KT1999Express

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

jfmorris

What you are describing on the AC issue sounds like the mode actuator under the dash is not working properly. If that is vacuum actuated, a vacuum leak could certainly impact it and the acceleration issue. BUT, I don't think that AC mode (vent, floor, defroster) is vacuum actuated on a 1999. My 1996 C1500 has electrical actuators for the mode and blend doors in the HVAC system.

That said, here is some info on your system.

https://charm.li/Chevy%20Truck/1999/...re%20Controls/

Here's a diagram if that helps:

https://charm.li/Chevy%20Truck/1999/...HVAC/Diagrams/

Based on the diagram, there may in fact be a vacuum line that is common to all those actuators. So I stand corrected on my initial thoughts...

KT1999Express

Thank you so much for providing all this information. It'll give me another area to investigate into. It's frustrating that none of the mechanics I've brought the Express to has even attempted to articulate what they thought was causing the issues. I've actually brought a printout of possible issues for them to look into, but that didn't help them either.

I appreciate your help!

Kevin

Triaged

See this recent thread on a similar issue. I'd start by looking at the vacuum lines and that check valve going to the vacuum reservoir (round ball).
https://chevroletforum.com/forum/exp...-issue-117027/