RPM's pulsating with Heater On.

[5.0L V8]

So this winte rI realized with the heater switched to any heater setting on any fan speed, it makes my engine bob and weave up and down. The RPM's pulsate to such and extent, that tonight when coasting downhill with the heater on I actually noticed the vehichle slightly lunging forward with each pulse.

What is up with this? It ONLY does it when the heater/defroster is on, not the A/C.

 

[deweyville65]

Vacuum leak possibly

 

[koda2000]

Vacuum leak possibly

+1 on possible vacuum leak. could be in HVAC controls as it didn't doesn't do it with the heat off.

 

[5.0L V8]

+1 on possible vacuum leak. could be in HVAC controls as it didn't doesn't do it with the heat off.

HVAC being? Sorry man, still learning.

 

[deweyville65]

HVAC The heater/air conditioning controls in the center of the dash. Pull the trim off from around the radio,HVAC controls, then you can remove the control and there will be electrical and vacuum connections visible. I don't know that is the trouble area, but a good fairly easy place to start looking.