Therostat/Check Engine Light

[Scott Sargent]

1998 Volvo V70, 65k miles. Temperature gage acts like a stuck-open
thermostat, operates in bottom 1/4 unless putting a load on the
engine. With a load on the engine, such as climbing a hill, the gage
will go up to 1/2 but as soon as you come down the other side the gage
drops back to 1/4 very quickly. When this first happened the check
engine light came on. I drove the car to my destination and back
without problems. Could these two problems be related? I can't get
to a service shop until next week. Any comments would be appreciated.

 

[Mike F]

1998 Volvo V70, 65k miles. Temperature gage acts like a stuck-open
thermostat, operates in bottom 1/4 unless putting a load on the
engine. With a load on the engine, such as climbing a hill, the gage
will go up to 1/2 but as soon as you come down the other side the gage
drops back to 1/4 very quickly. When this first happened the check
engine light came on. I drove the car to my destination and back
without problems. Could these two problems be related? I can't get
to a service shop until next week. Any comments would be appreciated.

The temperature gauge is not linear. If the engine temperature is
anywhere near normal, it stays in the center, just so customers don't
get too exited about fluctuating engine temperatures. If it is reading
abnormally, the temperature is a long way from optimum.
The fuel ECU expects the engine to warm up in a fixed amount of time.
If it doesn't, (as when the thermostat is stuck open) it sets a code
regarding the engine temperature sensor, and turns on the check engine
light. Most garages change the sensor and thermostat at this point to
avoid comebacks, but 99.9% of the time, only the thermostat is needed.