Cold air intake

[johnemac]

Last winter I felt my mpg really suffered. I know cold air helps empower combustion, but is there a point where the temp is too cold and actually becomes counter-productive?

[padgett]

In general, down to freezing everything is fine. Below freezing, extended warmups (the 3800 is a cool running engine when everything is optimum) will have a significant impact on MPG particularly if trips are short. Reattae are not very efficient in town.

Until coolant temp passes about 150F you are in "Cold Enrichment".

[johnemac]

Usually I start it and go without an extended warm up and I travel to work about 55 miles freeway all the way. Talking about NW Missouri(Misery) with temps from 0 to 25 Degrees.

[wws944]

The general answer to your question is that the computer has a lot of sensors to help it make decisions. Two important ones are the the intake air temperature sensor and the MAF. So temperature and altitude are taken into account. Another important one is the O2 sensor - which has to be hot in order to work right. The O2 sensor provides feedback to the computer so that it can optimized the fuel/air ratio based on the Real World.

When the engine is cold, it runs in "open loop" mode - where the computer uses preprogrammed data tables rather than feedback from the O2 sensor. At some point, when it is felt the O2 sensor data can be relied on, the computer switches to "closed loop". The faster the system can get to closed loop, the better. For example, in my Suburban, this happens when the coolant temp is over 130°. Takes about a mile of driving. In the RDV they programmed it to switch really quickly. Like about 30 seconds after start up! (This is easy to see with an OBDII scan tool. I don't know about our Reattas OBDI system.)

So if the MPG is suffering, you might want to check all the sensors. In particular if the O2 sensor is the original 20 year old one, and if it has more than, say, 75-100k miles on it, it could be bad.

Edited October 4, 2009 by wws944 (see edit history)

[Guest sintid58]

I used to haul gas in the St Louis Mo area (9500 gallon trailer) and there are two different grades of gas pretty much nationwide, one for winter and one for summer. Reid vapor pressure is the value used to control this by the EPA, in the summer it is lower to prevent gas from evaporating too quickly and it also helps with vapor lock. Also the lower RVP gives better gas mileage than the higher RVP used when the weather turns colder. The epa has rules stating when and where these grades must be used. Southern states will have a higher RVP year around than the northern states. In the fall of the year this RVP will start to drop off as the gas in storage from the summer is used up and replaced by winter gas. Of course if you live in really cold climates like I do then a lot also has to do with burning enough BTU's of fuel to warm the engine to an efficient temp and people dont like to turn their cars off and we often start them and let them run for extended periods. Here in South Dakota when it gets to below 0 power steering pumps moan and anything with grease is very stiff for the first few miles. It all takes more power to move. You can look these values up for your location at http://www.epa.gov/oms/volatility.htm if you are interested.

Edited October 4, 2009 by sintid58 (see edit history)