98 chevy truck heater will not work

Status
Not open for further replies.
Joined
Jan 4, 2008
Messages
59
Location
Mississippi
I hope somebody can help. My dad gave me a 1998 chevy truck with a 4.3 V-6 engine. The truck runs great but the heater does not work. He recently got it and gave it to me. The prior owner said the heater core was just replaced. When I got it I noticed the temperture gauge would fluctuate between 100 to 190, but mostly staying around 150. The heater also would only blow luke warm air. I thought maybe it was a bad thermostat. I changed it and the bottom radiator hose and replaced coolant. I immediatly noticed a gurggling sound coming from the heater core and the truck would still not maintain the correct temp and the heater still only blew luke warm. I ideled the truck for a long period of time and got a lot of air out of the system and added more coolant. The heater then blew hot but the temperture gauge never got above 160. Put thermometer in radiator and it read 155. When you let it cool off the heater will not work again. you have to take the radiator cap off and run it for a long period of time to get the heater to get warm. Every now and then you can hear a little gurrgaling in the heater core. The heater hoses are old and are not the correct ones. They are peiced together with different size hoses. Could it be the water pump, head gasket, air in the system, blockage, or what. All of the hoses get hot. Sorry for the long post but I am really frustrated. Any ideas? Thanks.
 
You may have a collapsed radiator hose, coolant hose, partially clogged radiator, or even a bad thermostat. If your needle fluctuates but never goes above 160F, chances are, your thermostat is still faulty afterall. do not discount the possibility of a water pump failure or slippage for that cause problems also. Lastly, purge your system for air blocks and bubbles carefully and you should be fine afterwards.
 
Engine not going above 160 not all that unusual in cool weather. when engine is up to 160, grasp one hose at a time and see if they are warm,hot,one hotter than the other (good place for infrared spot thermostat). Both hot=water travelling through core. one hot=probably a restriction. check it out and get back.

Bob
 
if the dex cool was mixed with regular green antifreeze, the system can mud or clog up, clogging the heater core primarily.

JMH
 
Thanks everybody for responding. Once the truck gets to 155 0r 160 all hoses are hot. The heater hoses are not o.e.m and are not molded. They are peiced together. One makes a pretty sharp turn and is not crimped but not fully open. Again though, all hoses are hot, even this one before and after the turn. It did not have the original Dex col in the radiator, it had the green, but it was clean. When I repolaced the coolant I put the Peak Global Lifetime. Could it be the sharp turn in the one heater hose? I still hear a little gurgaling in the heater core at times, so maybe there is still some air in the system. Maybe I should replace the thermostat again. How can you test the thermostat to see if it is working. I heard something about boiling it. Please keep the ideas coming.
 
I am thinking there is a water shutoff valve to the heater in that truck. I have a service manual as I have a similar truck - I'll see what I can find on that.
 
If both hoses are approx same temp, that means water is circulating through core, but heat isn't being passed to air inside heater. One of the vent doors in heater isn't opening or closing properly would be my guess.(really Common)

Bob
 
i second bob. also, do you have the all digital heater control or the one with the slide levers? the all digital ones have a step motor that controls the flaps the control the airflow. there should also be a motor that controls the water valve that controls the flow of coolant through the heater core. either of them could be bad and causing the problem. so check the hoses for temperature and then check the air flaps.
 
I checked my Chevy/GMC Factory Service manual, and while the diagnostic procedure refers to the presence of a water valve in some 98 Chevy/GMC trucks, none of the diagrams show one. As to the door actuators, there is a "Mode" door motor (on the left end of the heater box), and a "Temp" door motor on the center front of the box.
 
You mentioned those bothced up heater hoses and the one kinking with a sharp turn. That may be your issue - cutting off flow and all. Replace the heater hoses correctly.
 
Thanks again for all the responses. I have not had a chance to check all the things you guys have suggested. I did replace the thermostat and still the same thing. The temp gauge inside the truck still registers about 155. Sometimes it will rise to 195, but will only stay there for 15 to 20 seconds, and then drops down to about 150. Now there is almost no pressure in the system. I replaced the thermostat last night. Drove the truck about 30 miles this morning, got back to the house, and there was no pressure in the system. The cap cam off with no spew, spillage, or anything. What else could cause the truck to run cold? Maybe I am wrong, but I thought if the water pump was bad or there was a blockage in the system it would run hot. The heater hoses get pretty warm but the top and bottom hoses do not get very hot at all. If there is air in the system, would this cause no pressure and running cold? Please keep the ideas coming. Thanks!
 
All good ideas here. Could be any of them. Try replacing those kinked heater hoses and bleeding the system again and see what happens. If it still doesn't blow hot, check the blend door, linkages, etc for proper operation. Also, consider getting an OEM thermostat if possible. I've seen a lot of junker aftermarket t-stats that cause temperature fluctuation by opening too soon.

Also, how are you bleeding the system? Sometimes getting all the air out can be tricky.
 
Last edited:
for there to be no pressure in the system with the truck warmed up, you have to have a leak in the system. its not in the cab/heater core or it would fog up your windshield and you would probably smell it and there would probably be a puddle on the floor. lets assume that you did a proper job of installing the t-stat and the leak is not there. that leaves the radiator, the hoses and clamps, and the head gaskets/intake manifold.

GM had big problems with bad gaskets in intake manifolds leaking coolant into the intake and it disappearing out the tailpipe. a shop should be able to pressurize your cooling system at the radiator cap and see if there are any leaks evident. also, get one of the prestone in-line hose flush things and put it in your heater core hose and use it to flush out the possible air pocket/debris that you have going on in there.

just my 2 cents.
 
If you still have the original radiator cap on it, be sure to replace it with a new Stant 16# cap, otherwise that could be the source of pressure loss. There could also be a faulty Engine Coolant Temperature sensor at work. You did use a 195* thermostat, right?
 
Check for a vaccum leak,it is common on this engine, the vacuum hose rots and you only get a little heat from the defroster
 
I replaced the radiator cap today. The system is pressurizing to some extent now. As suggested, I am going to take it to the shop and let them have a look. It is not loosing coolant. It is just not getting hot. After 45 minutes of ideling, you can put your finger in the radiator and it is just warm. When the truck is running the fluid in the radiator looks motionless. If you rev the engine the coolant drops a little in the radiator, but when you let off the gas it goes back to the top. When the temperture needle does go to 195 for a few seconds, the coolant level will drop, but when the temperture gauge drops back down to 160 the coolant level comes back to the top of the radiator. I am not a mechanic by any means, but this is really strange. I am about ready to perform an exorcism on this truck! Maybe the temperture sensor or gauge is bad, the temperture at the radiator is much lower than the temperture in the engine, and something is wrong with the heater doors and the vacum lines inside the truck. All the information and ideas are greatly appreciated, and if you can think of any more please let me know. I will let you know when I take it to the shop. Thanks!
 
get a meat thermometer and stick it in the radiator fill hole and go by that. dont trust your gauge. i bet a new temp sensor cant be more then $20 for one of these. less i bet.

the rising/falling of coolant sounds strange to me. sounds like an air pocket. it could be dangerous. no cooling happens wherever the air pocket is, it could cost you a new head. have you ever used "bars leaks" on it? those pellets will find their way to the heater core and plug it. i suggest a complete drain followed by a in-line radiator filler installed on the heater hose that comes out of the engine at the lowest point. then you can vise grip off one of the heater hoses at a time and power flush your heater core. cost you about 6 bucks. fill it back up with water. you are in MS and dont have to worry about it freezing too fast. its worth a shot.
 
Status
Not open for further replies.
Back
Top