Heat works at idle, but not at speed

[RonW]

PROBLEM: Cabin heat works fine when at a stop or moving slowly, but goes away when at speed.

SOLUTION: Replace or rebuild heater valve.

The heater valve has a black plastic body, and a square metal top about 2" on an edge, with a short metal cylinder projecting from the top. It is in the engine compartment on the left side, up against the firewall. On the 535i, it's out in the open. On the 528e, it's lower down and not quite so accessible.

I've found it easier and less messy to replace the valve's core with the valve in place than to replace the valve. It's certainly cheaper. The core is available from several sources.

With the engine cool, release the coolant pressure by opening, then closing the cap on the overflow tank. I squeeze a radiator hose as I close the cap so the system is under negative pressure. This reduces the amount of coolant spilled.

Pull the plug on the valve's electrical connector.

Undo the two screws that secure the valve to the firewall.

Undo the four screws that hold the square metal top on the valve.

Pull out the valve core and solenoid coil by the short cylinder.

The metal square is the top of the coil assembly, which you reuse. The coil is surrounded by a metal cylinder about 1.5" in diameter with a thick metal washer at the base, both of which you also reuse.

The narrow metal cylinder that sticks out the top of the metal square is the core, which you replace. Slide it out the bottom of the coil. It has a thin flat washer and a wavy washer, which the new core should have replacements for.

Notice the rubber diaphragm on the bottom of the core? On the old one it's probably torn, which is why the valve doesn't work.

Now reassemble the whole thing using the new core. On the new core, put the thin flat washer, the wavy washer, then the thick washer and the metal cylinder surrounding the coil. Make sure the gap in that cylinder fits into the underside of the electrical connector on the metal square.

When you screw the whole thing back together, the wavy washer will make it difficult to push the metal square all the way down. Just keep screwing, and you'll get it. The electrical connector goes away from the firewall, toward the front of the car.

Now screw it back to the firewall, plug the electrical connector back in, and you're done.

HOW DO I KNOW?: I and others have done this many times.

DIFFICULTY: On a 535i this is easy- you can easily do it within an hour without spilling much coolant. On the 528e it's a little harder, because the valve isn't quite so out in the open.