My 99 had the same problem and my mechanic found that the honeycomb in the catyletic converter had broken loose and was lodged at the opening of the pipe sideways, limiting the exhast flow. Once I replaced the catyletic converter the problem was solved.
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problems with speed sensor for transmission. Computer uses a speed signal to shift transmission through the gears. Without speed input, the computer doesn't know when to shift the transmission. Either the speed sensor is bad or wiring from the ECM to the speed sensor is bad. Usually 2 wires lead to the sensor. The sensor generates a AC voltage signal with higher speeds generating more voltage and frequency. The ECM (computer), looks at the signal to see how fast the vehicle is traveling.
This is a rare one and can be tricky. Since your rpm gauge rises back up when you increase your acceleration slightly but you feel no jerking of acceleration then this tells me that a speed/rpm senser either has a bad connecton or the sensor itself is going bad when the engine reaches an optimum heat while driveing on the highway. Since you feel no acceleration when you slightly press the accelerator down but yet the rpm gauge is increasing back to normal, the throttle body sensor may be compensating for the failed speed sensor/failed sensor connector. Please get your uplander diagnosed properly first to confim this.
This car has both options of auto transmission plus manual gear **** so the only reason it can shudder when the clutch is not fully released and thus takes some time, that's when it shudders, but on accelerating this will not happen cos the centripetal clutch has enough torque such that the clutch is pressed hard to the cam & therefore no shuddering. On cruising speed the probability of clutch slipping making the car shudder. Thus changing the clutch assembly ought eliminate this problem....sodeep
This is a potential solution but if you are traveling down the highway and take your foot off the gas (or if you are on cruise no gas is required to maintain your speed) then the instant mpg calculation is similar to this 60 MPH / 0.0 GPH so effectively you end up with infinite gas mileage. But because the motor has to idle it's always using a little gas so it puts a very small number for the denominator which ends up giving you a high MPG. I've seen this happen in many makes of cars and I don't think it's anything to worry about.