Engine overheating

In another thread I started recently to identify an unknown reservoir in the engine compartment, among other things it was mentioned that water pockets could have been the culprit requiring me to add another 2 quarts to get the coolant up to the proper level. Yesterday my engine temperature suddenly began to quickly rise right after I had turned on the heater to eliminate a possible source of water pockets (mentioned in another thread I had started), so it happened I was watching the temperature gauge closely at the time. I was in town, only doing 35 mph. Fortunately I was close to home and immediately turned around and kept a close on the gauge. I arrived in the garage without the temperature rising much over 200. I looked for leaks (none), checked the coolant level (good) and pressure when removing the fill cap. The engine didn't have any unusual noises and sounded normal. The radiator had been recently serviced, topped up yesterday and the serpentine belt had been replaced and a sundry list of other items not related to this issue weeks earlier I had just left the house, engine was cold, and it usually hits operating temp after about a half mile after starting up. I see two possible issues: 1) Radiator low speed fan(s) resisters had failed. The previous night it happened to be raining hard with the temps in the mid-60's. It was possible one of the resistors was hot and the internals broke after being struck by the water. So replacing the failed transistor will resolve the issue. Best to replace all of them at once than piecemeal, after all they are the original parts and resistors do deteriorate over time. 2) Thermostat isn't opening properly. Here replacing the thermostat and water pump as an added precaution (since it has to come out to replace the thermostat) since I don't know how old it is and when it was last replaced. Can anyone else think of any more possible areas that might pose a problem? I can test the resistors, I can't test the thermostat (that I'm aware of), and I don't want to go through that expense only to discover there is another issue that is the root cause. Thank you all in advance. I'm a virgin in the Porsche scene and I'm not yet familiar enough with all the foibles to watch for. My mechanic apprentice days were years ago working on MG's and Triumphs.

When in normal operating conditions in traffic the needle should be in the middle of the gauge, around 200. Most of the times above 180. If air is coming in because you are driving on a cool day, you will see the needle lean towards the left, to 180. You need to update the water pump and the thermostat. A lot of people suggest a low Temp thermostat. Secondly you should remove your front bumper, remove the air conditioning condensers and clear your radiators. You should do this twice per year. After autumn and before summer. Because of tree leaves, etc. If you switched on the heating today, probably there was some air in there as well. Drive the car near your house with the maximum temp in your cabin make sure that it'll never goes towards the right because this is abnormal. You should hear the radiator fans starting, this means everything works fine. Check the photo below to see what is normal. It is a normal procedure for a couple of days to have that metal switch behind the water bottle on the upper position. I am talking about this metal clip on the photo below. Move it up so that it stands because this lets the air get out. It is supposed to be at the highest point. I usually lift the rear of the car in order for the air to move from the radiators towards the back. Clear your radiators. And verify that you don't see any milky stuff in your oil dipstick or cap. If you see water missing after a couple of days, concentrate on the smell. If you are familiar with burning water coolant smell, you might identify that the water is being burned somewhere and this is why you don't see it under the car.

Check this video for your radiators: You Tube Also buy a new cap for your water expansion tank. This part only, this specific code identical to the last digit and only from Porsche : 99610644704

Turning the heat on and off has no impact on the cooling system. The lines to the heater core are not valved - coolant is running through the core regardless of what you do with the switches in the HVAC head unit. I would pull the serpentine belt to make sure the water pump is turning smoothly and replace the thermostat (you do not need to remove the water pump to replace it). Would also say that I don't think dirty radiators would cause an overheat - I cleaned mine for the first time in 20 years yesterday and it didn't really change anything (they were disgusting).

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Unless 996 works differently and I am not aware of, I though that increasing the temperature allows hot water to run in the heater core... I just gave some generic advice, sorry if I am wrong

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As long as the engine is running, coolant circulates through the heater core and it gets hot (or gets, at least, to the temperature of the coolant) - there is no valve on the lines. The HVAC controls manipulate the blend doors that direct air through the heater core to get hot air or around it to get cool air. If your car is overheating, blasting the heat in a 996 will still contribute to cooling because the heater core effectively becomes a third (albeit very inefficient) radiator.

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Yes... In remember fixing the blend doors and solving the problem warming the cabin...

First off the engine is not over heating. The fans dont even turn in until the dme sees 204f. U can test low and high speed operation with a decent scanner. If u turn on the ac the fans should kick on as a quick test. The dash guage is at best a refference and not very accurate. Get a scanner and read what the dme sees. We know u probably have air in the coolant system....it still may be working its way out and an air pocket in the wrong spot can cause the gauge sensor to read steam temp if there is no flow.

If the temp just went over 200 it may be normal. If you have AC on, the fans should be running low speed so it is easy to test them by turning on the AC and checking the cooling fans are on low. If the fans do not operate with the AC on, then you may have a bad resistor. I think you can jumper the relay for the fans and make them run all the time if you want to do that, as well. There are threads and videos. I am actually going to do this mod so I can turn them on at will - I am just waiting for the center console switches to ship to me. I can let you know how easy it is afterward. You have some good hills in Greenville You can find a good steep incline and get the back of the car uphill from the front, and it will help get air bubbles to the reservoir if there are any. When I flushed my system I carried a gallon of pre mixed coolant in the frunk in case I had bubbles, so I could top it off if needed. It has not been necessary, but I was paranoid. I am in Beaufort - happy to get on a call with you and discuss and try to help you. Message me if you want me to get on a call with you. I wish I saw this last Thursday, as I was traveling by on my way back from Tail of the Dragon in Deal. I could have stopped by. Good luck. Keep us posted.

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Actually, the heater has it's own drains to empty and is separate from the cooling system. The heater has to be turned on to for the water to flow through to generate heat. This is only done after the fluid has been changed. This is documented in Wayne Dempseys 101 Projects for the Porsche 911 (1999 to 2005). Section 4, Project 30 Coolant/Flush Replacement.

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It turns out that my temps were the same as an earlier post. I'd never seen it get that high before and it concerned me. Today I took it out for another run in town and then hit the Interstate for a 10 mile high speed run with the A/C on. Then I returned to the house and the temp had reached the same area as the posted picture. I walked out front and could hear both side fans (I don't know if the center radiator for the Triptronic has a fan though.). So my newbie panic has subsided.

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This is the diagram of the HVAC system. Which part is the valve to direct coolant into the heater core?

No valves going to heater core. It gets hot coolant all the time. The drain on the heater box is a condensate drain for the air conditioner evaporator.

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Good to know that all is ok. Trust the common knowledge: update your thermostat and your water pump with plastic blade; clean your radiators and update the blue cap with the right part number. It is all you can do about cooling.

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How would I know? I only have Wayne Dempsey's book to go by. It's not to hand at the moment, but it appears I misread that part. I admit to skimming the Project because changing my coolant isn't due for 2 years. The only thing I am sure I am remember is advice to use an aftermarket surficant to the coolant to minimize foaming and damage to aluminum parts, he mentions that the 50/50 mix doesn't adequately provide quite enough protection. Mind you, this is his opinion and with his background and experience, who am I to question it. My only experience with this is with MG and Triump's working 2 summers in a foreign car shop. That was 44 years ago. Can't claim I can remember all the details from that period if it were for my father having one of each that I did DIY work on. According to your diagram it appear that I was completely wrong in my insertion. I'll keep that in mind for the future if I ever comment on the subject again. I can learn from my mistakes

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So I am finding out and eating crow.

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No worries, here�s some pics from my recent heater core swap. You can see the two hoses (inlet and outlet) that run into the heater core, then a picture of the (bad) heater core itself. Just a simple loop with no valving.

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Is it a good idea to have a bypass valve during summer? Manual you switch it on and off, twice per year...

...No, because the coolant will bypass the branch to and from the heater core anyway when its valve is shut.

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I feel like you guys are just screwing with me at this point

NO coolant control valve in the hvac heater core, 100% not there. Confusion about this is from many many different good quality vendors listings "Heater Control Valves" for 996. Guess that Wayne Dempsey's book, and being co-founder of Pelican Parts, having a listing on Pelican for a "Heater Control Valves" for 996,,, doesn''t help the confusion on mislabeled valve. Maybe its the language translation from German....? The "Heater Control Valves" for 996, listed by many vendors is not for the HVAC system that has no valve and controls heat with flap blend doors. Full flow goes thru the heater core all the time. Maybe a good thing as it acts as another small radiator to help dissipate heat/btu. Not sure if the 996 does the same as other modern cars and once in full a/c mode introduces fresh air only into the heater core chamber to cool it. Anyhow, the "Heater Control Valves" for 996, listed by many vendors is mounted on the back of the Tiptronic transmission. It controls the coolant flow to the transmission oil cooler only. Should be listed as 'Transmission oil cooler coolant control valve' not "heater control valve". But thats how it is, and probably why all the confusion. It does look exactly like a heater control valve, even with a vacuum line connection. jmho

The tip coolant valve is actually a 928 heater core coolant valve. Thus the confusion.

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Not at all. Irony is often lost in written words, so I don't bother. With the info coolant always flows through the heater core, a bypass valve wont do anything wrt cooling the engine but my previous point still stands. Having a heater core valve is generally worthwhile as it prevents (radiant) heat soaking into the dashboard etc. Eventually that heat lowers effectiveness of the A/C a little.

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Actually the initial confusion arose from a comment I made to another poster. It turns out I had misread Dempsey's book. I had skimmed over the project it mentioned because I had the service done recently. I do appreciate the additional education!