98 dodge ram 1500

I have noticed the heat isnt warm until you are on the gas a little bit. Also today i have noticed when you get it close to 2 thousand rpms on the interstate it will switch from vents to defrost.. ITS ONLY WHEN IN OVERDRIVE. I can be going 70 take overdrive off and the heat will come back to the vents..
I am sure i have a vac leak somewhere any ideas?

 

do a search on wild vents for the defrost/heater thing.

check that your cooling system is full. Does the engine temp get up to where it should be?

 

coolent is good and engine temp is correct

 

burp your cooling system. Gotta park the truck at an odd angle. Passenger side front need to be pretty elevated to get all the air out of the heater core. Not one of the best designs I have ever seen......

 

As far as the heat, if its been awhile try changing you t-stat and flush your cooling system and heater core.

I had the same problem with the vents and i had a vacuum leak caused by the plenum gaskets leaking. I would start there, take the throttle body off and with a flash light look inside the intake and see if you have any oil pooling on the belly pan. if you have oil pooling inside there that is most likely your problem.

 

My vacuum leak was in the transfer case lines -- they can be just about anywhere. Might be under the dash, might be under the hood, might be under your butt. The only sure way to find them is to break the system and eliminate major areas while isolating just one as at fault, then once a major area is found, break it into minor areas, and so on down to the problem. Thought it wouldn't hurt to just go through the whole darn thing and replace all of the vacuum lines.

Also, any air in the system is likely to lodge in the heater core so do as HeyYou said and get the truck onto a crazy incline before burping the system.

 

my truck does the exact same thing! it drives me crazy!!

 

allright so a long time of looking at the vac lines and fixing stuff and test driving.. Finally figured out the problem. It was a check valve . Now for my other problem Why when i am not on the throttle is my heat get cooler and blow less?

 

Air bubble in the heater core. It is, unfortunately, the highest point in the cooling system. (smooth move dodge...... hence, the reasoning behind getting the passenger side of the truck higher. )

Some blower speed variation at idle is to be expected. not a lot mind you, just, some. If your blower speed varies with engine RPM.... something else is going on. What you might also want to look at, is if the air intake for the heating system isn't plugged with leaves and such.

 

I had this problem until I burped the system after I drained it with the plenum install.

ALSO, it could be a vac leak. Vac lines determine how the air is routed, so you could have a vac lean and the air is not blowing through your heater core (or most of the air isn't blowing through).

So 2 things:
1. Burp your cooling system with your front end jacked up
2. Check the vac lines.