* Transmission Problems: This is a very common cause. At 40 mph, the transmission is likely shifting between gears, and a problem with the transmission itself (worn clutches, low fluid, faulty solenoids, a failing torque converter) can cause jerking. A slipping transmission is a serious issue that needs immediate attention.
* Driveshaft Issues: A worn driveshaft U-joint or CV joint can cause vibrations and jerking, particularly at specific speeds.
* Wheel Bearing Problems: A worn wheel bearing can create a vibration that feels like a jerk at certain speeds.
* Tire Problems: Uneven tire wear, a tire out of balance, or a warped rotor can cause vibrations and jerking. Check your tire pressure as well.
* Engine Misfire: Although less likely to cause jerking specifically at 40mph, a misfire can certainly cause rough running and hesitation. This could manifest as a jerk if the misfire coincides with a gear shift.
* Vacuum Leaks: A vacuum leak in the intake system can affect engine performance, potentially causing jerking.
What to do:
It's crucial to have a mechanic diagnose the problem. They can perform a proper inspection, check the transmission fluid level and condition, and test the various components mentioned above. Ignoring the problem could lead to more significant and costly repairs down the line. Don't continue driving the van extensively while experiencing this jerking, as further damage to the transmission or other components is possible.
