Last post on Jan 28, 2009 at 7:41 PM
You are in the Chevrolet Blazer
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Chevrolet Blazer, SUV
#4 of 23 NOT THERMOSTAT
Oct 29, 2008 (9:27 am)
It is seldom the thermostat that causes heater problems. If your engine is running at the correct operating temperature, the heater core will also be hot if it is properly connected to the engine with an in and out heater hose.
The problem may be bad heater fan motor and can be tested. If the fan motor is good, it is a kinked or a disconnected vacuum line, a disabled heater control switch or it could be that the linkage that opens the register (heater door) is disconnected. The register is located inside the heater plenum and the vacuum lines and controls are located under the dash If the fan is operable and if the register in the heater plenum does not open, warm air from the heater core cannot enter the pasenger compartment.
If the heater core has recently be replaced, the person who did that work accidentally disabled the register controls.
#5 of 23 Re: NOT THERMOSTAT [duntov]
Oct 29, 2008 (9:38 am)
ok. The heater core hasn't been replaced. Only work that was done on it recently is that we had the fuel pump replaced. Just bought the blazer 2 months ago. When we test drove it we tried the heater and it blew out nice hot air. Now, it is not and it is getting very cold here. I don't know what to try first and I don't want to have to take it to a shop. Thanks for any help!
#6 of 23 Heater plenum
Oct 29, 2008 (10:59 am)
Whenb the engine is warm, does the heater blow cold air? If it does, the heater fan os OK. After the engine warms up to operating temperature, turn on the heater and feel around on the heater plenum under the dash and see if it is warm. If it is warm, the hot air from the fan through the heater core is being blocked by a closed register. You must find out why the register is not opening. Eith vacuum line is disconnected or the switch to the register linkage is binding.
#7 of 23 Re: Heater plenum [duntov]
Oct 29, 2008 (11:48 am)
Yes, when the engine is warm it blows cold air. I will check under the dash to see if it is warm. What is the heater plenum? Thanks again.
#8 of 23 Plernum
Oct 29, 2008 (12:38 pm)
The plenum is the plastic air box that is under the dash that has heater outlet vents and is attched to the heater fan and core assembly that is located on the engine side of the firewall.It contains directional doors that are controlled by the vacuum diaphrgams and those are contriolled by the heater contol switch.
#9 of 23 Re: Thermostat Replacement [sheps47]
Oct 29, 2008 (1:05 pm)
If the engine is operating normally, but there is no heat in the core, it is very possible that the core is plugged up. These vehicles do not use a heater control valve, they always run the water through the core and use a baffle to bring warm air into the vent system. Check to see if the feed and return lines to the heater core are both hot. If one is noticably cooler than the other, the core needs to be flushed. If it is plugged, probably a good idea to replace the Dexcool. Use Dexcool again, don't mix with the green stuff!
If both feed and return are hot, you have a baffle not operating in the duct work. Haven't worked on these before, but there are others on the maintenance and repair board that have.
#10 of 23 Re: Thermostat Replacement [jlflemmons]
Oct 29, 2008 (2:16 pm)
Ok thanks. I don't have warm air coming out of the plenum. I will check to see if the lines are both hot or not. What causes this to happen?
#11 of 23 Re: Thermostat Replacement [sheps47]
Oct 30, 2008 (8:51 am)
Dexcool turns to pudding after about 4-5yrs or 60K miles.
#12 of 23 Re: Thermostat Replacement [jlflemmons]
Nov 02, 2008 (11:33 am)
Thanks for all the help and suggestions! We flushed the heater core and it blows HOT now!
#13 of 23 Re: Thermostat Replacement [sheps47]
Nov 03, 2008 (11:06 am)
Glad to help. Just keep an eye on it for a couple of days to make sure no leaks appear.