Vacuum leak or TPS?

'96, 4.0, auto

Erratic idle.

May idle much faster than normal, then dip right down so that it is stumpling and struggling to stay running. Will also stumble badly sometimes when pulling away from an intersection.

The kicker is the heater.... My heater controls default back to defrost when it begins stumbling and struggling. Under load (like a long hill, with 3 passengers) the heater will also switch back to defrost. This is what makes me think it is vacuum.

I've tried tracing out the vac lines with WD 40, but haven't noticed any change in idle when doing this.

Ideas?????

Thanks,
Geoff

You may have a cracked vacuum canister. If it is changing you heater to defrost under a load. It really sounds like a vacuum problem. Instead of WD-40, try carb cleaner, just be careful around hot engine parts. (I've never had a problem, but don't want you to be the first!)

Happy hunting!