burp the air out of the cooling system. need to get the passenger front of the truck up nice and high. getting all the air out these trucks is a pain....

 

Does your temp gage fluctuate? If not probably not air in the system. Are you running straight anti-freeze if so that can be an issue anti-freeze does not heat up like water. it should be a 50/50 mix. If that is not the issue I would think half stuck thermostat or pluged heater core. Sometimes crap falls into the heater box from the cowl and can block the flow of air from the heater fan. I would check this as a last resort as it requires removing the dash.

 

I have flushed the heater core and it was clean. Brand new thermostat, new radiator with the 50/50 mix and the temp gauge does not fluctuate other than going to its normal operating place.

 

Not to hi-jack this thread but my truck does the same thing. My temp gauge does fluctuate a lot and has luke warm heat as the OP stated so I will have to give that a try to remove air. I thought maybe I had a bad temp sender as to why the gauge moves so much. What does everyone recommend to bleed the coolant system? As any one tried to pressure systems to force coolant back in? Any info would be great to help both of us out here.

Jeff

 

Could be a stuck blend door.

 

How do I check that and if it is "stuck" how hard is it to replace?

 

I don't recall if the 98 blend door is cable operated, or, if you have the little motor on the bottom of the heater box. Have a look on the passenger side, just to the right of th trans tunnel, there will be either the little motor, or, a very little bit of shaft sticking out the bottom there. If you don't have the little motor, you can put your finger on the end of the shaft, and see if it moves when you rotate the temp dial.

 

Well I didn't see a "shaft" so I'm guessing that it has the little motor. So do you think the motor is bad?

 

Spin your dial fast enough and you can hear the doors slapping if it's cable controlled.

Sometimes these cores need assistance when being flushed...Disconnect both ends of the core, add additional hose to keep crap from ending up all over the engine bay. Fill core with water and then use air compressor to evacuate rapidly. I used 90psi. As soon as core is empty refill. Repeat a few times.

Did mine 3 or 4 times, reconnected the core, lifted the front end, burped a few times and now get roasted out when it's below 25.

After the first Air compressor evacuation look at the discharge hose, if you see chalky brown sludge dripping your on the right track. If not....different issue.