Results 1 to 2 of 2

Thread: Worse MPG At Lower Engine Temps?

  1. #1
    Junior Member
    Join Date
    Oct 2009
    Posts
    27

    Worse MPG At Lower Engine Temps?

    I've needed a new thermostat in my 2003 Passat for awhile. The engine has been running 30-50 degs cooler than normal (at least according to the gauge). The replacement is getting done now.

    But what I've noticed is that for months now my MPG has been below normal. I know that many things affect MPG and a colder engine is one of them, but my question is how much of a difference would 30-50 degs on the cool side make? I've seen my mileage go down about 2 MPG since this thermostat problem came about.

    Also, a related question, do cars get better MPG in cold weather or warm (all other things being the same)?

  2. #2
    Senior Member grouch's Avatar
    Join Date
    Jan 2008
    Posts
    2,195
    Quote Originally Posted by REM View Post
    I've needed a new thermostat in my 2003 Passat for awhile. The engine has been running 30-50 degs cooler than normal (at least according to the gauge). The replacement is getting done now.

    But what I've noticed is that for months now my MPG has been below normal. I know that many things affect MPG and a colder engine is one of them, but my question is how much of a difference would 30-50 degs on the cool side make? I've seen my mileage go down about 2 MPG since this thermostat problem came about.

    Also, a related question, do cars get better MPG in cold weather or warm (all other things being the same)?

    On fuel injected engines, the computer controls the amount of fuel the engine gets. On an older engine, you have a choke. The fuel injection just puts extra fuel into the engine in what's called "open loop mode". By running a cooler thermostat, or having one that never really shuts, your engine stays in open loop mode and you waste a lot of fuel. You NEVER use an alternate temperature thermostat in normal usage. On a carbureted engine, it will give a little richer fuel charge and give extra power. On F.I. engines it wastes fuel.

    On two trucks I've owned, a 1989 C3500 with a 454 engine and a 2001 Ram 2500 I currently have, the fuel economy made a large increase after replacing the thermostat. The 3500 was getting 6-8 mpg (A dually 1 ton likes the gas) before and got 11 afterward. It had a Failsafe thermostat that had failed open. The current Dodge got 8-9 mpg and after replacing the too cool unit with the proper 192 unit, I'm now getting 12 mpg. This is combined city/highway driving on a rather heavy 4X4 truck.

    The most drastic change was a '94 Dodge Ram 1500 with the 3.9 V-6 and automatic. I was content with 17 mpg as it was a truck. I replaced the thermostat with a new unit and went up to 19. However, I had a lot of timing chain noise so I replaced the worn chain. The engine then warmed up to the proper temperature. I never really had good heat and afterward it would cook you. My mileage then went up to 23-25 on the highway. I thought this was a fluke but I got it on a regular basis.
    Honk if you love Jesus.

    Text if you want to meet him.

Similar Threads

  1. Municipalities lower the speed limit as they please
    By dieseleverything in forum Fight Traffic Tickets/Driving Issues
    Replies: 3
    Last Post: 03-31-2010, 04:47 PM
  2. KS - Lower Speed Limit Unlikely to Pass
    By swamprat in forum Fight Traffic Tickets/Driving Issues
    Replies: 9
    Last Post: 08-23-2008, 12:10 PM
  3. Volvos built to withstand -40 temps...
    By Eric in forum Automotive News
    Replies: 0
    Last Post: 01-28-2008, 07:56 AM
  4. A (semi) serious suggestion to lower gas wasting
    By mrblanche in forum Motor Mouth
    Replies: 38
    Last Post: 05-22-2007, 10:31 AM
  5. Clean engine - happy engine!
    By Eric in forum Motor Mouth
    Replies: 0
    Last Post: 05-12-2007, 10:34 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •