I have a 2012 R56 N18 with 68K miles. On a drive I first received a temp warning that would show for 5-10 seconds at a time then disappear. I had the water pump and thermostat replaced ~ 5000 miles ago last June at the dealership. I also had a P0136 code that showed up on this same drive so I ordered a NTK O2 sensor and plan on replacing this. Could these 2 things be related? What would the first thing you would check as far as the cooling system goes? I don't see any coolant leaking. I plan on testing the cat to see if this could be causing extra heat in the engine bay and perhaps be the cause of the P0136. Looking forward to any input.

 

No, the two issues you are having are not related.

To start with the cooling system. 1. Check your fluid level in the recovery tank and make sure it's at the min level. 2. Connect a scan tool and pull up engine coolant temp (ECT) in the live data menu under the DME menu and go for a drive. Coolant temp should be 221 when engine reaches full operating temperature. Coolant temp will drop to about 185 under wide open throttle. If you don't have a scan tool, got to Amazon, and look for a Foxwell NT530 for BMW.

The P0136 code is for the downstream O2 sensor; Chances are good that the sensor has failed, but you may have another issue. Remember the downstream O2 sensor is a narrow-band sensor (4 wire). so you will be looking at the voltage signal. If the catalytic converter is working properly, you should see a steady voltage around 750 mV. Again, you can connect a scan tool and go for a drive. The voltage should spike to about 800 mV under wide open throttle, and drop to about 200 mV when during deceleration. Yes, even the downstream O2 sensor will switch ... it just doesn't switch as often as the upstream sensor. If you notice your sensor output voltage doesn't change during WOT or during deceleration, check power and grounds to the sensor; if those are good, replace the sensor.

 

I appreciate the response and will need to see if my generic ODBII scanner can read temps and if not purchase the Foxwell. After doing some exploring, I found that the serpentine belt is is terrible condition and the water pump pulley outside rubber is cracking. I can't believe the Mini dealer would put this back together with these low cost items in terrible shape. I am going to try this first to see if this resolves the issue due to possible belt/pulley slippage.

As far as the O2 sensor, I will see if I can read voltages as I know it can do live stream data, I just have never used it for that. I also read that you can compare temps on the front of the cat to the back of the cat. Temps shouldn't vary much more that 50 degrees or so. I will need to check this once I replace the belt and water pump pulley.

Also found a leaking strut so will be replacing these next week.

 

Comparing catalytic converter inlet and outlet temps is only needed if you have a P0420 DTC. On a normally functioning cat, the outlet will be hotter.

 

Thank you. I will read the voltages as previously suggested once I have this back together. I found that the water pump pulley could be spun pretty easily by hand so hopefully once I replace this along with the belt this will resolve the overheating issue. Along with a deteriorated lower motor mount, I found a blown strut so am replacing these as well. Can of worms..... I look forward to getting this back on the street this weekend.

 

I changed the belt and water pump pulley but I can still turn the water pump pully pretty freely as it is only making slight contact with the friction wheel. The tension release tab is pushed all the way in. No change on the overheating issue. Would my next step be to replace the friction wheel assembly? Have these been known to fail by not providing enough tension to turn the water pump? I'm in the states so my understanding is that the water pump should always be turning. It isn't making any rattling or other noises I've read about.

Is there a spring inside of the metal portion that my be weak or broken?

There appears to be good tension on the crank side of the pulley, it is just not making good contact with the water pump pulley.

 

Is the water pump pulley spinning? If it is, I wonder if there is damage to the water pump impeller or if there is some foreign object inside the system blocking the flow of coolant. You might also have a defective thermostat that's not letting water flow to the radiator, or there is a clog or restriction in the radiator.

Have the cooling system pressure checked to make sure it will hold pressure. Pressure increases the boiling point of the coolant.

Did you have the overheating problem before you had the thermostat and water pump replaced?

 

The water pump pulley is spinning. From what I have read, the thermostat should give a code and CEL. I plan on getting the radiator flushed and as you suggested, a pressure test. It's narrowed down to a coolant circulation issue, just need to find the culprit now.

 

Wanted to provide an update in case someone else runs into a similar overheat issue. I replaced the thermostat, the expansion tank cap, removed and inspected the water pump, removed the radiator and flushed, water pump pulley, serpentine belt, pressure tested the cooling system and vacuum filled the coolant. The culprit? The expansion tank. When I pressure tested the system, it held pressure fine but the tester had a conical shape that went inside of the opening instead of having a cap. The pressure was either escaping through the threads or the opening of the expansion tank became too large to seal. I could tell once I put the cap on the new tank that there was resistance that I hadn't felt before.