91 E30 M42 318i HEATER problem

[pdxh20]

I've replaced the recirculation doors & links, checked the solenoid actuators, verified that all controls are functioning properly and still have a problem no one can solve: The air from all three levels (floor, dash & defrost) is perfectly hot (140F with my HVAC thermometer) once the car is warmed up and the temp needle is at high noon AS LONG AS YOU ARE DRIVING AT LESS THAN 50 MPH. over that and it cools down until it blows 60F or so, whether or not you have the recirculation on or not. Slow down and it's hot within 5 seconds. The heater core has been bled, the control valve works perfectly (at least when driving slower than 50 mph). It's not RPM related. This has got to be a water flow/ temp at the heater core only (the temp gauge stays right at 12 o'clock) vs road speed issue... ANY IDEAS ? THANKS !

[Jeron]

I cant think of a good explanation so I'll just share some thoughts.

If it isn't RPM related then the only thing causing it is the headwind. Assuming it is the headwind then I can only think of three things that are happening due to headwind.

1) The radiator gets cold from the airflow
2) The wind hitting the fan blades might cause the fan clutch to disengage or the water pump speed to change
3) The fresh air system pushes air into the cabin

The items in your post eliminates most of the idea I had.

I would check the fan clutch with a piece of rolled up newspaper. Then try a leaf blower on the front of the car and at the fresh air vent to see if you can get any ideas. Then pull the mechanical fan temporarily and drive it to see if the condition changes.

[pdxh20]

Thanks for the input. I reread all the cooling system info in the books to check on your ideas about the fan / clutch, but I can't see how any change in the fan(s) could affect the pump speed, since it is 100% mechanically driven by the belt. The outside air pushing into the cabin was what I originally suspected, and that's why I rebuilt the recirculation flap system. I also tested that theory by completely blocking off the doors from the outside with rags after removing the intake grilles just aft of the hood, with no change in symptom. There just isn't anywhere else air can enter the system from outside. This morning I also tried blocking off the radiator with cardboard, but the symptom's still the same, and the coolant temp remains right at 12 o'clock. That eliminates a bad thermostat or other usual issues at speed with more forced cooling like a stuck fan. The only thing that makes sense is some kind of partial blockage in the heater hose(s), control valve or heater core, that only manifests itself in colder air temperature at speeds above 50mph, but instantly vanishes when I slow down. The biggest part of the puzzle is why 100% of the time it's hot as soon as I slow down...I'm grateful for any other ideas that come to mind !

[Jeron]

I was assuming a stuck fan clutch and a slipping belt for the fan to effect pump speed. I know its a stretch but...

"The outside air pushing into the cabin was what I originally suspected, and that's why I rebuilt the recirculation flap system."
--I figured you rebuilt the recirculation flap for that reason.

You blocked the fresh air intake that eliminates another possibility.

In the last couple of days there was another thread (E46?) where the heat to the core was stopped by a high temp protection device. I can only assume it is in the heater valve and I don't know if it was in place in 91 and it doesn't seem to fit your problem unless your temp sensor is bad because you are not getting hot. There is another possibility that the heater valve is coming apart internally and flow is somehow moving loose parts around. That happened in another recent thread.

You have already eliminated many of my ideas in you posts such as:
"This morning I also tried blocking off the radiator with cardboard, but the symptom's still the same, and the coolant temp remains right at 12 o'clock. That eliminates a bad thermostat or other usual issues at speed with more forced cooling like a stuck fan."
Which I thought of in your first post but it is hard to explain and obviously you already thought of it.

Maybe I can think of something in the morning.

[pdxh20]

Again, thanks for your continued help ! I read about the valve internal rubber parts and thought about that possibility in the hose etc. Also this is the first cold season I've owned the car, so I don't know how long this problem existed, and the previous owner is unreachable. The car is near perfect in every other way, and I just had my ( very experienced independent BMW only) mechanic do a complete engine rebuild. The big mystery is still why it works so well as soon as you slow down. There are also (rare) occasions when it seems to be fine even at high speed for a while (10 minutes ?) further adding to the mystery...I've looked at, photographed, and investigated an E-30 heater box + all ducting and controls at my local BMW wrecking yard both in and out of the car, and my mechanic is also stumped...Today I'm going to try blocking off the radiator completely to try and raise the temp higher, but I really don't think that's it...

[Jeron]

Based on your tests so far it seems everything on the engine side of the heater valve is good. If you are sure you've eliminated fresh air flow then the only option left is the heater core is not getting flow at speed. The biggest mystery is how can it be speed related and not RPM related. I'm sure you've tested holding a steady 3-4k RPM for a minute while parked.

The only option I see is the heater valve.

Have you tried partially disassembling the dash, having a driver drive at speed while you look for issues and feel the temperature of the core?

[pdxh20]

Thanks. I'm pretty sure I've positively eliminated the rpm vs vehicle speed question, but I'll double check it as you suggest. To verify the heater core temp at speed, if nothing else works I'm going to tape a temp sensor to the incoming heater hose at the firewall where it is easily accessible and hook it to a gauge in the car...As I looked at the hoses I noticed that the heater hoses at the rear of the head go to a T with a smaller jumper hose between the in & out hoses. I assume this is so when the temp control is in the cold position, closing the heater control valve, that some flow can bypass the heater core, correct ?

If the issue were related to SUSTAINED higher RPM as opposed to vehicle speed (I drive pretty conservatively to get better mileage so on the highway in 5th I'm still only around 3K), what would that make you look at ?

[pdxh20]

Progress, and an apology for not verifying this better before...I just ran it with my thermometer in place parked at 3500 RPM and the temp drops right off just like at high speed. The blower speed makes no difference or maybe even makes it hotter at high fan speed, so the air issue is OVER. This is positively a water temp at the heater core at higher RPM issue...If you know anything about that little jumper, I'll clamp off the little jumper hose and let you know if that's the culprit but I don't want to blow the heater core if it is there as a pressure relief ! My mechanic thinks it was a dealer installed setup that the factory recommended to prevent core blowouts...Do you know ?

[Jeron]

I read recently there was a recall that added overheat protection to the heater core. Maybe that is what you see.

For RPM related loss of heat I would start with air in the system. On E36, air in the system usually presents as no heat at low RPM and thermostat jumping to red at high RPM. The E30 it may show up like your problem so I would start by carefully re-bleeding the system. While bleeding air make sure to have the heater set hot and rev the engine. I've heard elevating the front of the car helps too.

[pdxh20]

The bleed with the front end elevated and heater hot was my very first step when all this began...Thanks for the info about the overheat jumper, that must be what it is. I'm now pretty sure it must be the heater valve rubber internals coming apart, that somehow block the flow at high RPM enough that the jumper functions as intended. If I block the jumper with a failed valve at high RPM I might just finally pinpoint the problem and blow the core at the same time !

[Jeron]

Your first post seemed very knowledgable or else I would have suggested bleeding the sytem first thing.

I dont know for sure what the jumper is but the heater core protection is only to protect it in case your engine overheats. I dont think you are goinfg to build too much pressure and blow the core if the engine is at normal operating temp.

I would take the valve off and inspect it and remove the internals if it is damaged.

Heres some good stuff:
http://www.automallusa.net/1991/bmw/3-series/recalls.html

[pdxh20]

Turns out it was the jumper hose, or rather the little valve it is was attached to. I pinched off the hose and the heat works like a champ. My mechanic knows of lots of them ruunning around with no valve and has never had a heater core blow...Thanks for your help and happy holidays !