I've been noticing recently that my temps have been going up and down while driving. To try to fix the problem, I put a new thermostat in it and it still has the same problem. I tried to bleed the system for air pockets as well, but still no luck. When it first reaches operating temp, it shoots up to about 230 degrees (according to ultra gauge) then goes back down to 195ish after a few seconds. Then it will jump back up again, then back down. I have no clue to why it is doing it, but it is really driving me nuts. If anyone could help, it would be appreciated.

 

Does the temp gauge in the dash fluctuate as well? I know they're controlled by the computer, but I'm thinking the sensor is going bad.

 

I am agreeing with MKoch right now, when mine went bad it would go from -40*F to +250*F but the needle on the dash never moved.

 

Yep, with two sensors (one for CPU, one for gauge) it could be the sensor. Hope the stat is not sticking. An IR thermometer pointed at the thermostat housing might show a back up to what is going on.

 

id start there, with the IR meter if you have one
otherwise if the dash gauge is solid then im betting on a bad ecu temp sensor.
interesting that that wasnt caught by ecu and isnt setting a light, but i think ive come to find the OBD logic on these lucas controllers is a bit of a joke.

 

I can unplug mine and it will show -40F, but no light comes on or code set.

 

Well...when the temp occasionally spikes above 230, the needle will go up to the 10:00 position, but back down to normal before the ultra gauge does.

Also, the temp doesn't jump to 230 degrees instantly, it works its way up and down, but quickly.

 

Slipping belt, failing water pump are the first things that come to mind.

 

truly a wonder how they got these trucks to pass initial cert.
that along with the reports on here of cats being removed and it not setting efficiency lights, etc...

 

How can I check to make sure that the water pump is working correctly?