No heater at idle

[pontiff]

Hi all,

Okay I am at my wits end I do not know what to do next. The problem car is a 98 6n1 auto which only had a heater when you are driving over say 3-4k rpm, let it come back to idle and it just starts to blow cool again.

Things I have done so far

1) flushed heater matrix (it is/was clear and does not appear to have any blockages) and changed unit for good measure.
2) removed heater box and repaired mixer flap (the foam had all come off) (not a job for the faint hearted, its a PITA!
3) changed thermostat
4) removed and inspected water pump. It had previously been changed (not by me) and now has a brass impeller version as opposed to the plastic ones normally in these cars
5) flushed entire system with pressure cooling thing that you vac down then inject coolant into
6) flushed radiator (no blockages)

and yet it still does the same thing.... what have i missed? How is the water pushed through the heater matrix? It looks like the inlet and return pipe from the matrix are just connected to the coolant regulator housing so what actually pushes the water 'through' it as from what i can see there is little to force the coolant through.

What have a I missed????? I'm just so confused.

[sports racer]

The heater works by taking water from a high pressure area and returning it to a low pressure area. There is no actual pump for the water going through the heater.

With the engine warmed up and idling, feel the rubber hose at the top of the radiator which goes to the motor. It should be warm. If not then your cooling system is working too well, could be the fan is permanently on? Or your thermostat isn't opening properly - does your car overheat?

Now feel both rubber pipes going to the heater. Are they both warm, is one warm or are they both cold? They should both be warm. If only one is warm then there is a blockage somewhere in the line, probably in the heater.

If they are both cold then there is no water going to the heater so you need to check what's stopping the water coming out of the engine. Turn the engine off and take off one end of one of the rubber hoses that go to the heater. Start the car and see if water is being pumped out of the connection you just undid. There should be a steady flow of water. If not then there's probably something wrong at the engine side which isn't forcing the water through.

You know the heater works at higher revs so my guess is you have a blockage somewhere that is only overcome by higher water pressure as the engine turns the water pump faster.

Let us know how you go.

Cheers

Paul

[pontiff]

Hi Paul,

The heater hoses were hot going into the heater matrix and cooler (but still not) coming out of it and as I say, i had changed it with a good unit. In the end what I did was let the car sit overnight (as it was about 2am by the time I finished), when i got out to keep looking at it in the morning I noticed that the water in the coolant reservoir has dropped about an inch or so. I topped it back off again and started the car and it started working fine, so I can only assume that there was an air blockage that by cooling down and letting it sit and what not fixed the issue? i thought that running the car with the reservoir lid off was suppose to work through any air blockage that could be stuck?

[sports racer]

Good to hear it's all fixed.

You know you do have a car that's older than 10 years so it's bound to have a few idiosyncrasies.