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Q: shifting at higher rpm's than normal in hot weather

asked by on

When it is 90+ outside and running the ac on i have shifting issues from a stop. The engine temp is just above normal, 200-210. I take off and the engine revs up louder and shifts around 3000 rpm for the first and second gear. Again only when it is hot and ac is on. In the mornings I have to let it warm up for a few min or I get the same issue. I have changed the transmission oil, filters, pressure sensor and shifting module but no change. No error codes are displayed The oil looks fine in the tranny and no smell. Durring winter the only issue is i have to let it warm up for a few minutes.

My car has 95000 miles.
My car has an automatic transmission.

A: When the temperature is hot outside, the ai...

When the temperature is hot outside, the air density is low in comparison to the wet cold air of winter. This means there is less power available because there is less air to combust. Your car needs an air fuel ratio of 14.7 parts air to 1 part fuel. With less air molecules, lower air density, your car will inject less fuel in order to maintain the 14.7 to 1 fuel ratio. This means less power output. On hot summer days like you are describing, not only is the air density less, but your cooling system will need to work harder to keep the motor cool. This means the cooling fan will need to push more air and this is the sound you hear when you describe louder revving. This will be particularly noticeable when starting from a stop light.

One of the ways a vehicle produces more power is to increase RPM. With the lower power output available due to a lower air density, your car needs to shift later in order to produce the same amount of power at lower ambient temperatures.

The phenomenon of air density is also a factor of altitude. Before we had fuel injected vehicles that can adapt to changing atmospheric conditions, we would need to change carburetor jets if you were to move to a higher altitude city. Race cars that still use carburetors are constantly making adjustments to optimize their power output. Nascar and drag racers must constantly monitor the barometric pressure and temperature to calculate air density or they will not be competitive. It is critical to success in racing.

If you'd like a second opinion on this or feel like your shifting is truely abnormal, consider YourMechanic, as a certified technician can come diagnose your shifting issue firsthand and fix it from there.

Good luck!

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