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I have to change the radiator thermoswitch in my car. Seems like I just changed it yesterday, but I guess that was about 25 years ago. Cheap West Germany junk. I'll do a little write-up on that, but in the meantime, I'm trying to decide what coolant to use these days. For reasons I cannot re-create, I seem to have Prestone Dexcool in the motor now. It's phosphate-free, so that's good, but otherwise I can't figure out why I didn't use a G-05, G40, G48 or Pentosin NF coolant, all of which seem to be a better match for the 944/968 motor. So... who here has geeked out on coolants for these cars and can school me a bit. I'm leaning toward Zerex G40 because it's readily available and apparently won't cause gel if I don't get 100% of the Dexcool out. My plan is to flush it a few times with distilled water, but still...
@dr bob -- have to believe all 928 coolant advice applies equally
If you do in fact have Dexcool in there, I strongly recommend that you remove it ASAP and do some serious detergent flushing to get it all out. I have a couple personal experiences with Dexcool that were "inconvenient" only, if you can consider replacing head gaskets merely "inconvenient". Fortunately not on a Porsche. GM ended up taking a pretty serious financial and PR hit when their miracle universal coolant was eating head gaskets. Mine wasn't a GM car, but even the German Ford head gaskets were susceptible.
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The 928 folks have shared a lot of research and experience on which coolants are appropriate. My car came to me and had a full service at the dealer in Denver prior to the ride home. They installed plain old yellow-green Prestone coolant. That came out pretty quickly, system flushed, and G05 installed. I've been using G05 since then (mid 1990's) with no problems or concerns. I did go through a learning experience with a stuck thermostat, one that included removing the radiator for cleaning and inspection. Looked (and still does look) brand new inside. It did turn out to be a sticking thermostat, and the replacement effort showed no (like zero nada none) corrosion on the exposed internal components. A subsequent intake and cam covers refinishing project had the water bridge removed for PM resealing, again showing no wear or evidence of corrosion anytime in its history. A look into the top of the block and barely into the heads showed the same. The 944 doesn't have that water bridge between banks, but I'm sure has a similar fitting where the thermostat lives and the radiator hoses connect.
The most-current recommendation from experts, including Greg Brown last time I looked, is for G48. He buys it from Porsche for some reason, rather than trusting the potential vagaries in the aftermarket versions. I do use G48 in the early-2000's E55, as that's what was in it when purchased, and seems to be highly recommended by the Mercedes crowd. No issues with that system, always looks clean and corrosion-free. The engine block metallurgy is very similar, but the manufacturing process slightly different. No good reason to change from the G05 in the 928 though.
G05 is almost water-clear, and is sometimes a challenge to see levels in the plastic reservoir in the 928. Similar to seeing new clear brake fluid level in a new clear reservoir. Takes a more careful look sometimes.
The 928 is on a 2-3 year coolant change regimen, and it gets regular (annual) pH checks just because it's really easy. Wisdom among the gurus is to make sure it stays above 8.0 at all times. It goes in in the high 8's and stays north of 8.5 in the three years max it lives in the system at a time. It gets 2x distilled water rinses when I change it, and the block galley drains come out for each cycle. Since moving to where it actually freezes in the winter, it gets a 50% concentration with distilled water. In Cali, it got ~30% and a couple doses of Water Wetter for the summer temps. Except for that sticking thermostat episode, the car has always run right at thermostat set temp.
That sticking thermostat was a head-scratcher. Symptom was on hot (90º+) days at super-legal cruise speeds, the temp would slowly creep up from normal. Slow to 75 or so and it would slowly drop back down. Turned out that the position was limited by something internal, and worked OK in the mid ranges. Anyway, a new thermostat instantly solved the symptom. Plus it causes the engine to warm up much more quickly, a handy feature now that we see summer morning temps cooler than saw in SoCal "winters". I use the original-temp thermostats too, BTW, and waste no time with "colder" versions that don't force the engine to warm up sufficiently. The seal behind the thermostat gets changed at the same time.
The thermostat adventure padded my spare-parts inventory, with a NIB Behr radiator in the attic plus a new thermostat and seals in the parts drawer. Just In Case. So long as those spares are so readily available here, the parts in the car will perform flawlessly.
dr bob wrote: Thu Dec 04, 2025 12:31 pm
The most-current recommendation from experts, including Greg Brown last time I looked, is for G48. He buys it from Porsche for some reason, rather than trusting the potential vagaries in the aftermarket versions. I do use G48 in the early-2000's E55, as that's what was in it when purchased, and seems to be highly recommended by the Mercedes crowd. No issues with that system, always looks clean and corrosion-free. The engine block metallurgy is very similar, but the manufacturing process slightly different. No good reason to change from the G05 in the 928 though.
Great info, thanks!
Porsche's current spec is G40 and Pelican lists it as compatible with our cars, so that's what I've been running. And am about to refill with Zerex G40.
However, G40 is not compatible with G05 or G48 (which are compatible with each other), so it is definitely worth checking what you currently have and doing a full flush if uncertain.
Thank you both! I am no stranger to changing the head gasket. The first time I did it (on my original 2.5 motor), the composite part of the gasket was literally eaten all the way through -- I just had the metal parts and what was sandwiched between the block and head left. I chalked that up to the original owner managing a small coolant leak by filling the reservoir with the garden hose once a week. I then changed it maybe a dozen times between the old engine and the 'new' 3 liter -- all in an effort to impress the dyno. I'm over that now though.
Since I currently seem to have Dexcool (through some lapse if judgment I'll blame on the last solar eclipse) I am inclined to go with G40 as well, since all of my AI friends tell me it will gel less than G05 and G48 if there is any residual Dexcool in there. My block is the later 104mm bore (2.7/S2/968) so there is no block drain. As such, I'll drain it via the radiator and lower radiator hose -- encouraging drainage with gentle air pressure and vacuum; then fill it with distilled water and heat cycle it; then drain and repeat. Once the coolant runs clear, I'll fill with G40. The cooling system was working very well -- zero issues even on the hottest days of summer -- so I'm reluctant to use a chemical flush unless there's a real reason to do that. Is there a real reason to do that?
Fasterfaster has good advice that any doubt flush that motor, and frankly if it has not been changed for a long time a full flush is really a good idea even if replacing it with the same antifreeze product, imho.
Wow, Dr. Bob, that was an impressive read and I think plenty of us learned alot from your post - I know I did!
As far as flushing, I'm a hater of most commercial chemical flush products, mostly because they tend to be pretty caustic, intended to remove mineral scaling and metal more than just getting the remnants of old coolant out. So long as I'm replacing coolant "in-kind", a few cycles of distilled water will dilute the old stuff enough to keep the additives happy and the pH up.
In previous cars especially where there was a radiator breach and oil in coolant, the weapon of choice was some liquid Tide laundry detergent in distilled water. Drive with that for a few days maybe, then at least a double distilled water flush before new coolant in distilled water. The Tide seemed to offer the best oil and other contaminants removal, without leaving anything corrosive in any remaining coolant. The 928 got that treatment when I swapped out the dealer-installed Prestone and went to the then-current G05 flavor.
Among the bigger lessons over the years is to NEVER use hose water in the system, not even for a cold rinse. Most of my adult life has been based in southern California, where tap water is some blend of ground water from too close to the ocean, and imported/stolen water that had been, um, concentrated during transit through canals and lakes/reservoirs before getting stuffed into pipes for the last mile or so. Seriously high mineral content. Fiddling on power generation projects over the later decades, I've seen some even more serious water contamination in sources initially presented as very clean. Our local water here in central Oregon varies slightly, but importantly it has never actually met the wellhead purity claims from the very-local water company. TDS and conductivity regularly test at more than 2x what their twice a year published analyses show. The whole system is plastic pipe with lined metal storage tanks, nothing I can see as a logical source. The water company does not do their own analyses, instead taking grab samples and shipping them to a local lab. I did a demo analysis for them in their own offices, and they poo-poo'd the results. Apparently I'm an amateur at this, even though I've done this stuff a lot more than they have. Regardless, no matter how clean and mineral-free you might think your hose water might be, don't scale up your heat exchangers with it.
For grins, if you have one of those handy bore-scope cameras for your phone, slide it into the lower hose nozzle on the radiator while the system is flushed. Mineral scale drops to the bottom and plates out on the metal where the cooler water lives in a flowing radiator. So we expect to see first deposits where they are easiest from that bottom hose nozzle.
More interesting water stuff --
WalMart has quietly been at the forefront of disaster support, particularly hurricane relief over the years. They have a storm and disaster-prediction center that includes staging semi trailers loaded with bottled water to get to stricken areas in a hurry. What does that mean for us? Their gallon water jugs are stronger and have great screw caps that actually reseal when you tighten them. Contrast with 'normal' grocery-store plastic water jugs, flimsy with one-time seal flip caps. Maybe stupid details, but I tend to keep at least a partial jug of distilled water in the car for the first few drives after a cooling system service. A learned lesson. No worries about bottle leakage in the car. Ditto at home, where there are a couple dozen gallons of distilled water in the emergency lockers. Not so big a worry here now that we are out of earthquake-preparedness zones. Those get rotated when the cars get coolant changes every few years.
Back to work. The 928 is getting ready for winter hibernation, and will get placed on tall stands this morning. I spent the last week or so eradicating a mouse infestation on that side of the garage, so hopefully it's safe to store the car that way. It will have mothballs in the engine trays and in the valley, in case any mighty-mouses are tempted to try and jump high enough. Bucket traps placed, and snap-traps are littering the areas where they were evident a couple weeks ago now. None have, um, taken the bait in the last week-plus. 928 has been on the lift for a little under-carriage care and cleaning, so safely out of easy reach for them.
Tom, IME not sure if dexcool can be viewed as a lapse in judgement. I have been running Prestone Dexcool (gray jug with orange cap) in my 951 for years with no problems. It seems to go in orange but turns a pink color shortly after. When I replaced my water pump awhile back, it looked flawless inside the block ports. In the beginning, it was the only coolant I could find that said phosphate-free so I've stayed with it for ages. Always distilled water with 3-4 year drain/fill intervals. I can sit in 100F stop and go traffic and watch my fans cycle on & off so can't believe it has eaten my head-gasket??
I believe the issues Dr. Bob mentions (pls correct me if wrong) were incompatible gasket materials GM used in the 90's with the 2-EHA formulation (which Zerex G48 & others contain). Plus, owners would top off with old school green IAT & compound the problem. The dexcool issue is a topic we have followed for decades as my father purchased a Firebird new in '97. I would like to hear & welcome anyone's input if I should flush it out and go with a blue G48 mixture? Good post.
Off topic...Dr. Bob for your garage mouse problem, go with poison in pet friendly bait stations. Everything else is a waste of time IMO. Your problem will be solved in a few weeks plus you can see when they eat it to gauge effectiveness. Been there, done that, at my old garage.
Thanks guys... Poking around a bit, I see a lot of people are using Dexcool in the 944 and 928, so 'lapse of judgment' may have been the wrong choice of words. But, there is/was some controversy about it corroding gaskets (whether valid or not, I don't really know) and -- importantly, neither Porsche nor any of the hardcore experts recommend it, so since I have my sleeves wet already anyway, I'm thinking I'll switch over to something Porsche endorses -- either G40 or G48. My new thermoswitch is in and the fans are working as designed again, and I have my first distilled water flush done. Kind of surprised how orange the fluid still is, and I'm now thinking it will take at least 3 rounds, maybe 4 or 5?, to see clear water coming out. I hate the idea of putting chemicals or even soap in there, so hopefully enough flushes with distilled water will do the trick. Depending on how well I can flush it, I'll either go with G48, which seems to be recommended by Porsche /GregB, etc., or I'll go with G40 since it seems to be an acceptable alternative per Porsche for the 944 and since it it supposed to react (gel) less with the Dexcool.
flush-progress.jpg (188.2 KiB) Viewed 787 times
Used coolant on left. First round of all distilled water flush on the right. I'll keep at it until it would look at home in a snow globe.
p.s., jug-quality is not a trivial issue at all! I need the screw-on caps in order to have something to store the old coolant in, so I can dispose of it in a duck-friendly way...
I'll do a little write-up on that, but in the meantime, I'm trying to decide what coolant to use these days. For reasons I cannot re-create, I seem to have Prestone Dexcool in the motor now. It's phosphate-free, so that's good, but otherwise I can't figure out why I didn't use a G-05, G40, G48 or Pentosin NF coolant, all of which seem to be a better match for the 944/968 motor. So... who here has geeked out on coolants for these cars and can school me a bit. I'm leaning toward Zerex G40 because it's readily available and apparently won't cause gel if I don't get 100% of the Dexcool out. My plan is to flush it a few times with distilled water, but still... 
