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Re: MPG vs outside temp Posted by Kevin M [Email] (#1193) [Profile/Gallery] (more from Kevin M) on Mon, 17 Mar 2008 20:23:41 In Reply to: MPG vs outside temp, Aom, Sun, 16 Mar 2008 09:45:47 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Sorry, dealt with the old ones too long. However, the conclusion is the same. The ECU uses the output from the MAP, absolute pressure, and RPM to yield the Mass of the air entering the cylinders. Mass of the air is proportional to the air DENSITY which is proportional to the ABSOLUTE PRESSURE at the manifold and INVERSELY proportional to the TEMPERATURE. WOW, I SAY IT LOUDER!
The warmer the air the less fuel is required to maintain a stiochiometric ratio of fuel to air. Conversely, colder temps yield greater air density which requires more fuel to get to the magic ratio of 14.7:1 or 6.8% by volume.
So, most likely, the MAP is feeding incorrect numbers to the ECU and when it is slightly cooler OAT, it thinks you are driving in Fairbanks in January!
I hope this helps somewhat. At least, I can see how a bad MAP sensor could cause the symptoms. Have you pulled the codes- any CEL?
All the best,
K
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