Heater only warm when engine reved

[franksand999]

Once the engine is warm, the heater only becomes hot when you rev the engine, at idle top pipe going into core is warm, bottom luke warm -air comiong into car is cold.

I have flushed the heater core, (attaching hose pipe to exit hose), but still problem remains.

I suspect some 'Carlube' radiator sealer, when I poured it in the bottom of the bottle had all congealed, maybe some of that has blocked something.

I assume the heater core vents any trapped air by itself ?

[zollaf]

water pump maybe ? what engine ?

[DarkBunny]

water pump maybe ?

+1 on water pump... Sounds like it's only pushing the fluid through under a reasonable amount of load.

[franksand999]

Thanks for the replies.

Its a 1.4 petrol, only 50K on clock, could it really be the pump ? if that was failing would that not effect the engines temp - which sits right on 90.

I started to lose fluid from the cooling system, I was topping up over the summer, just a small amount each time, but decided this winter to throw in the Carlube Rad Sealer, then issue seems to have cropped up.

I read lots of comments on other threads saying its difficult to properly flush the heater core if it gets blocked - could my attempt at flushing simply pushed the blockage rou9nd somewhere else. Someone suggested using some kettle descaler

[zollaf]

disconnect the heater hoses and put hosepipe on one end, study flow of water. good, not good ? that will give you an indication of whether anything is blocked. where was the leak before that made you think radweld would be a good idea ?

[franksand999]

The flow seems fine.
The heater works fine at idle, but only a good 15/20 minutes after the engine has warmed up, before that I get the problem where its warm only when revving the engine.
Have never thought Radweld was a good idea, it was losing just a small amount of fluid every 1/2 months, the amount you need ot top up before the warning light comes on.

[zollaf]

do you have a flow of water through the little return pipe going into the top of the coolant resevoir ?

[franksand999]

I've not had a chance to check that, yes I wondered what that little pipe was for, could someone enlighten me ?

Anyway, it seems to have sorted itself out, maybe the flush I gave it helped to dislodge the blockage ... working A1 now, thanks.

[itavaltalainen]

the small pipe is so the system can vent. if you have a jet of water coming out of it water pump is probably ok (in idle the 'jet' won't be very powerful).

much more suspect that both:
a) heater matrix is coated with the radiator leak stop *****... even partly blocked.
b) thermostat is gunked up with radiator leak stop and does not fully close anymore (won't show in dash temp gauge since that shows 90 as long as temp is approx 75 - 105).

so.... flush whole cooling system (drain, fill with water, add some dentures cleaner, like kukident - 3 tabs, run for a while, radiator should get hot!).... then drain, fill with water and flush, flush, flush.... repeat last step about 5 times.... observe any leaks in the cooling system and FIX THEM. then fill with 50% G12++ or G13 (or equivalent) : 50% water mix....