Resistor inline with coolant temperature sensor on M20B27

[tomstin]

I have a resistor soldered in line with my coolant temperature sensor. I have found some speculation that this is a trick for more fuel or an official BMW modification. I can't seem to find any official documentation, only speculation. Does anyone have anything "official" on this modification?

[McGyver]

That would make sense, adding a resistor would tell the ECU that the engine is cold, which would trigger the Warm Up enrichment.

Per the Bentley:

[Gregs///M]

I believe on the M20 B27, there are at least three or four coolant temp sensors. One for DME, one for gauge, two for cold start system. You will need to figure out which one its for, I doubt its factory.

[Andre3127]

I believe on the M20 B27, there are at least three or four coolant temp sensors. One for DME, one for gauge, two for cold start system. You will need to figure out which one its for, I doubt its factory.

M20B27 had a sold start switch in addition to the gauge and ECU sensor.

Yes, technically, the ECU would change fuel and timing based on coolant temp but this has issues and is not the way to tune your car. But, depending on the stock lambda targets, it may trim fuel back to stoich anyways.

To add fuel and timing, it would be significantly better to simply change the tune in stock motronic. This can be done by a number of qualified individuals or yourself for around $60 if you accept the risk.

[tomstin]

Maybe this will help the next guy on a 85 M20B27. Below is my understanding of the various temperature sensors (I’m sure I’ll get corrected if I’m mistaken):

The sensor at the very front of the engine, has a large brass base, is the Thermo Time Switch that controls the Cold Start Valve.

The next one back, two prongs with slip on connectors, is the Coolant Temp Switch for Idle Speed Control. Closed above 113F.

The next one back is the Coolant Temperature Sensor for the Temperature Gauge which has a single spade connector.

Finally, the one closest to the head is the Coolant Temperature Sensor for the ECU. This sensor, on my car, had the resistor inline on one of the wires. I have to run this for a while to draw a conclusion, but the initial test drive seemed a bit smoother without the resistor.
.
One other tip, when I was struggling with an almost impossible cold start, my Thermo Time switch was bad. Also, the connector fell apart in my hand. Those connectors can be replaced with Bosh Fuel Injector connectors. A little soldering skills and some shrink wrap will work well. These Bosh connectors worked on the Coolant Temperature Sensor.