torquing the sensor to end up the with the tabs on the connector facing forward. I don't remember the orientation of the original sensor.

Other possibility - air pocket trapped right at the sensor causing it to read badly??? I'm reaching.

Please help. I want to drive my truck again.
 

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Did you consider that you may have the incorrect temp sensor? Or, counterfeit aftermarket sensor.
 

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Discussion Starter · #4 ·
Upon further review... Rock Auto says that Standard TS380 or TS380T is correct - probably searching the same database that O'Reilley is using. Searching StandardBrand.com says TX61 for my engine. Hmmmm??? I'm heading to the FoMoCo dealership while I'm out getting more antifreeze.
 

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Discussion Starter · #5 ·
Okay. With a new MotorCraft sensor installed, things are looking better. Temp gauge reads cold on initial startup. Climbs slowly to middle of range. Heat gets toasty warm. Gauge stays in middle of range. All is well. I just need to drain and refill with my new Rotella ELC.(currrently on 100% distilled H2O). Shameless plug for Tractor Supply - Rotella ELC Concentrate is currently on sale at $20.99 / gallon. That's about the regular price for the 50/50.

I didn't test the resistance of the new MotorCraft sensor before I installed it (racing the daylight), but I did test the original equipment vs. the Standard TS380. At room temp, the Ford original was ~41.5 kOhm. Standard TS380 was ~4.6 kOhm. That probably explains why the TS380 was showing me middle of the gauge on cold start up and climbing to idiot light status before the coolant got to even normal operating temperature.

When I have more time, I'll bench (stovetop) test each one and plot some response curves for them.
 

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Don't discount the possibility that you have a short to ground on the sensor wiring that was caused by you having it disconnected and moving it around. A short to ground would be an immediate overheat signal, but if the short is intermittent, it could act like you're describing.
I would check the wiring to be sure it isn't rubbing on something.
 

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Glad you are having positive results after so many set backs. Like you I use and favor the Rotella ELC. I also get mine in concentrate from the Tractor, I like to keep 5 gallons of concentrate in my shop in case I have a big leak event and have to start from MT.
I agree with klhansen, check the wires, likely they have been a high traffic zone lately.
 

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Discussion Starter · #8 ·
I'm much more comfortable with the wiring / connector now. When I was contemplating removing the sensor, the locking tabs were facing away from me and hard to see and access. From what I could see, it looked like the tabs were damaged. I turned the sensor about 90 degrees with the connnector still attached and gained good visibility / access to remove the connector and swap the sensor. Connector was in fine condition and gave a good positive click when I connected the new MotorCraft sensor. Wires look good - at least to the point that they enter the plastic loom and disappear into the spaghetti farm near the passenger side valve cover.
 

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Over the years, I have come to rely heavily on klhansen's and RT's advice. I bet either will be more than happy to help further if you need them.

I run Rotella ELC also and have been extremely happy with it since I have been using it. Like you, I followed Gooch's flush procedure and have not had any problems.
 

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To any 7.3L owner who has encountered this problem: IF your wiring harness to the temp. sending unit has a gray connector, you must find and use a temp sending unit manufactured with a gray connector. The black connector units will cause all the above listed problems. There is no difference in Ford part numbers that I could find. Find a sending unit with the gray connector when harness connector is gray and a black connector for black wiring connector. There is no other reason for your gauge to move to middle as soon as trans temp gauge starts to move then max out when trans temp is at normal operating level and then temp gauge needle will drop to bottom of gauge and stay there after the first time the thermostat opens.
 
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