The coolant temperature gauge on my '99 Dakota R/t is malfunctioning. When I first noticed it it climbed to about 230, then dropped down to normal. Then rose again and the engine was obviously becoming affected by the computer thinking that the temp was above normal, it would idle at a higher RPM. I changed out the coolant temp sensor but now when it goes on its journey upward, it goes all the way to the point of setting off the check gauges light. It then drops down to normal range but then goes up. Again, the computer thinks the coolant is hot, but it is not. I checked the temp at the radiator with a fluke lazer thermometer, pointed it at the hose on the intake. pointed it at the manifold where the temp sensor is, simply put, the car is not running 1 degree above the thermostats normal operating range, 175 in this case but the gauge says it is running hot and the computer believes the gauge and it does runs differently because of that.

 

Did you reset the PCM after the new coolant temp sensor was installed?

 

It's acting a bit like a blown head gasket, or maybe you have air in the system.

Did you do any cooling system maintenance before the problem started?

 

I disconnected the battery cable (black), put the key in start postion for 30 seconds. Then reattached the cable and started it, if thats what you mean by resetting the PCM.

 

The cooling system is functioning properly, the coolant is not hot even though the gauge says so. But it doesn't stay reading high, it goes down faster than it gradually goes up. I'm thinking the wires between the sending unit and the gauge could be the problem.

 

I'm gonna go with air pockets and/or bad head gasket or water pump going out...Laser guns are not accurate for this purpose. Everything touching the water is a heatsink, and will always be cooler than the water itself. The only true way to know the water temp is a wet sender.

 

You may be correct but if an engines coolant temp was 250 or higher like the gauge reads, steam would be coming off, like I have stated, it is not over heating. Thats the part that you(me) have to get your head around. You can touch a hose without immediately pulling your hand away. Also, the coolant would pretty much be boiling if it was pushing passed 250 degrees. I've seen this truck overheat before,(10 years ago) I cannot stress enough that its not overheating. The gauge would not drop 100 degrees in under 30 seconds. I had a '65 GTO with a mechanical sending unit for the temp gauge and wathced it rotate all the way around, past 300 degrees but it wasn't overheating, that was the sending unit going bad. This reminds me of that and I replaced the sending unit on the truck today. Did not make any difference. Actually made the gauge peg out faster. But again, it then goes down to normal temp on the gauge in 30 seconds.

 

The thing with a bad head gasket is that as gases build up, the water stops flowing. It's not like the whole engine is overheating, it's localized. Then the water flows and the temp drops sharply. It may stop doing it once it warms up or it may keep repeating the cycle.

Check for exhaust gases at the radiator neck with a sniffer or Autozone carries a kit that turns colors in the presence of exhaust gases.

 

I would say the next simple step is replace the thermostat and check for exhaust gases or oil in the coolant at the same time

Check for collapsing old hoses also check your radiator is it getting plugged up then opening up on you somehow.

Have you flushed the system out in the last two years if not do it see what happens.

This way you are checking off cheap possability's

 

My sister had a problem like this after we put in a new rad it would get very hot then open up and be fine the go through the same thing over and over again.

Turned out the new rad was defective replaced it and no more problem.

One word of caution regarding any rad you get flush it out real good with fresh water a few times before you install it just put something over the out hole fill with water the open it up zoom a simple flush.

I say this because I have seen sand and grit come out of these rads from getting rebuilt as almost all rads are rebuilt rads I had about half a pound of sand come out of the last one I purchased that was a surprise.

Can you guess how that could destroy your water pump.

Never assume anything you buy is clean always flush it out