Running Hot (coolant temperature)

Advice Needed Please! I have a 2001 996 Carrera Cabriolet with 133,470 miles and a coolant temperature issue (see photo of gauge at 75F ambient temperature after sitting idle for a few minutes) This is what I have replaced related to cooling system within the past 5,000 miles: [*]Water Pump[*]Thermostat & Housing (stock thermostat)[*]Coolant Expansion Tank and cap[*]Coolant Level Sensor[*]Coolant (Pentosin)[*]Left Side Ballast Resistor[*]Cleaned both Radiators Both front radiator fans come on at low and high speeds and both ballast resistors are working fine. What could possibly be the cause of this problem? Do I need to replace the coolant temperature sensor? Or is there another possible problem? Also, I noticed that my deck lid fan doesn't ever come on even though I tested it and got it to come on by touching the engine compartment sensor to a hot engine component. The engine compartment sensor is properly seated in the passenger side manifold as it's supposed to, but even at high temperatures the fan will not come on. Any ideas? Thanks in advance for your help!

My guess is you have a trapped air in the collar system. Did you purged it properly?

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+996 on an un purged system. Open the bleed valve and follow the 'burping' procedure or drop the coolant and refill it with a vacuum refiller.

I see you posted over on Renn-Tech as well. Something to consider - not all thermostats are the same. In other words, they fit within a range of values. So for instance a manufacturing run is done for a 186 degree stock thermostat, and the allowable variance is 10% (made up). That means they will operate from 168 to 204. Maybe you got one on the high side. Also check out low temperature thermostats from LN https://lnengineering.com/products/o...led-stuff.html

Suggest you replace the coolant temperature sensor with an OEM unit. Also, get yourself a ScanGauge or similar device that plugs into the OBDII port with the capability to read the coolant temp from the engine management system to rule out any possible temperature gauge malfunction.

My car runs that hot all the time. It's probably around 102-104C. My guess is the sensor, although, it could just run that hot by design.

I didn't want to bother with overheating ever again. When I did the job, I changed to a low temp T-stat, and added the center rad option. If your car is a 6sp manual you can add the center rad pretty easy. With the low temp thermo, it made a world of difference. Best investment ever. Now, you need to know several things. First, is your gauge reading accurate? Use a IR temp gauge to test your water coming in to the rad. If the temp gauge checks out, I would switch to the low temp thermostat, and see what that does. If it still isn't right, you may have some internal corrosion in your rads that is preventing them from exchanging heat fully. Which means new rads because I don't think you can remove the end tank for rodding them out. Getting all the air out of the system is a bit of a challenge as noted. It took me a few days driving, with a few hi rev passes to get all the air out.

These "overheating" or "running hot" threads have been a recurring theme in the 996 section for years. The car is only overheating or running too hot if the car is operating at a higher temp than it was designed for and the excessive heat is causing, or in danger of causing, damage to the engine. Does anyone actually know the max operating temp the engine (in this case the coolant temp) is designed for under normal conditions, like heavy traffic on a brutally hot summer day. Or, to put it another way, at what temp is hot too hot?

I read an advice of a well known local indy here that one way to properly fill your coolant system without proper tools is to run your engine at 3000rpm, and have somebody fill the coolant system (expansion tank) to the brim, then screw back the cap, and only then drop the rpm to idle again. Do you have the air bleed open (little metal thing on your filler cap up)? To let the air out? When I had my waterpump and TC replaced I ran the car for a week with the air bleed open. Every day I had to fill up the tank. In the end I poured almost 2 liters of coolant into the system although it was filled properly. I did not see traces of coolant escaping. You mention your fans run properly. I normally always keep the aircon on so the fans run all the time at low setting. This really helps in keeping temperature stable. Is yours on normally? If not, what you see is quite normal. The low speed fans otherwise only come on at 104C which is ball park where your gauge is sitting. Check the sticky thread on things to know on your 996; you will find how you can more accurately read out your coolant temps via the center display. Your car (2001) should be capable of this.

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From what I understand, it's somewhere around 235-240F with 250F being the absolute max pressure that the cooling system can handle before the cap lets out. These are water temps. Oil temps can be anywhere from 10-30F higher, so when regular street oil exceeds 250F, it's not good. That said, heat is the enemy of reliability and longevity, so that's why folks aren't comfortable with these temps.

Thanks guys, all your advice has been helpful. I think my problem was improper purging, as it now seems to be at a happy temperature. I watch my temperature gauge more than my speedometer and now I'm happy that I won't have to worry about overheating as much. Thanks again!

Thanks guys, all your advice has been helpful. I think my problem was improper purging, as it now seems to be at a happy temperature. I watch my temperature gauge more than my speedometer and now I'm happy that I won't have to worry about overheating as much. Thanks again!

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lols wut? 250 degrees pressure...

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Lol, sorry. Water boils at 212F or 100C at 1 ATM (14.7psi). But the cooling system, combined with Coolant chemical, can build pressure to raise the boiling point. There are many factors, but if you take a look at this chart, you can get Coolant temps beyond 240-250F if you apply between 8-12psi of pressure: https://durathermfluids.com/pdf/techpapers/pressure-boiling-point.pdf From another poster (Loren): "The pressure relief valve opens at an excess pressure of approx. 20.3 PSI; the pressure then slowly decreases again, and the valve closes at 14.5 PSI."

Look - I don't care what the mfg says is 'ok' for max running heat. Heat kills engines. All engines, all the time. the only reason we run our cars so hot these days is combustion efficiency for smog. There is no reason other than complete combustion to run an engine at 194F, or whatever Porsche specs. Sure it'll run just fine with that temp. You see anecdotal stuff all the time like this; 'well - I've never touched my cooling system and I have 100k miles on it, doing just fine.' So I guess that makes it all right, because some people succeed at the factory temp. I've run enough air cooled aircraft engines at 420F head temps to know that cooler = longer life. So - you gurus who just run it right up to near red line, go ahead. For me, I'll stick with the lower temps and the slightly higher CO reading(but - still in certified range), and longer engine life. YMMV, objects in mirror, pro driver - closed course, contents have settled and may cause **** leakage.

I wonder what the failure rate is for m96 engines where 1) coolant temps are kept below 230F & 2) have a well-serviced IMS system. From what I've read on here the last year and a half, those two conditions cover all but a relatively small number of cases involving chain or c-rod bolt failures.

Agree^ I have mentioned that biggest factor is the engagement od the fans. I there were a mod the engaged the fans at an earlier time, heat would be controlled.