Engine stumbling,very odd

[Chubbs]

Coolant temp sensors on older engines should be replaced since they can start to fail by losing their accuracy slowly over time. This can cause the engine to run poorly. A $30 part that has a significant effect on the engine. I have repaired multiple engines of different brands over the years both gas and diesel by installing a new temp sensor. Of course this has no connection to the sensor connected to the dash gauge for engine temp.

good tip.

 

[retiredsparky]

One more point I should have made: One reason a temp sensor can affect an engine only when it is cold is that if the sensor falsely tells the PCM that the motor is warm when it really is cold, the PCM puts the control system in "closed loop" mode which is the normal mode when at normal operating temperature.

This activates the signals from the O2 sensors and leans the mixture (a warm motor needs less fuel). So a cold engine running lean is gonna barely run and the O2 sensors cannot compensate for the lack of fuel.

The exhaust temperature is also too cold to make the O2 sensors work correctly (some O2 sensors have built-in heaters, but won't make up for a cold motor at first start-up). I hope this makes sense.
Larry