2EF4 code with Service Engine Soon
 

I got the SES light and it was time to service the cooling system (194,000 miles) so I changed out the water pump and the thermostat. The SES light came back in a few days with the code 2EF4 (mechanical thermostat fault).

I actually do not know what else can be bad? I will change out the coolant temp sensor today but is there anything else to check?

That sounds like the engine harness issues the 8cyls had years ago. Hopefully it doesn't come down to that or you'll be trying to find a cheap harness.

Is the engine temperature and coolant temperature where is should be (with the new pump and thermo)?

Is the new thermo operating as it should?

Its always possible the new thermostat is malfunctioning.

A while back I called the product manager for Pierburg that makes my water pump. He told me the reason those pumps and thermo's go bad is owners use the wrong coolant or don't mix it properly.

No I am not going to change the harness, I would just get rid of the car. I should check the wiring harness signals.

I did a bit of troubleshooting today. Hooked up my Launch scanner and started the engine at idle. For more than 10 minutes, the Coolant Temperature at Engine and Coolant Temperature at Radiator Outlet climbed slowly to 52C and 47C respectively. That means (to me) thermostat is stuck open and does not allow engine temp to raise to operating temperature. It should get to 75C or 80C in 10 minutes (??)

I will crawl under the check for 12V at the Thermostat connection, and if not there, try to find a fuse for that Tstat line. Where is that fuse?
If 12V is there, then I have to assume that the new Thermostat fails right out of the box.

Although the SES light was there BEFORE the thermostat was swapped, so chances are the problem is with the car, not the part.

It can take a while to get the coolant up to spec, especially if it's a cool(ish) day. I'd try cranking the AC to max to help it along.

But yeah, if the thermostat is "stuck open" it's going to take forever or longer to come up to full temperature. I suppose that's a possibility with the N52 thermostat (assuming it's controlled by the DME rather than having a simple (and reliable...) metallic spring to turn the coolant flow on and off. But to make up for the unnecessary complexity, BMW helpfully mounted it where it's really, really difficult to replace. ;-)

And the other relevant question is - what brand is the thermostat? There are a lot of no-name options that I wouldn't trust, especially for a part this integral to the longevity of the engine, and due to the PITA factor for replacing it. Also, there are a host of counterfeit parts floating around, so if you got it from anything other than a well-trusted source, you could have a no-name thermostat (that came in a BMW box).

Inspect the electrical connections related to the cooling system components, including the thermostat, coolant temperature sensor, and any related wiring harnesses. Ensure that the connections are clean, secure, and free from corrosion or damage. Faulty electrical connections can sometimes cause false error codes or incorrect sensor readings.

I will need to wait til tomorrow to measure the voltages. I am too busy today putting an engine back in a Ferrari F355 F1.

The T-stat is a Mahle-Behr version, and bought from Pelican so all are good names. Plus, the SES light went on BEFORE I swapped out the Thermostat so I am thinking it's with the car, not the Thermostat. That thermostat is in a really inconvenient spot to work on. What stupid designer did this ?

OK,

1. Ignition ON, engine OFF, Thermostat connector disconnected, there is 12V on one pin, and 4V on the other pin (this is likely the pin from the ECU that is an open collector circuit).
2. Ignition OFF, engine OFF, thermostat connector disconnected, the 4V signal goes to zero since there is no more electrical current any where
3. Engine on and running, thermostat connector Plugged back in, the pin that was 4V before now bounces around from 12V to zero and then steadies out at >13V (battery voltage when car is running)

All I can conclude is the main ECU is controlling the thermostat by turning it on and off at start up. The thermostat seems to be dead (out of the box).

Since it is only $100 ish, I will replace it and see where we are.

I found this video on line explaining how it works.
he showed the thermostat has failed in the Closed position. I had thought that it was designed to fail in the Open position so that the engine won't overheat. Comments anyone?