Heater/AC Blower Intermittent Issue

The Heater/AC blower is acting up from time to time on my 98 XJ.

It can work fine for days and deliver very cool air as expected but all of a sudden the blower becomes very weak even on highest setting. I can hear the difference in the fan speed but very little air comes out. The little air that is coming out is cold so the AC seems to be working.

Turning AC off and switching between the different modes does nothing but later on it will come back and act normal as nothing has happened.

I've also noticed that when accelerating fast the AC is stopping until I let of the gas.

Anyone experienced similar issues?

Quick answer, and I am no expert, loss of vacuum.
Does the fan switch to blowing from the defrost/windshield vents?

Hmm don't think so there's still air coming out from the vents but very very weak. Have to check next time it happens.

Any info if there's a vacuum leak?

A vacuum leak will result in the system defaulting to defrost (blowing up the windshield). If you have a partial vacuum loss some of the air could be going to the defrost and some to the vents you selected. So feel for airflow up at the windshield to see if some of your airflow is going up there.

If there is no airflow up there, then pull out your blower motor and see if it's full of leaves and debris, pretty common.

FYI, if it does seem to be a vacuum leak....follow the vacuum line from the firewall to the intake manifold and check for cracks. Goes into the firewall right near the refrigerant lines.

All the vent actuators are controlled by vacuum, the hvac head unit controls the vents with a little vacuum manifold on the back. If system vacuum drops, the selectors wont work, and hvac goes to the defrost vents by default.

Cruise control won't work either, since it also vacuum driven

Do you still have the vacuum canister (football) under the front bumper? Are the lines intact?

Thanks for all the suggestions.
Will do some more troubleshooting next week since I'm out of town this week.

I have a 96 and found the connector on the back of the blower speed control switch was burnt up and starting to arc really bad. High speed would cut in and out. Also found the resistor pack on the heater box had a loose connection on it that would cause the fan to act up at different speeds. 3rd, the blower motor had a lot of thread from a mouse that had made it its home and was wrapped around the blower shaft. Plus , I also found the vacuum line under battery tray was rotten and broken in several places leading to none of the controls working. Last, I found the vacuum actuator for the recirculating damper was bad and is not available any more. I was able to adapt a GM actuator to work. Good luck! :wierd: