New build - initial start up - ECU Coolant temp vs. Rear Iron Coolant temp question
 

Hi,

Finally fired up my new engine today. Idle is perfect and engine sounds perfect.

I was using Lisle funnel to burp the coolant system and temp reading from the ECU was raising slowly as expected. At around 185 to 190 degrees F bubbles started to show in the Lisle funnel which was also expected. Right when this started I noticed that the factory coolant temp gauge was 100% pegged and oil temps were steadily raising. I let the car continue to run for few more minutes but got spooked since the oil temps had gotten to 250 degrees F. ECU coolant temp remained at 185 to 190 the whole time.

My guess is since the factory water temp gauge gets it reading from top of the rear iron there is simply air in the system but wanted to get some professional opinions before trying to let burp out. If oil temp had not gotten so high I probably would have ignored the factory water temp gauge and burped as much air as possible.

Any insight would be much appreciated.

Your oil temp got to 250F just from idling? If that's true then I would definitely keep a close eye on it next time you start it up. You may have an air bubble restricting flow to the radiator.

Yes, just from idling. This was literally the first time this motor has been started.

In my 1.6L 4 banger track car, it takes me a good 20 minutes of 8500rpm redline shifting to get the oil to 250F. I know rotaries are different because oil goes inside the rotor and is exposed to combustion heat moreso than a piston engine, but still that seems really excessive for just idling. It makes me think that your engine was indeed quite hot. I think it is plausible that the sensor at the front of the engine could have skewed readings due to an air bubble blockage, while the sensor at the rear of the engine is reading what the actual temp in the keg is. I ran into this issue with my turbo MR2 that had long pipes running from the mid engine to the radiator at the front of the car. I didn't bleed the system properly and an air bubble cut off flow to the radiator, causing the needle to climb while the car was at idle. I'm not sure how you could burp the system any better than you already have, but that's my 2 cents.

At idle, you have nothing cooling your oil. 250 is not excessive.