91 Ls400 Heater Not Getting Hot. Only Warm. Why?

914lps · January 10, 2014

My 91 LS 400 heater is not working right. It is only putting out warm air when sent on maximum.

I have played around with the settings etc... To see if I could figure out why.

Now I am in Arizona and only dealing with outside temps no lower then in the high 40's.

As I move the temp setting up and down the system seems to work. The vent and fan settings change as I change the temp setting with things set on auto.

The only way I can get some warm air coming out is to go into manual mode. Set to recirculate air. And a/c off.

Vent and fan speed seem to make no difference. Any ideas on things to check, etc????

The cars temperature gauge is where it always is after the car warms up. Dead center of the range. Car engine heats up fine. Car runs fine. No indication of a cooling loss/leak.

Normally this car puts out really hot air when set on high... So something is not working right.

As always thanks to all.

steve2006 · January 10, 2014

First thing to check is the coolant level, even slightly under the max level can cause heater problems.

Also check the operation of the heater flow control valve located on the engine firewall, it is mounted almost centrally at the top and should have a black plastic cover on it.

billydpowell · January 10, 2014

if that dosen't help, do a search and you will find a lot of post on this... good luck

landar · January 10, 2014

Ditto on both counts. On checking the heater flow control valve, it is controlled by a cable coming out of the firewall on the passenger side. Check the location of that lever going into the valve when calling for heat. It should be either fully down or up (I forget which) but you can 'play' with it manually by disconnecting the cable and moving it.

Sometimes (usually if low on coolant), you will get cool air when idling and then hot air with the engine revved. This could also indicate an air pocket in the heater core.

If none of these issues applies, you could have a plain old clogged heater core. Let's hope not. Usually it is something simple and that is what you should look for.

914lps · January 10, 2014

Thank you. I will check coolant level and the valve fiction.

914lps · January 11, 2014

Coolent level looks to be good at reservoir. Turned temperature up and down valve moves full range.

I will use infered thermometer to check temperature diferance between upper and lower hose connected to valve when open.

landar · January 12, 2014

You know I was just thinking that perhaps your issue could be related to a blend door. Could be that the heater core is being bypassed via the ducting and that is why you have very little heat.

914lps · January 12, 2014

You know I was just thinking that perhaps your issue could be related to a blend door. Could be that the heater core is being bypassed via the ducting and that is why you have very little heat.

Can you give me more information on this?

landar · January 12, 2014

I was afraid you were going to ask that. :unsure:

There is a so-called "blend door" or chamber behind the glove box that mixes the hot and cold air into your main duct work. You might want to explore that area a bit more. It is usually fairly easy to remove the glove box and start looking around if you have ruled out all of the other issues.

Here is a thread that may help. You will have to go off on some of the 'rabbit trails' in the thread but hopefully it will give you some more ideas of things to check -> http://us.lexusownersclub.com/forums/topic/68099-heater-control-valve-or-a-cable/