Q: where is the diagnostic port for this van? on 1993 Chevrolet G Series Van (G30)

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The van "bucks" at freeway speed only when maintaining speed- stops if I give it any gas or back off the pedal. I'm thinking a vacummn leak. Has throttle body injection.
(3) Answers
it is under the left hand side of the dash. Don't rule out worn Spark Plug wires, very common for bucking jerking ( misfires) Lots of heat under that 'dog house' that covers up the motor.
here is a link to repair info for your car
My van was doing this whenever I was coasting. It felt like I was pulling a trailer on a ruff road. I replaced the O2 sensor and it quit...I gave up on trying to remove what looked like original equipment and replaced the Y and O2 both...about 175 bucks for both, but it was worth it.