Chevy & GMC Vans

Hi everyone. I'm having trouble figuring out what's going on with my 1999 Chevy Express 3500 5.7L, and would love any feedback or input you guys could share. The vehicle has less than 70K miles, and drives almost perfectly most of the time. AC, heat, everything works well. But it does have two recurring issues that could pop up expectedly, and sometimes, when expected.

Issue 1: AC - sometimes there’s no air coming out of the vent, even on high. Or the air comes out of the wrong vents.

Issue 2: Acceleration - sometimes when gas pedal is depressed, vehicle doesn’t accelerate. It tends to be much noisier at this point. Sounds like it’s not getting enough air or fuel, or it’s not catching the right gear.

While going uphill, when the acceleration issue occurs, I would lose speed. By releasing the gas pedal and stepping on it again, the vehicle does slowly gain speed, but still struggles quite a bit.

While driving on leveled road, when the acceleration issue occurs, I don’t necessarily lose speed, but the acceleration is very weak. Releasing the gas pedal and stepping on it again gets the vehicle moving a bit faster.

A lot of times, both issues happen at the same time, especially while going up hill. But sometimes, the AC doesn’t work while the acceleration works perfectly fine, and vice versa.

Pattern: One pattern I’ve observed is while going uphill, the acceleration would struggle and the AC blower would either stop, or it would blow out the wrong vents. Once I’ve made it to the top of the hill and begin my descent, the acceleration and AC work, again. This pattern could repeat for hours during drives through mountainous areas.

On long trips, sometimes, I would drive 8 hours a day for 3 days straight and not encounter any issues. Then on the 4th day, both the acceleration and AC would struggle continuously for at least a few hours. Or sometimes, the issues occur on the first day of the trip, but disappears during the rest of the trip.

I was told that the vehicle could be losing vacuum, or it has a bad check valve. It could also have issues with air or fuel intake. I believe both AC and Acceleration issues are related.

Has anyone experienced something similar with their vehicle?

Thank you in advance for your help!

Kevin

This sounds like classic vacuum system / lines failing. I am only speaking of the similar 4.3 V6.
Sounds very familiar when the vents quit blowing upon accel, but wish I could say that is what is affecting acceleration in general.

The vacuum reservoir / ball, that check valve and the line running from the bottom of the intake to the resv. is usually the culprit.
If you find a bad spot, clip a bit of it and show it to your guy or gal at your LAPS. Most GMs ( at least in your age range - late 90s ) use 1 size of hard line ( going into dash ) and 1 size of larger soft rubber or silicon.

Vacuum is an easy fix to many common issues. Replace every piece you can get to and the check valve. Eliminate that as a possibility and have one less gremlin in the van.