Odd heater behaviour

totta crolla
Club4AG Pro
Posts: 651
Joined: Tue Mar 26, 2013 5:21 am
Location: Oxford U.K

Odd heater behaviour

Postby totta crolla » Wed Feb 03, 2021 8:24 am

AE86 standard heater without air conditioning. Idling heater is ok, fan blowing hot/warm air. Driving, the heater suddenly goes cold after an indeterminate time and stays cold. Any ideas what this might be?

User avatar
jondee86
Moderator
Posts: 2910
Joined: Fri Jan 11, 2013 10:21 pm
Location: Wellington, New Zealand

Re: Odd heater behaviour

Postby jondee86 » Wed Feb 03, 2021 4:40 pm

Only logical explanation is that there is no longer any hot water reaching the heater core.
Image
The outlet from the heater connects to the suction side of the water pump so unlikely
as it may be, perhaps you have a hose that gets soft enough when hot to collapse and
block the flow ?
Image
Other option is some problem with the heater tap. As you can see the direction of flow
thru the tap is such that it will hold the valve closed. Maybe if the cable has a lot of slack,
or the return spring is broken or the valve can somehow vibrate into the closed position,
you could lose the heater flow. So make sure the valve does actually open wide and stay
open, assuming you have the standard mechanical heater tap of course :)

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

totta crolla
Club4AG Pro
Posts: 651
Joined: Tue Mar 26, 2013 5:21 am
Location: Oxford U.K

Re: Odd heater behaviour

Postby totta crolla » Thu Feb 04, 2021 7:21 am

Hi Jondee, thanks and all of those things are very plausible and I will check them carefully. Indeed, both of the large diameter radiator hoses are soft, I wonder if that could be it?

User avatar
jondee86
Moderator
Posts: 2910
Joined: Fri Jan 11, 2013 10:21 pm
Location: Wellington, New Zealand

Re: Odd heater behaviour

Postby jondee86 » Thu Feb 04, 2021 1:02 pm

I was meaning the heater hoses. A collapsed radiator hose would not make the
heater run cold... more likely just the opposite.

Some basic checks would be if you stop the car (but keep the engine running) after
the heater starts blowing cold, does it return to blowing hot air without you touching
or changing anything ?

If the engine is up to temperature and idling (heater hoses hot) and you rev the engine,
do you see either of the heater hoses change shape or flatten out ?

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

totta crolla
Club4AG Pro
Posts: 651
Joined: Tue Mar 26, 2013 5:21 am
Location: Oxford U.K

Re: Odd heater behaviour

Postby totta crolla » Sat Feb 06, 2021 1:59 am

Feed and return heater hoses are hot at idle and the heater is working. Weather is a bit crap so I'll need to wait until I can take it out to test them when the heater goes cold. Thanks

allencr
Club4AG Expert
Posts: 394
Joined: Thu Jan 10, 2013 9:59 pm

Re: Odd heater behaviour

Postby allencr » Wed Feb 10, 2021 10:01 am

IIRC, I could give the heater a quick feel while driving, but you're RHD. Just a one time confirm the heater's venting is open & what's blowing is what's happening in the core.
The heater's outlet goes right to the thermostat and cools it down, slowing the flow. That's as much as I'm able to guess.
HATED that Mickey Mouse design inlet side thermostat with a little disk to restrict flow when open, though the big pipe from the rear seemed like a good idea to keep 4 from overheating.
Cardboard for the radiator & rag on the oil cooler stopped my head from hurting.
Good luck.
/////
Also soft hoses when hot? It ain't warm or bad cap.

totta crolla
Club4AG Pro
Posts: 651
Joined: Tue Mar 26, 2013 5:21 am
Location: Oxford U.K

Re: Odd heater behaviour

Postby totta crolla » Wed Feb 10, 2021 10:24 am

Thanks for the input guys, I have still yet to get out and test it further. I will report back asap.

totta crolla
Club4AG Pro
Posts: 651
Joined: Tue Mar 26, 2013 5:21 am
Location: Oxford U.K

Re: Odd heater behaviour

Postby totta crolla » Sat Feb 27, 2021 11:18 am

So an update... finally managed to get the car out for a run and it looks like the heater core is blocked. Inlet hose is hot but the return hose is stone cold. I really hope that it can be flushed out. Any recommendations for which way to flush it, i.e with the flow or against the flow? Thanks for any help.

User avatar
jondee86
Moderator
Posts: 2910
Joined: Fri Jan 11, 2013 10:21 pm
Location: Wellington, New Zealand

Re: Odd heater behaviour

Postby jondee86 » Sat Feb 27, 2021 7:30 pm

Just checking... but you have checked that when the cabin control is set to full heat
the heater tap is also wide open (spindle fully up) ? If the core was clogged I wouldn't
expect the heater to work fine for a while and then suddenly stop working.

None the less, I would suggest reverse flushing in the first instance.

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

totta crolla
Club4AG Pro
Posts: 651
Joined: Tue Mar 26, 2013 5:21 am
Location: Oxford U.K

Re: Odd heater behaviour

Postby totta crolla » Sat Feb 27, 2021 10:27 pm

I will double check that Jondee. The actual hose from the tap was hot as l remember it.