I have a Lexus LS430 with 125k miles. Heat sometimes works then blow cold air. Sometimes it blow cold air then heat, then back to cold. What do I need to look at?
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I find the Diesel Cabin Heater will not operate if you have less than 100-120 miles of fuel left in the tank. Next time it doesn't work look at the miles left in the tank if if it is below this figure then top up and you will probably find it working again.
see if the feed pipes are hot ,if so then its a fault with the operating flap control ,1992 it could be a cable and not vacumn ,just cannot remember all the cars i work on sorry
When you say the heat doesn't work are you meaning the fan doesn't blow out hot air or just doesn't blow out any air? if it blows cold air sometimes when you have the heat on then it could be poor circulation through your radiator or a weak water pump. make sure your radiator is clean on the inside, not too corroded or nasty inside. You can buy a radiator flush thing from wal-mart or any auto parts store. See if it helps.
It sounds like it is just going into the defrost cycle. Go outside and see if it is iced up. If it is, it reverses itself to melt the ice. Heat strips should come on if unit has them
With the car warmed up,feel the heater hoses,are they both warm/hot near the firewall in the engine compartment?If they are both warm/hot,then the heater core should have circulation through it and it may be a "blend door" issue in the heater a/c assembly.If they are not warm/hot then the heater core could be dirty and plugged up and needs to be flushed.
If the above does not help,a drain and vacuum of the cooling system and vacuum refill will remove any trapped air.
Sometimes,clamping off the top rad hose and holding the engine rpm at 2000 rpm will force trapped air out of the heater core.(Watch temp gauge when trying this)
Lastly,a NEW thermostat doesn't mean its working correctly.
after checking the heater core hoses they were both hot I went in to look at the blend door and noticed it wasn't moving on hot or cold so I simply disconnected the lincage and moved to hot and its working ok. but its not completely repaired
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