I'm pulling off of my street when the water temp gauge goes from blue to red then back to the middle? My gas mileage has sucked horribly, like 16-18 mpg.

I'm guessing that means my thermostat is stuck open? Would this be a correct assumption? I was thinking about picking up one of those Peake tools to read fault coded.

The question is, will a stuck thermostat throw codes of there are NO dummy lights on? TIA..

-John

nope that doesn't seem like an open termostat ...
more like a stuck water-pump ... or air in the
coolant ...

an open thermostat would've kept the car
around the blue zone ...

and i don't think the thermostat
will throw any codes either :)

If you have a CE light youve got other problems.

.. for the replies. Air in the coolant system is definetly a possibility... but would that decrease gas mileage?

I will have to bleed it, I know UNDERSTEER.com has a how to on that if I'm not mistaken.

I don't have any CE lights either, thankfully. Thanks again guys..

-John

Actually on an OBD-2 car a stuck open thermostat will throw a CEL for the coolant temps being too low. OBD-1 cars won't throw a CEL until the o2 sensor fails from being carbon fouled due to running the engine too cold :)

The gauge going from normal to hot to cold sounds more like an electrical problem though. Is it reading normal temps now?

The gauge going from normal to hot to cold sounds more like an electrical problem though. Is it reading normal temps now?

Yes, it is reading normal temps now. It only had done the decribed fluctuation once before. Thanks again for the replies..

Do you know the code related to your current check engine light. I too had a similar situation on my 98 740IL where I noted that my temp gauge went from normal to hot in a split second then receded back to normal. The next day I got the check engine light on. I pulled the code P1622 that appears to be "Electric Thermostat Control, Final Stage", whatever that means.

After reading several posts from this and other boards it appears as if their is a relationship between the thermostat and the CE light. I assume that the thermostat may not be shutting or opening all the way and their is a sensor that notes this condition through the coolant flow. Again just a guess on my part.

BTW I reset my code and it came right back on after the reset. This tells me their is a condition that I need to fix. I am assuming that it may be the thermostat only.

Any observations from other members to help diagnose.

Thanks,
DJ

Do you know the code related to your current check engine light.

I don't have a current check engine light.. :dunno

Interesting that you had the same problem tho. Is your heating/cooling affected at all as far as temperature control in the car? My A/C sucks, even after a re-charge.. wonder if it's related?