No heat at higher speed.

[Tken]

Hi
I just picked up an 88 325i sedan, 120k, the car is in mint condition, everything works perfect, as i thought. But when i drove the car home, i notice the heater stop blowing hot air when i go above 30mph, then when i get up to the freeway, it was blowing freezing cold air. It was 35 degrees out and i was driving home freezing my butt off. However, when i get to a complete stop, the heat work again. Where do i start diagnose this problem? Thanks
guys

[Simon S]

That is a head-scratcher..

Well, starting simple – there are three mechanical things I can think of that effect on/off flow of hot water through the heater core: water pump, thermostat, and ‘heater valve’ in heater core plumbing (cabin side of firewall).

Assuming temp gauge for engine was fine (water pump and thermostat would have to be working) – then I would look toward replacing the heater valve.

But what would cause the heater valve to open/shut per vehicle speed is beyond me (?)

BMW did incorporate a pressure-relief valve into heater core plumbing on the engine side of the firewall. This was done as a recall issue - as a measure to prevent steam burns to leg from a failing heater core in cabin. A wild thought of mine is that that relief valve cuts off circulation – for whatever reason in your scenario – to the heater core while car is in motion. Note that I’m thinking out loud – and that I’ve never heard of that relief valve failing. Fwiw, many enthusiasts – including myself – remove ‘unneeded’ plumbing from cooling system – mostly as a means to simplify it and reduce possible leak points.

Maybe someone else has experienced this and can chime in..
Good luck.

[tonywonder]

i wonder if its possible that the windchill is enough to cool the engine that theres no hot coolant flowing through the heater core...is your heat cranked all the way? and is your AC off?

no clue if any of this would affect that just thinking out loud also.

in 50 deg weather in socal ive seen my coolant gauge drop at 70mph but i also have a larger radiator. and i dont run heat so no clue how that affects the heater.

[Wh33lhop]

Likely the problem is that the coolant hoses that attach to your heater core are reversed.

Try something: stop your car and put it in neutral, then rev it up to 3000rpm with the heat on. If it blows cold when you rev it up then your heater core hoses are switched around.

[u3b3rg33k]

I have to ask, is there enough coolant?

[Tken]

Updates: Talked to the previous owner and he said the the thermostat was replaced not too long ago, he thought the thermostat causing the malfunction in the heating system. On the way home, the car runs beatifully, normal temp, no coolant smell, nothing!!
Then I start playing around with the car and this is what i found out. Heater works fine at idle, then i start driving it at low speed, i heard some flapping noise inside the dash board, then it stop once i got the car up to 30-40mph and that's when the heater start blowing cold air. So I start all over again, at idle, start driving, same noise, but this time the noise waffleswaffleswaffles not stop, and heater working good, took the car up to 80mph and still hot air. Then i start playing with the temperature control dial, the noise stop and here goes the cold air again.

[Ceeker]

The flapping noise could be a loose air duct door. which is usually letting fresh air in all the time. I would try using the recirc button until you sort out the problem. Try it again with the recirc on and see if you get the same issue.

I would suggest checking coolant level first as an obvious.
check heater core hoses like someone mentioned.

Make sure the system is BLED PROPERLY- very common overlook after changing thermostat.you might have vapor lock in the system.

I have a similar problem and after changing EVERYTHING with the exception of the rad my suspicion is that my rad is partially plugged. upper hose gets hot - loser hose I can hold with my hand after driving for a long time still.

the update heater valve in the engine bay does clog from the thin hose. But it is a pretty simple mechanical set up and I haven't had issues with it.
I replaced the heater control, heater tap, core, valve, thermostat, bled the system numerous times, and still I have this problem. so it lastly has to do with the rad in my case.

[Killacortes]

This EXACT same thing happened to my first e30 when i drove it home. i changed the blower motor and resistor that goes along with it, after that it was just dandy

[e30trooper]

i has no heat ever.... maybe heater core valve? heater core hoes gets kinda hot ??