Truck Trend Garage: 1999 Dodge Ram 1500 Laramie SLT Loss Of MPG
Question: I have a 1999 Dodge Ram 1500 Laramie SLT (Quad Cab). I keep my speed just below 60 mph, and I set the cruise control, which made the fuel economy 20-22 mpg according to the computer. Over last month, mpg dropped to 14-15. The truck has 225,000 miles and still runs great except it seems to take longer for it to shift during acceleration. I keep rpm below 2000 during acceleration. Could this be caused by the transmission filter or torque converter?
Answer: The problem likely isn't caused by the filter or fluid, but make sure they're not archaic and that the transmission fluid is filled to the proper level (check it when it's hot, idling, and in park). Have a technician scan the system for diagnostic trouble codes, proper incoming data, and outgoing commands -- especially if the late-shift condition began at the same time as the fuel economy issue. Shift timing is determined by the PCM turning solenoids on and off within the transmission.
You can help with the diagnosis by communicating the shift condition to the tech as accurately as possible. Make sure the transmission is shifting through all four gears, and that the lockup torque converter is being applied. Application of a lockup torque converter feels like a fifth gear engaging, but it's actually a clutch within the torque converter physically locking the crankshaft to the transmission's input shaft. The lockup converter not working will have a big effect on highway fuel economy. A good lockup test can be performed during highway driving (about 65 mph) at normal engine operating temperature. Cruise steady, and gently depress the brake pedal (just a hair, not far enough to apply the brakes) with your left foot, while maintaining position on the gas pedal with your right. When depressing the brake pedal, observe the tachometer reading.
If the torque converter clutch is operational, it will disengage with brake application, and the rpm will rise as you touch the pedal. Other simple things like a thermostat that's stuck open will prevent the engine from reaching normal operating temperature. This, in turn, will affect the overall shift pattern and prevent application of the torque converter clutch. Of course, this is a high-mileage Ram, so the list of causes of lower mpg goes on and on, including O2 sensor operation, engine compression, and more.
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