How do I diagnose no heat

I have 1996 Caprice Classic (350 LT-1 V8 engine) & I just noticed no hot air when trying the heater/defroster. (Its August and hot outside, the heater should have run me out of the car within minutes)

The Temp gauge reads normal, the Car does not appear to be overheating. (a/c works fine) I had the Heater Core Changed out approx 4 years ago. (GM dealership did the work)

The water pump was just replaced last winter. (GM dealer again)
They flushed and filled with fresh dex cool. Car has approx 95K miles on it. (car was under GM extended warranty but its not anymore...)

I was thinking it may be a stuck T-stat. The T-Stat appears to be easy to get to on this car so I was going to take a pot shot and change it.
Then I realized this is a sealed cooling system. (how do you drain & refill it.. I don't see any where on the Radiator to drain.. and there's no Radiator cap to refill...)

Should I try to chg the t-stat without draining? Won't it spew coolant all over the serpentine belt if I try that? (when the water pump went bad the coolant got all over the belt and caused it to slip on the pulleys,
caused intermittent pwr steering failures etc.)

If I pull the lower hose I'll most likely loose all the coolant. How do you refill if that happens... no radiator cap (if its sealed, wouldn't I put a nasty air bubble in there if I open it up?)

I used to be pretty good a fixing / tinkering with older 1970's 307's and
350's, 305's, but these Gen 2 350's engines seem pretty intimidating to
me. (I'm a shadetree mechanic at best..)

I don't even know if the T-stat is the problem... These 1994 and newer vehicles have lots-o-new gadgets on them. (vacuum switches, control
modules...oh my....)

Anyone got any suggestions on How I can diagnose / correct this?
(be nice... I know my questions sound pretty dumb but I really want to
take a shot at fixing this if I can...) Seems like common sense ain't quite
enough to get the job done these days....

Find the T-stat... if its anything like the older style ones (vaccum powered) try unplugginf the vaccume line, and open up the valve with your hand. If it doesnt wanna ope, try to break it lose with some pliers or channel locks, some WD-40 and if necesary a little bit of heat. One you get it lose move it back and fourth many times, and constantly lubricate it with a little bit of lithium grease, or even motor oil. Then open up the valve again with your hand, and plug the vaccum line piece with your finger, and let got of the valve. The valve should stay open with the vaccume line plugged, but as soon as you let go of the line it should close again.

Find the T-stat... if its anything like the older style ones (vaccum powered) try unplugginf the vaccume line, and open up the valve with your hand. If it doesnt wanna ope, try to break it lose with some pliers or channel locks, some WD-40 and if necesary a little bit of heat. One you get it lose move it back and fourth many times, and constantly lubricate it with a little bit of lithium grease, or even motor oil. Then open up the valve again with your hand, and plug the vaccum line piece with your finger, and let got of the valve. The valve should stay open with the vaccume line plugged, but as soon as you let go of the line it should close again.

Thanks for taking the time to respond to my question.
Here's what I found...

The T-stat has no vaccum lines. Goose neck is bare. Its located on top of the water pump. (not on the intake manifold like the older 350's.)

I drove the car out on the highway today and oddly enough, I did get some heat, but only when I put the setting to VENT and temp dial to hot while driving 60 mph. It would initially come out hot, then gradually fade back down to luke warm.

If I turned the dial to OFF for a few minutes and then back on to VENT setting, & the Temp dial set to hottest , the same thing would happen. (Starts out hot, then gradually fades to luke warm...) I tried this again, but under hard acceleration. No difference.

When I set the dial the HEAT, the airflow switched to the vents located under the center of the dashboard just above and to left and right of the tranmission hump. The air that came out was actually cooler when I used the HEAT setting than when using the VENT setting. The air would
get luke warm (if even that) and then get cooler again.

I also tried the DEFROST setting but the air that came out of the defroster vent was also cooler. I tried turning off for few minutes (while still driving 60 mph) and back on again but setting it to HEAT. The air was still cooler out the bottom heat vents than it was when I set the dial to the VENT setting. (front vents facing passengers) Ditto for Defroster. :screwy:

When I got home with the car, I popped the hood and felt the heater
hoses. One was hot, the other was very hot.

Sounds like maybe the flapper door is not closing/opening correctly. Are the doors on the newer cars controlled with vacuum, a slider or something else???

Sounds like maybe the flapper door is not closing/opening correctly. Are the doors on the newer cars controlled with vacuum, a slider or something else???

I used to own a 1980 Buick Regal Ltd (231 v6), and I currently have a 1987 monte carlo ss. (305, but its a pooch) Those cars have flappers that are controlled with a wire attached to a slider control. I know what your talking about with those older style 70's and 80's GM products.

The 1996 Caprice Classic is a different beast. My guess is that the flapper door is opened / closed via vacuum pod, or thru some other electronic control or both. (but to be honest I don't know)

I don't hear any tell tale hissing. I can tell you that the air flow does get re-directed from the front vents to the vents underneath when I turn the dial from VENT to HEAT. That would seem to confirm that at
least some of the vacuum pods are working correctly.

The controls consist of 3 dials. 1st dial is FAN SPEED, 2nd dial is TEMP (cold to hot), and the third dial is the AC / HEATER defroster selector. (several settings, OFF, MAX, A/C, BL, VENT, BLEND, and DEFROST)

The thought of blindly tearing out the under side of the dash does not seem very appealing. If I knew what I was looking for I'd be more apt to give it a go... I wish I knew where I could get my hands on a Chilton manual or something equiv. I can't seem to find any USEFULL information on the web without being asked to deposit $$$ from my credit card. (anybody got any cheese to go with my wine...)


I appreciate your response though, thanks for your post.

Check your heater control valve. It should be at the back of the engine, spliced into a heater hose. It it's partially frozen or stuck, that could do it. You should be able to manually move the arm that opens up the flow to the heater. If it hangs up you either get little heat, no heat, or heat all the time. When the car is fully warm, the heater hose between the valve and firewall should be hot.

Bob

Check your heater control valve. It should be at the back of the engine, spliced into a heater hose. It it's partially frozen or stuck, that could do it. You should be able to manually move the arm that opens up the flow to the heater. If it hangs up you either get little heat, no heat, or heat all the time. When the car is fully warm, the heater hose between the valve and firewall should be hot.

Bob

I must have a goofy version of Caprice Classic because there is nothing
spliced into the heater hoses near the back of the engine. I see no valve. The only splice is on the upper heater hose. Its a plastic tee.

There's a hose that runs from the tee to the underside of the spill tank.
The spill tank also has a small hose 3/8" (inside diameter) running from the top of it to the top of the front of the motor along side the t-stat housing on the drivers side of the engine, but it disappears under the air intake stuff.

[Sounds like the water line valve to me too. I thought they were solinoid activated on the newer models., Track the heater hoses back to the block, both of them if you dont know which is in and out. Clean up or replace the valve if you cannot get it to flow, Heres another option Ive had blockage before in the exchanger, You may be in need of an enema!!!!, Take the beast to a car wash pull the hoses and using a rag ti help seal the fitting squirt the wand ( low pressure first, high if low doesnt work), You may be able to push out the block, Squirt both dirtections till clean....P.S. Take a piece of pipe and a couple of hose clamps., If the core is rotted you could blow it out and need to bypass untill you can get a new or used replacement.

I guess they did away with the traditional heater control valve at some point in time. I apologize for any confusion I may have caused. Take a trip to your local library and look for a Mitchell's auto repair manual to help you out. These books will be in the reference section, you can't take them out, but you can copy all the pages you want.

Bob