Well I thought I knew most of the 928's HVAC foibles, in fact I'm often on here helping other people resolve their problems. But I've just had oddest experience and I can't figure out what caused it.

Have suffered leaking radiator so today I finished installing a brand new one, together with new top and bottom hoses, and new sealing washers on the block drains.

When it came to fill she took 14L, and that was with expansion reservoir at the halfway seam. The level didn't drop for a couple of hours and I thought it would drop when the heater matrix filled.

I left her idling to check all ok warming up to operating temperature, with heater full on (today it's 0C in UK). However even with the temp needle at normal operating position, there was no warm air coming out of the vents. I confirmed the setting motor was moving to the full hot position , with mixing flap arm in place. The coolant level in reservoir hadn't budged.

There were no leaks (hooray) so I suspected a jammed hot water valve (not the actuator as with no vac they are open I believe). Thought that was unusual, especially as I had changed the valve as PM a couple of years ago. Last time I checked I had no vacuum leaks.

Took her for test drive and almost the moment I started moving welcome hot air came out of the vents and continued for 10 miles or so test drive. The temperature was under normal control of the temp lever. When i got back the coolant level had dropped about an inch below the seam (I'll top up when she's cooled down).

So what could cause the heater to not work when car standing at idle, but operate fine when I drove off? All I can think of is that my drive where I had let her idle to operating temp is a very slight slope i.e car pointing upwards very slightly. Could that have prevented the new coolant from flowing towards the hot water valve?

Thanks for your diagnosis. I am really pleased the new rad is plumbed in OK, but this is puzzling!

 

I have a similar problem on my 99 Dodge Avenger. I can be driving the car and it be fully warmed up. But when I come to a stop I lose heat. It isn't until I give it gas that the temp comes back up. Now the air at idle will be warmish but no where near as hot as when driving and the rpms are above 2k. Not sure if this is a similar thing but I will be watching this as I want to correct it on my car as well. Maybe the resolution to this might be similar to my car.

 

what you saw was the water pump filling your heater core, the pump has to turn faster than idle to fill it correctly,
thus running at idle coolant wont be forced into the core .

To fill the 928 coolant system
with the car level,
add 2 gallons of coolant and 2 gallons of distilled water.

NOTE mix 1/2 gal water and 1/2 gallon of coolant for the last bit.

with the cap removed ,
and the heater turned full hot
run the engine till the fans come on .

add coolant to the bottle as necessary to keep it above 1/2 full.

Then replace the cap and drive it for 5 miles let the engine cool off.
Top up as necessary with the last bit of 1/2 and 1/2 water coolant mixed.

 

I thin k it will be a different issue, based on the particular circumstances for my 928 that I had drained the block (and presumably this drained heater matrix?) but when refilled and idled the coolant was not reaching the matrix.

But I hope you sort the Dodge while winter clings on!

 

It's not that big of a problem for me, I just keep my rpms at around 2k when sitting at a stop light. Good thing my car is quiet but I'm sure some people have given me some weird looks for it.

Have you tried to use the heater now that you have driven it? Maybe now it has filled the matrix and will pump properly?

 

Brilliant thanks Stan. I wondered if it was something to do with the pump at idle.

I did mix coolant and distilled water 1/2 and 1/2 in a 2L jug for the fill (I'd seen info somewhere that there are problems if you don't premix, as the coolant is heavier than water).

So if the pump can't push fluid to the matrix at idle, what is the thinking behind setting the heater to full hot when running engine till fans come on (or are you meant to apply light throttle at this stage?).

Thanks a lot for solving mystery,

Adrian