Questions and Answers

BMW 318i Runs Hot And Overheats Q. Hi, I have a 1984 BMW 318i, 1.8, five speed manual, two door. I have been restoring it for the last year and a half. I have done most of the work myself, except the motor. I had Reid Van, a very reputable shop in St. Louis, check it out. I live in St. Louis. They said it had a burned valve and bad valve guide seals. I had them fix it and am happy with their work.

Here is the problem. The car, after I had the motor fixed, ran fine through the months of March, April, May, but in June when I turned on the A/C, it over heated. Before then, the water temperature would always stay just below the 90° mark on the gauge. The temperature gauge gave me problems in the past. I had the SI board rewired and the batteries replaced. I bought a new thermostat, but the car overheated again, but this time immediately.

I took off the water pump and it was okay. The radiator and all the hoses have been replaced in the last nine months. The radiator is new out of the box last October. I made a thermostat (flow through) out of pipe fittings and the car did not over heat. I think that the thermostat I was sent was defective.

I have replaced both of the temperature switches at the bottom of the radiator that turn on the auxiliary fan and all of the relays. The auxiliary fan only comes on when the A/C is on and does not go on high. I hot wired the fan so it came on high when you start the motor and stayed on.

Then, I turned on the A/C and during the day it ran cool. The temperature gauge does not show the actual temperature but it ran a bit below 90° or the middle mark, but in the evening around 6 pm, with the A/C on, the car ran past the middle mark and half way to the fourth mark. I do not want to over heat the motor.

I have sent off for a new thermostat but can not figure out why the car has a wide range of temperature readings and why the auxiliary fan does not come on until the A/C is on and why it does not go on high.

A. First off, cooling systems on BMWs can be difficult to get all of the air out of, there is a bleed valve located near the thermostat housing which should be opened to bleed air out of when the vehicle is near operating temperature anytime after the cooling system is opened.

It usually takes me two or three cycles of doing this as a minimum to get all of the air out of the system. Air pockets near the various temperature sensors render them virtually inoperative as they must be immersed in coolant in order to function properly.

Additionally since your engine came from a junkyard I would suspect the possibility of blockages in your cooling system from bugs and small animals nesting in the cooling jacket. There very well may have been nesting material in there which wasn't initially a problem but broke loose after operation of the engine and now is causing blockage.

I recommend a thorough back flush using a commercially available flush additive such as Prestone Superflush or other and drain and refill the cooling system then follow the cyclic bleeding process described above.