Strange cooling behaviour

Hey all, I wanted to check with you something regarding the temperature management of my Porsche 996 C2 Manual. Before I had the engine rebuilt, I had some 'erratic' behaviour of the coolant temperature. Therefore I decided to have the water pump and thermostat replaced by GT3 versions when they did the engine rebuild. Unfortunately the car still behaves the same as it comes to the cooling. No change at all with the new GT3 water pump and GT3 low temp thermostat. The issue is as following: - In colder weather (around 65F) it takes incredibly long to get up to temperature. While driving on the highway it can even happen that the temp never goes above 180F. - On warmer days (around 83F) within a few miles the temp gauge is nice in the middle just over 180F Now the bigger issue which I was hoping would go away with new cooling components: - In traffic or 'mountain' roads on warm days, the temp really goes up all the way to the '3rd' line. See attached example image. Sometimes it goes even a bit further, but not over the line. - On the highway the temp goes back to normal again Just so you know: the cooling fans are both working on both settings (high and low). Now normally I would suspect the radiators to be old and dirty. So I cleaned them from leafs and dirt etc. Also in case the radiator is bad, the temp would also go up on the highway or on colder days, correct? I've searched this forum and I read that it can be normal for 3.4s, but is this also normal with a low temp thermostat and a new upgraded water pump? Or should I indeed suspect and replace the radiators? I was really hoping the 3.4 would run a bit cooler with GT3 cooling parts on it . It just doesn't feel good to drive around like this. Since the temp goes slow up on highway and cold weather I'm almost thinking the brand new OEM thermostat is bad, since to me this behaviour is something for a bad thermostat.

If you have access to a Durametric or another diagnostic computer, I'd check the temps via that. I think the gauge is fairly accurate but not exact. Could potentially by a gauge issue but I doubt it. It's true about 3.4 .1 cars showing a bit higher temps. Maybe other 3.4 owners can chime in with what's the highest temp they seen in similar conditions. What has me a bit puzzled are the GT3 t-state and water pump. GT3 had a very different engine and I didn't think those parts were interchangeable. M96 and Metzger are not the same. The first thing I'd check is the T-stat.. There is a way to test it that it's opening and closing as designed.

The water pump is one of the 2 parts that are compatible between the Mezger engines and the M96. The other is the exhaust lifters. The Mezger water pump is actually an overdriven water pump compared to the M96 pump, it spins faster for a given engine RPM. The OP's issue could be either a defective thermostat (yes some new ones are defective right out of the box) or clogs in the piping or radiators or lots of air in the system or a bad coolant temp sensor.

Or when is the last time u cleaned the rads?

A faster running water pump might be moving coolant too fast through the system and not allowing heat transfer at the radiators and in the engine block. It's a lot like running the engine with no thermostat. This sounds like an experiment gone bad. I would go back to stock.

I'm having similar issues on the high temperature side after a rebuild. I'm not having the low temperature issues you're having. In traffic or more aggressive driving my needle also kisses the third mark. Looking at the actual coolant temperature with the needle there I've seen as high as 225 degrees F. From what I've seen, it looks like the third line is meant to be 230� F. This seems higher than before the rebuild. Although it always ran hotter than I'd like (I have a 1999 3.4). I see people from Arizona with later model 996's post that their gauge stays nailed to 180 even if they're driving around in 110� weather. In hot weather with aggressive driving I can get mine to 230�. The high speed fan seems to stop the temperature from rising any further. I talked with the shop that rebuilt my engine. He put a stock thermostat in it where I had the low temp in before. He swapped a low temp back in for me but so far it doesn't seem to have made a difference. I did the exact same drive in very similar conditions before and after the thermostat replacement. The needle and the displayed temperature from my OBD dongle were the same. Some things he told me that some people may or may not agree with: -They have tried all sorts of things to keep the temperature down: Low temp thermostats, aftermarket & third radiators, etc. The only thing that seems to make a difference is keeping the radiators clean -He also advised me to look for dirt in the fins. Shine a light through the radiator and condenser and make sure you can see through each of them. Use compressed air to blow dirt out back the way it came in. I tried that this weekend. The jury is still out -The 1999s seem to run hotter. He also mentioned that the newer temperature gauges or sensors seem to be less sensitive. i.e. they don't register changes as quickly or as much -He doesn't like to see the needle leave the "0" in the 180. He advised me to pull over and let the engine cool if that happens Like the others I'd suspect the GT3 water pump throwing off the system and also a faulty coolant temperature sensor. As an aside, I mistakenly ordered a turbo 996 water pump when I replaced it before the rebuild (the car didn't go any faster). I didn't see it make any difference in the temperature.

I think this is just how the 3.4s run...

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Everything you describe seems totally normal to me. The last white line on my �99 is very close to 100C on my car. And yes, in the winter, you have 9 liters of oil and many gallons of coolant. That takes a long time to heat up. My 88 Carrera was the same way, as it took 15 mins to heat up in the winter. My car runs close to 83C on the highway in the extreme cold. In the summer or at low speeds around town, I can eventually get to the last white line, although again, that takes time as well. I don�t think you have any problem at all. Porsche changed the coolant temp gauges in the later cars because these swings bother too many people. It�s to the point on the 997 and 991, the gauges are pretty useless.

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Thanks a lot for all your support! To be honest I cleaned the radiators not long ago, but I didn't use compressed air (don't have an air compressor). Only removed the dirt by hand. So I'm actually thinking to buy a bottle of compressed air and clean it better. ​​​​​ Well actually this reminds me of a chat I had when I was calling around to pick the company for the engine rebuild. One of the companies told me they refuse to use the GT3 or other 'high performance' water pumps on normal 996s. The guy told me the exact same thing as you. Of course I was stubborn and decided to move ahead with the GT3 water pump. So let this be a lesson for other people who want tho 'upgrade' their water pump Again, thanks for your help. The car is now in my garage for the winter sleep. So during the winter I will clean the radiators again. In spring I will go to see a mechanic to check the thermostat etc. If I won't forget, I will report back here what the outcome was of the checks in spring (March / April)

Just for clarification - a low temperature thermostat does NOT make the engine run cooler. It just opens sooner. Those who install a low temp thermostat with the hope it will make the car run cooler are mistaken.

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I've found this to help but not really do a good enough of a job to clean the radiators. You get rid of the bigger junk that blocks the air flow but what's left behind is small particles and insects that embed themselves in the fins of the radiator. This will often lead to suboptimal cooling performance. Even more so, the junk lodged in the fins will expand with over time and cause rust and eventually cost you the radiator. The best way I found to clean them is by pulling off the bumper and plastic vents and using a pressure washer and a stiff bristle brush. You can gently brush away a lot of the stuff lodged in the fins and then wash it out with pressure washer. Drying it with compressed air is also a good idea if you have access to it. You will be surprised how much more stuff comes out!

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so basically this can also cause the car to heat up slower, right?!

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Yes and its even slower on col days. It starts opening at 165 instead of 185. This starts coiling the motor sooner, eventually the motor does get heat soaked and stays steady at a normal operating temp. btw my 3.4l acted similar to yours and i dont think u have a problem.

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Please don�t do anything until you�ve connected to the DME and actually recorded your temps, instead of using the gauge. Otherwise you might be chasing a problem that doesn�t exist� Everyone thinking they have a cooling problem and always cleaning their radiators are only getting single-digit percentage increases in cooling efficiency. Porsche did not design the rads to beyond 85% margin. Over time, yes, efficiency drops, and yes, debris contributes, but again, until you actually measure DME-recorded values, I wasted a LOT of time on my own car until I realized that 99s almost all do this (show beyond 180F on the gauge) and nobody really knows what the temp is since the gauge doesn�t have numbers past 180 on it.