Hey guys, my truck has had a problem surging over the past couple weeks. It only does it when the heat is on. If I leave it in park to idle or stop at a stoplight with the heat on the rpms will jump about 500 rpms every 30 seconds or so. Turn the heater off and it doesn't do it at all. Any idea what could be causing this? It will stand the back end of the truck up in drive when it does surge like you're trying to power brake it. It seemed to develop after I sprayed the motor off at a car wash about a month ago. Maybe I got something wet I shouldn't have? I already changed the IAC valve thinking that was the problem but it wasn't. Any suggestions would be appreciated. Thanks!

 

several positions of the "heater" turn the air conditioner compressor on and use it to dehumidify the air coming into the cab. Let the truck set at idle in park and get out and look at the compressor when the RPM changes. The computer raises the motor RPM when the compressor starts........ see if this is related to your problem.

 

Yeah I know it's supposed to raise the rpms a little bit but I've owned the truck for 3 years now and it was never bad enough to the point where it was moving the truck from a stop. It does have a squealing pulley sometimes though and it sounds like it could be coming from the ac compressor, or idler pulley or the tensioer. Could that be the problem...unless my brakes are getting weaker? I've had brake problems with this truck since day one. It had no brake pressure at all when I first got it. After changing the booster, master cylinder, checking all of the calipers, and finally replacing the abs box under the hood I got a brake pedal. The truck had an abs light on but after changing the box the abs light went out, and the brake warning light came on and has been on ever since. The abs vibrates constantly right before the truck comes to a complete stop, have not been able to figure out what's causing this for the life of me

 

The surging with the heat on is probably caused by a vacuum leak.
Could be the famous PCV line elbow between the rear of the intake manifold and the fire wall. Could be a vacuum leak in the heater control assembly.
Or could be both are leaking.

 

You have a wheel bearing that is going out. It allows the toner ring to fall out of alignment with the wheel speed sensor at low speed That causes the wheel speed sensor signal to drop out and the ABS unit does its thing.

 

Ok I've heard of the PCV valve issue before. And I just changed the drivers side wheel bearing this summer so I hope it's not that one. Must be the passenger side one maybe. Thanks guys