Cooling Issue - Street & Track

[sapote]

I would start to question if the coolant temp sensor is good.

[bimmer]

I would start to question if the coolant temp sensor is good.

I’ve replaced both the aux fan switch and the coolant temp sensor under the intake manifold as I thought they were the culprits but didn’t make any difference unfortunately.

[sapote]

Double check if the O-ring seal in the thermostat housing still there?

[Samsonite79]

Have you checked to see if the coolant is flowing? if your not scared to put your hands on the lower hose and upper hose while the engine is running, compare the two see if one is overly hot or stone cold. if you have a pyrometer, use that instead of your hand. Not questioning your work but make sure there isn't a plug or cap that got left in there or maybe obstruction. Also if you haven't try a air lift or vacuum bleeder system to evacuate all the air and voids out to fill the coolant system instead of just pouring in the coolant/water. Good luck!

[bimmer]

I actually went as far as replacing both the water pump and thermostat again entirely (thanks FCP Euro) just to make sure I wasn't victim of faulty parts (that's the sort of luck I usually have) and replaced the tstat o-ring, water pipe o-rings etc. during that process.

I believe the coolant is definitely flowing... the heat comes out piping hot, lower rad hose was burning hot, and I know it's flowing during the bleed procedure. I don't have a pyrometer or vacuum bleeder but I'll pick those up later today as I've been meaning to get them anyways. Always up for new tools that cost less than shop labor!

Will report back with findings...

[bimmer]

Drained the coolant and vacuum filled with no positive results. Still the same issue. The system did hold vacuum at -22 so I know there isn’t a leak somewhere.

Pyrometer temps varied at different places on the rad hoses but lower rad hose ranged from 145-161F and upper rad hose from 170-181F. This was at normal operating temp.

[sapote]

Did you double check to see if the Tstat housing still have the rubber o-ring or seal that keeps the hot coolant from bypassing and flow straight back to the head? With no seal then part of the hot coolant not flow to the rad, but gets back into the head again and this would cause the overheat.

[bimmer]

Did you double check to see if the Tstat housing still have the rubber o-ring or seal that keeps the hot coolant from bypassing and flow straight back to the head? With no seal then part of the hot coolant not flow to the rad, but gets back into the head again and this would cause the overheat.

Yep I am positive that it was there, as I replaced it again when trying the new T-stat since it comes with that o-ring. Part # 6 on this diagram: https://www.realoem.com/bmw/enUS/sho...10#11537831103

[Onefastsicilian]

Go back to an OE rad…. Problem solved

[sapote]

OP,
Using a wrong tank cap or leaking old cap prevents a proper pressurized system and caused inefficient thermal transfer. So check for a 2 bar cap.

[bimmer]

The cap is 1.5 years old and is the OEM BMW one that came with the new expansion tank: https://www.fcpeuro.com/products/bmw...m3-17112283344. It does not appear to be leaking.

[bimmer]

Go back to an OE rad…. Problem solved

The issue started while I had a 1.5 year old OE rad...

[sapote]

Either your special engine generates more heat than normal, or the fan somehow is not fast enough.
* If the oil temp is not proportionally hot as the coolant -- I don't see report about oil temp -- then I don't think engine generates too much heat is the issue. Do you remember the oil temp when coolant was at 242F?
* measure the electric fan controller output -- it's a PWM signal going from 14v to 0v switching, and you might need to use an oscilloscope to see. You reported measured voltage at 14v with engine running, but we need to confirm the voltage input to the fan. At very hot coolant, above 97C, the fan should be running at 90% PWM duty cycle (90% at 14v and 10% at zero volt). A bad connection somewhere could cause lower voltage at the fan.

[bimmer]

I don't have exact read outs on the oil temp but at that time it was a tad below the middle mark between 210F and 300F so my guess would be 250ish.

I'm going to have to educate myself on how to measure a PWM signal and get my hands on an oscilloscope, but I will work on getting that measurement and report back.

Also, very much appreciate all of the help!

[bimmer]

Welp, as much as I can't believe it, a brand new BMW OEM radiator fixed the issue. I am pretty shocked that the previous one crapped out after only 1.5 years, but I guess these things happen. Thank god for FCP Euro. Lesson learned on assuming new-ish parts shouldn't be the issue.