ECT's too low for mpg's? (Engine Coolant Temperature)
 

I drive a 2002 Saturn SL2 and I’ve noticed a bit of an odd condition with it; the running coolant temps seem very low. By “running” I mean fully up to temp, ‘normal’ operating conditions. I can’t help but wonder if it’s costing me mpg’s, although the car really gets excellent mileage seemingly ‘in spite of’ the odd coolant temps. :confused:

Here’s what I mean:
A week or 2 ago I decided to hook up my scanner and monitor a few things to try and track down a high idle speed issue that’s recently popped up. What I found is that at full operating temps my car was running between 154-158deg ECT. Seeing this I decided to go ahead and change the coolant temp sensor since S-series cars are known for crappy temp sensors and a new was is only $9 (man I love how cheap parts for this car are! :D ). I figured it had to be reading incorrectly. No dice, the temps are exactly the same, to the degree, with the new sensor. It shows none of the classic signs of a stuck T-Stat either; the car warms up fast (faster than any other car I own actually) and the engine temps are rock solid stable in freezing cold or blazing heat. The gauge has read exactly the same since I bought the car a couple years ago, I just never monitored it until now.

The thing is, I consistently get 40-41mpg in warmer weather and it’s been 37-39 since winter set in. Those are very good numbers from a ‘twin cam’ Saturn so it’s hard for me to believe that the coolant temps are costing me much. However, the logical side of my brain keeps telling me that those temps are too low for optimal mpg’s.

Anybody ever seen such low, but totally stable, operating temps?

Yes, replace your thermostat. When I started driving my saturn I was getting tanks around 37mpg. with a scanner I saw my coolant temp reading 153 usually. With a new tstat I would see 184-186, running the same route and speed, I then was getting around 40mpg.

I've watched old thermostats open early in a pot of hot water. They don't have to be stuck to be bad.

As said above, although your car warms up quickly showing the thermostat is closing, it obviously is opening too soon keeping the temperature too low.

When I bought my 95 SL2 the gauge rode at 1/4. For 2 years I ran like that because I didn't know any better. When I changed my thermostat and flushed my cooling system the gauge then rode at the proper spot (on a 95 it's half). I always checked my mileage and never saw a change from before and after the stat change. But everyone is right that you should change your stat so that everything gets up to proper temps like it is designed to.

FWIW on my focus the needle reads middle/normal once the engine gets to 115f but that thing is less gauge and more idiot light :/

We have two SOHC Saturns in the family, and both have a high idle issue I haven't been able to solve. One has a CEL for an emissions-based problem ("secondary air injection"), the other has no CEL, and both idle at 2000-2500rpm, warm or cold.

Thermostat didn't help, ECT sensor didn't help.

I think its all Saturn's that do that high idle thing. My girls, dads a Saturn fan, hes had a 94' 96' and now an 04' all that idle high. all automatic.

Check for a vacuum leak where the #1 intake runner meets the head, the corner w/o a bolt through it. Check pcv grommet and hose too