Hello again, I had the overheat near miss this past weekend. Sitting in traffic, about 85F. The gauge was climbing toward the top of the normal range, but no other indication that anything was wrong. I let it cool and took it back home.

I replaced the fan clutch, which failed the hot/cold spin test, and rinsed an alarming amount of mud and bugs out of all 4 radiators. The temp was still high at 217-220 on open road, and 226-230 sitting still.

Having run out of easy, obvious problems, I checked the temp cold after sitting overnight for 13 hours. The air temp was 61f and the intake manifold, next to the sensor, read 68 with the point and shoot thermometer. The temp read with the OBD scanner was 111.

So the question is, do I need to drain the coolant to change the sensor, or can I just get away with being quick and doing a bleed.

Any thoughts would be appreciated.

N

 

Open coolant cap to release pressure and replaced e c t

 

I was hoping it would be that easy, thanks Buzz

 

Back up and give us more to work with.
What year is the truck, what has been done to the cooling system since you have had it, how many miles on it, has the t/stat ever been replaced as well as the radiator?

 

Sorry about the lack of info, it is a 99 with 150k on the clock. I bought it in december of 2011, without doing quite as much background research as I likely should have, but that is a story for another day.

Aside from what I mentioned earlier, the electric fan was replaced when I discovered the original was seized up. As far as what the PO had done in the ten years he had it, I don't know. The radiator looks way too nice to be the original, and the hoses and coolant look great (even if it is the wrong color).

The PO was an aerospace engineer who bought nuts and bolts from the dealership, so I'm inclined to believe that it was maintained with an appropriate degree of mental illness.

Perhaps there is something I am missing, but doesn't the 40+ degree difference between actual and indicated temp point to a bad sensor?

Again, thanks to everyone for the great volume of knowledge.
N

 

You might want to send me your number, there are way too many things that can cause this and at this point, every time you let that engine get that hot, you are that much closer to making a boat anchor out of it.

 

The 40F difference between two cold overnight sensors is an issue. But as Mike says, it could be bad sensor, or it could just be bad connection or wiring, some corrosion across pins for the wiring will make it read higher than normal. It is a resistor sensor than goes lower as temp goes up, so any "parallel" resistance will make it seem warmer. So first up, remove connector for sensor and be sure it is super clean. While it is off, switch on key to get electronics to wake up, and see what scanner shows for temp with wire off. Mine (a different model) shows -40F. If you suspect sensor, changing it releases little coolant, but as always, you will need to bleed air from system.

 

pm sent
With the sensor disconnected, it read -40 via the obd port. Both pins and sockets in the connector looked good, but cleaned and greased anyhow.

 

Your sensor could indeed be bad.

The 40 degree difference in the morning is not a good sign whilst you are using your scanner to cross check.
A very good idea.

make sure the signal ground is intact.
I am suspecting this is the braided ground wire from the USA driver's side cylinder head to the firewall.

Also - if you are very low on coolant - you could have an air pocket at the sensor. If this sensor goes high in temp, the aux fan should come on.

Of course a head gasket going bad will put air into the system
and will cause air pockets at the top of the cooling system near this sensor. I had that problem when my head gasket was failed.

But, you probably just have a bad sensor - as confirmed by your smartly done reading at ambient temps against the sensor and the scanner.

The sensor is probably a thermistor. If your truck was overheated, I don't think that could damage a thermistor.

Depends on which way low resistance

Found this chart on another board .
Uploaded it.

 

While ground to block cable impacts many one wire sensors, this one has both wires direct to ECU, no header, so my money is on the sensor. If ground was off, seems like there would be many other problems. $25 or so part.