1991 318is Cooling Issues

[BobbyZ]

I have searched quite a bit and found that cooling issues are fairly common but easy to fix for the M42 engine. I must be stupid because I can't seem to figure out what is going on.

My engine is having a serious overheating problem, it takes around 15min from stone cold for the water to begin boiling over. However I am not getting any heat out of the cabin heater.

My first idea was thermostat or water pump. I completely removed the thermostat and the water pump seems perfectly fine (it was replaced with a metal impeller version about 6 months ago). Still the car is overheating.

Next I figured I must have a clog somewhere that is causing the lack of circulation. I used one of those fancy Prestone fittings in the engine bay where the coolant is supposed to flow into the heater core. I hooked up the hose and water came shooting out of the radiator cap. I seemed to have good flow.

My next idea is that the heater core is clogged or the heater valve is broken but I don't think this would cause the car to overheat, doesn't the coolant just bypass the system when the heater is off?

[KenC]

Are you certain there isn't a huge air bubble? These cooling systems are tough to bleed properly.

[Mike325]

Is your thermostat working correctly? Could be stuck closed. The best thing to do is to start the car and periodically check the lower hose coming off of the radiator to see if it gets warm. If it does not get warm by the time the gauge hits the middle mark, then I would say that your thermostat is broken.

Also, like mentioned above, you could have an air bubble. How are you bleeding the system?

[BobbyZ]

Thermostat is now completely removed from system. (I also checked it in a pot of boiling water, worked just fine)

I am really hoping its just an air bubble but I keep coming across people who say its easy so I am wondering how I am screwing it up. I guess I will give it one more try.

Right now here is the procedure I am following.

1. Fill coolant reservoir
2. Remove bleeder screw
2. Turn on heater to full
3. Start car
4. Leave the radiator cap off and keep adding coolant as the level drops in the reservoir
5. Once coolant is coming out of the bleeder screw replace the screw and tighten
6. Replace the radiator cap

Then the car overheats again...

[Mike325]

So you confirmed that coolant is flowing from the bottom radiator hose? Just thinking if there is a block in the radiator. How much coolant do you add before you are full?

[BobbyZ]

OK, so I just left work early and double checked everything again... in the rain :-(

I jacked up the front of the car to make sure I get as much air out as possible. The front of the car was a good 12" off the ground. I opened the bleeder screw (with heater on full) and kept the engine above 1500RPM's continuously replacing the water that was pouring out of the bleeder hole. After a few minutes of clear water coming out of the hole I replaced the bleeder screw, topped off the reservoir, and put the cap back on.

After no more than 30 minutes the needle was above the half-way mark. And still there was no heat in the cabin.

The top and bottom hoses at the radiator both seemed to be hot, the bottom one was cooler than the top one though.

One of the hoses going to the heater core was totally cool (the one with the valve on it), the other was very hot.

Also, the bleeder screw seems to be leaking pretty bad. Is it supposed to be relieving pressure in the cooling system? Or does the reservoir cap do that?

[monty23psk]

I can help as I have become a somewhat expert on the cooling system on the m42. First, your heater valve might be dead. It is on mine and has been since I have owned it. Not necessary for bleeding, but necessary for heat in the car.

To bleed:

1. park car on incline then make sure it is dead cold. Put cap back on and bleeder screw tight. If you are leaking from screw, then you need to replace it as nothing is suppose to come out. Only if you have to much pressure, would the radiator cap blow.
2. fill coolant tank just to a little over fill mark.
3. turn car on, if you know your heater valve is working, turn it to full heat.
4. let the car idle and warm up.
5. Once the car is warm, the top hose should get hot. Only when the thermostat opens, would the hose on the passenger side get hot. Open the bleed valve. You should see some coolant. If you get steam, you are low on coolant and just go to step 6. If you see coolant coming out, give it a few revs and keep it at 2500 until you see a steady stream. Just look at tank. If drained, go to step 6.
6. look at the needle and look at the coolant tank. If needle goes to 3/4 or you dont see any coolant, turn the car off and let it get dead cold.
7. repeat steps 2-6. The long time comes to the cooling the engine off. I noticed as long as you dont get the heat too high, in the red, it should cool quicker.

[BobbyZ]

If my heater valve is dead it still shouldn't overheat right? That alone can't be the problem.

[monty23psk]

Yeah, the heater valve has nothing to do with overheating. Mine does not work and I do not overheat. Correct, your issue is trapped air, coolant blockage or leakage or not enough coolant. Also in my case I dont have fan clutch but unless you are doing this in +90 degree whether, it wont matter.

[BobbyZ]

So should I be repeatedly bleeding it?

Everything I have read says that you just have to bleed it once.

[monty23psk]

Not sure what you have been reading, but everything I read on the m42 plus my own experience is at least twice if you are lucky. I just did a full coolant flush this past weekend and took me 6 tries. If you wait a full day for cool off, then maybe 2 at times. Once you have stream out of the bleeder screw, then you know you are good.

[BobbyZ]

I have a continuous stream out of the bleeder screw after a single bleeding. I left it overnight to cool and the coolant level has not dropped at all. I don't think there are any more air bubbles in there.

[monty23psk]

Great. Just make sure you have nothing coming out of the screw when it is tight. Also make sure that level is not over the fill mark when completely cold.

How is the temp guage now?

[BobbyZ]

That is one issue that I appear to be having. I am getting a small leak out of the screw.

Where is the pressure relief for the coolant system. Is it the radiator cap?

[BobbyZ]

If I had a clog in the radiator, could that keep water from getting to the heater core? I am having a hard time believing that these are all unrelated problems.