RX400h heater issues on battery only

Martin_Euxton · January 6, 2017

Hi Guys,

Had my wifes Lexus about 18 months now. Its a 2005 UK car, high miles but with full history and never missed a beat. The last month, give or take, its started having 3 issues which I fear may be related in some way:

1. mists up badly inside overnight, I'd even describe the windscreen as wet inside. If I give it a good dry / wipe out its fine again for a while

2. the round town MPG's have been dropping off, down from about 26mpg to around 19mpg now

3. The heater is blowing cold air when running on Battery only mode. Still nice and warm above 30mph when the engine can kick in properly but as soon as you stop at lights and the engine cuts its cold again, even after a 4 hour drive on the motorway, the second we hit traffic and Battery power it was cold once more.

It is of course winter here and we've seen the temperature outside drop, but the car was totally fine last winter.

Any ideas before I take it to Lexus with my eyes closed and my cheque book open?

Ta

steve2006 · January 6, 2017

Hello and welcome to the LOC.

Not sure about MPG question but could be due to winter temperatures but for the wet windscreen and poor heating check the cabin pollen filter.

This item tends to get overlooked becomes completely choked then affects air flow which in turn reduce the drying effect of the AC and the mixing of warm and cold air that comes out of the vents.

Martin_Euxton · January 7, 2017

Thanks for the response Steve. In theory, according to my receipt, the cabin filter was changed 5k miles ago at its annual service.

My current thinking is that the circulation pump is faulty, preventing the hot water circulating when in EV mode.

I think the damp is 3 fold - heavy condensation from the sunroof and void between the roof lining and roof, general wintry conditions and the heater malfunction means its not drying out said condensation as it likely did previously. A secondary possible assessment would be some sort of minor leak somewhere, maybe related to the heater issue or from the sunroof seal perhaps but thats me just guessing.

And finally, the MPG, from reading on here, is likely just the cold weather, possibly exasperated by the need for the blowers to be one full to dry the screen and the lack of heat meaning the heating system is trying to work harder.

I'm going to get on with checking the pump - will revert once I've tested it.

Thanks

Slooj · September 14, 2017

Did you fix your heater issue? I have the same symptoms where the heater blows cold when the speed drops to around 30mph and the ICE turns off. I'd be grateful of any suggestions or resolution.

Slooj · September 14, 2017

Mine also makes a very strange chatter type noise coming from the drivers foot well or maybe a bit more central. The noise is audible even after the car is turned off and the key removed. Could this be a ventilation servo noise? If so, might that be the cause of my cold heater problem?

Farqui · September 14, 2017

The ventilation servos are known to cause noise from the center console.

The drive cog works loose on the motor shaft which can cause chatter as the cogs turn out of sequence.

A servo may also have failed.

If the RX servo mechanisms are as complex as my GS then it's a real pain to work on.

Good luck setting it sorted.

Slooj · September 15, 2017

Thanks @Farqui. It's going to be too complicated for me to do, I'll get a professional to look at it. If it's gets sorted I'll be sure to post back with the remedy.

Farqui · September 15, 2017

When I researched servo issues, access on the RX looked much easier.

But I understand that it could be daunting task.

Let us know how you get on.

julian1969 · September 19, 2017

my rx has a weird heater 'issue' it's almost as if the temperature i set is 5 or so degrees too low when heating. setting to 22 will still blow cold air and to get warm air out i have to up it to the 25 - 27 range. Not really a problem more an aesthetic issue really but if something is deteriorating towards failure I'd be interested to know.

Britprius · September 19, 2017

There is also an electric pump that pumps warm coolant from the engine to the heater matrix when the engine is not running in all the Toyota/Lexus hybrids, as obviously the engine coolant pump is not running with the engine off in electric mode. If the electric pump is not running the heater runs out of hot water "coolant".

John.

Slooj · September 19, 2017

Thanks @Britprius I've tested all the modes from the climate screen and they all seem to work (air gets blown out of the correct vents). Do you think the noise I here from the centre console might be the electric water pump? Where is the electric water pump located?

Britprius · September 19, 2017

I am sorry I cannot help you with the position of the pump, but they are usually under the bonnet. I have a GS450H, and a Prius. Both have pumps to circulate the coolant when the engine shuts down. Try "googleing" it.

John.

sorcerer · September 19, 2017

34 minutes ago, Slooj said:

Thanks @Britprius I've tested all the modes from the climate screen and they all seem to work (air gets blown out of the correct vents). Do you think the noise I here from the centre console might be the electric water pump? Where is the electric water pump located?

In the engine bay according to this:

cb12250 · February 14, 2021

I don't have noises from any of the components, I just have lost heat now on and off - very strange. It works for a bit and then just blows cold air. Could this be the pump that was in an earlier reply? I love the diagram given, is this something that is a DIY thing or does it need to be taken in to a pro to fix? Any help or advice would be great. It is -5 at the moment and I really would like to have heat in my car.