Tails

SteveG,
All engine designs are slightly different and can infact have different combustion temperatures, depending upon the timing of the ignition of the air/fuel mixture and how long the burning process is carried on before the exhaust valve opens, so each engine designer sets the engine up to achieve the results they require. Advance the spark and you have a hotter engine combustion area. Note: this is an extremely basic description, as books have been written on the combustion process and timing and today with computer controlled ignition and variable valve timing etc., the engines can be tuned to achieve the designer's optimum results.

My research shows that Porsche have designed there 928 engines to run around 85 degrees C cooling water temperature at the bottom inlet to the radiator, however, the temperature adjacent to the combustion chamber in the cooling water passages will be higher than this.

The S4s fitted with cooling flaps are designed to control the flaps movement and fan speeds from 79 degrees C to 95 decrees C of the cooling water, so this is the range accepted for the efficient and effective operation of the later MY 928 engines (reference 1987 928S4 Service Information Technik (WKD 493.421).

As previously stated my engine, via the temperature gauge, runs aroung 89 degrees C, so I consider this acceptable.

Tails 1990 928S4 Auto

dr bob

For street engines left with coolant in the engine constantly, a minimum concentration of 25% is probably a good starting point. There is a little bit of issue with corrosion of the aluminum with straight water that's taken care of with buffers in the coolant. Our engines also need some lubrication for the water pump seal, so the combination of the light coolant load plus the dose of Water Wetter fills that need nicely. I think I use a gallon in the system, followed by distilled water (2.5-3 gals or so) plus a bottle and a half of Watter Wetter. The Water Wetter dosage is the Redline-recommended rate based on cooling system size.

Watter Wetter is a blend of surfactants and lubricant that reduces the surface tension of the coolant and water in the system. The big benefit is that the samller steam bubble sizes at hot spots in the jacket allow better liquid contact with those hot spots, carrying heat away better and keeping them from being hot spots in the end. Each bubble transition between liquid and vapor (steam) state is like a small explosion, eroding metal that's in contact with the bubble. Making fewer, smaller bubbles meams fewer, smaller explosions. It works.

Tails

Porsche 928 Coolant Mixing Table

Protection to

-25 degrees C /-13 degrees F Antifreeze 40% AF 6.4 lts H2O 9.6 lts
-30 degrees C /-22 degrees F Antifreeze 45% AF 7.2 lts H2O 8.8 lts
-35 degrees C /-31 degrees F Antifreeze 50% AF 8.0 lts H2O 8.0 lts

Tails 1990 928S4 Auto

Charley B

Thanks a bunch. Finally some definitive answers.

SeanR

Sorry for not reading all of the responses so far, but here is what I did on my '88 and it reduced the operating temperature by 1 bar on the temp gauge.

I replaced my radiator with a newly rebuilt stock unit. Ensured that my flaps were working and the fans were operational.

I had an old Devek in the car that was highly corroded, once that was replaced it went from above the last white line to the middle of the gauge in 90 degree days.

Charley B

What's your take on the commercial cooling system flushes that go in before flushing the system?

dr bob

Hmmm. A bar is where you hang out, having a beer while the engine temp drops. Or a measure of pressure. Coolant pressure should be limited to one bar by the reservoir cap.

Soory, didn't mean to give you the third degree over this.

dr bob

Most effective fluhes are also a little corrosive. With the aluminum block, there's a risk of leaving caustic material trapped in the edges of headgaskets, for instance, causing longer term corrosion and eventual gasket failure. If the radiator is clogged with calcium salts from old coolant, mineral-rich water, etc, pull the radiator and have it cleaned by a specialist who can disassemble, then reseal and reassemble after it's been cleaned properly.

If the block passages are so fouled with deposits that the engine still overheats, you have bigger problems that a simple chem flush isn't likely to cure.

Nicholbry

I've decided on the aftermarket aluminum radiator from 928Intl. I don't expect that alone will reduce my temps considerably, however, the mechanic is installing a freshly rebuilt engine with a new thermostat, etc. Collectively, I pray that my concerns about this will be resolved once I get the car and test it out.

From reading these helpful tips, I gather the consensus is to use Water Weter as recommended by the manufacturer and my climate. It, apparently, has additional benefits, like lubrication, as well.

The temps here range from 94-101 degrees with 100% humidity at times. Getting caught in a traffic jam in this climate would likely overwork most 928 cooling systems. Before it went to the mechanic, I would experience temps while driving 55+ to be just above the 1/2 way mark. However, those temps would quickly climb very close to the 3/4 mark after having to reduce my speed to a crawl in traffic. Perhaps I'm unfairly comparing this car to other more common cars I've owned and this could be normal for the 928??? Just watching that needle climb diminishes some enjoyment from the experience of driving her.

I'll be sure to report back with my findings once I've got the car back and can test it out. That may be a month or two away as Greg B. is a busy man.

Thanks to you folks for being so helpful and contributing to an important topic concerning many of us.

dr bob

Nicholas--

Did you already install a lower-temp thermostat? If so, your symptoms are quite typical. With plenty of airflow at speed, the coolant temp drops to the 75C (just above the middle gauge mark on my gauge) or so that the colder thermostat is designed to maintain. Drop speed and airflow, and the fans don't come on seriously until temps get above 85C (just below the next mark on my gauge).

The S4 engine really likes to run at the 85C design temp. The original thermostat, the fans and the flaps all conspire to keep the engine that warm and that cool. Adding the colder thermostat "allows" the engine to run cooler, but does nothing to manage temp once it's wide open. At that point the fans and airflow take over. I won't argue the need for the new radiator, but I'll warn you now that you are very likely to see similar gauge results if nothing else is changed. You will of course gain extra capacity with more surface area, but that benefit will show up in ways that won't be readily apparent. Biggest change will be that the fans will run less, and you are unlikely to notice that at all while driving. The car will lilely stay cooler a little longer when you transition from fast to in-town driving, but that will be a difference of a minute or two, not something you'll easily point to. It will stay cooler on track days, enough to allow you to drive the DE's with the AC running. That alone is a worthwhile benefit.

Meanwhile, your renewed engine should generate just a little less heat, especially if there is some restoration of lost compression. Slightly less heat load for the amount of work done, proportional somewhat to the improvement you'll see in fuel economy. Again, something you won't notice easily.

Nicholbry

I thought I had the 75 degree thermo and may have said such earlier, but I started to second guess myself. I went back through some receipts and realized I actually bought the 83 degree thermo. instead (unless they just sent the wrong one). Indeed, my concerns seem to mirror your observation of a 75 degree thermo.

dr bob

The differences between the stock temp thermostat and the low temp thermostat defines the gap between thermo open and fan operation, as you see it on the gauge. In a properly working system, there will be a variation between the two. The thermo starts to open at a specific temp and is full open by a slightly higher temp. Tne at some higher temp the fans start to run, with three fan speeds that step up as conditions cause engine temp to rise. With the stepped analog control methods, it's to be expected that the gauge temp will indicate across that range in normal operation.

I happened to look at a temp gauge in a 928 older than mine, and noticed that Porsche/VDO decided not to put the actual temp numbers by the graduation lines on the temp gauge. I think mine has 170ºF and 210ºF marks indicated, with another line in between that I can only assume is 190ºF. The red area indicating warning/overheat is to the right of the indicated 210ºF mark. So what's the right indicated temp for your car? Mine parks just above the 170ºF mark on cold days (SoCal cold is 50º...), moves two needle-widths above the 170º mark when cruising on warmer days, but goes half way or more towards the alleged 190º mark on warm days before the AC is turned on; With the AC on and fans at full speed independent of coolant temp, the needle drifts back down to a needle-width above the 170º mark.

The big question you would ask might be why your gauge doesn't indicate the same. Maybe it's higher. maybe it's lower. Don't worry. What you need to look for is a change in the reading beyond what's normal for your car and the conditions you happen to be driving in. Hot day? It will indicate a little higher. Slow driving and depending the fans for air circ in front? It will indicate a little higher. Got the AC on? Could go higher or lower, depending on how much heat the AC is shedding into the radiator, and how well the radiator sheds heat with the higher air temp. Normal stuff.

Concern starts when the temp is higher than the pattern you know for the car, and gets to be worry when the gauge creeps up close to the red area.

But interest should start now, before you even drive the car. Is the coolant fresh? "Regular" coolant is supposed to last a couple years. My coolant maintenance schedule says half the predicted life, so that would have it changed every year. I use the extended-life stuff so I can save on labor time and the Water Wetter bill, and change it every two years or so. I also hate getting stranded by Stupid Stuff, so the coolant hoses (ALL of them...) get replaced on a schedule. Coolant cap new with new coolant, maybe every other change. All this is cheap insurance, in the big picture. Min price of a screwup is getting stranded, inconvenience, a flatbed, field repair, maybe a rental car and a hotel while traveling. Could be as much as blown headgaskets, fatal engine damage, stuff like that. Like I said, cheap insurance. Takes an hour or so if I have to pull the belly pans, start to finish with the floor cleaned and tools put away.

daveo90s4

Hi Nicholas, on the 'digi-dash' 1990 and later 928's my experience is that at normal / ordinary / safe operating temps the gauge does not read in the middle, but reads towards the high end. That can be a perceived worry when in fact all is good. You could bridge pins 5 and 13 on the 19 pin plug and use the digi-dash diagnostics to give you a digital temp read out. That may ease your mind about the actual temps. You could also use in infra red gauge to check water temp, head temp etc to satisfy yourself the car is not overheating (or confirm that it is). Also you could put the temp sender unit in boiling water and, with a multimeter and a thermometer, see what signal it emits for given temps as the water cools down and check that against the specs. I did all this with my 1990 S4 and in the end was able to confirm that the apparent overheating was merely the analogue gauge:- Normal temp. equalled high end of scale. Then I simply 'recalibrated' the analogue gauge to read 'normal' when temp was 'normal'.

All my advice is that you do not want your car to run too cool. Too cool is as bad (or maybe worse) than too hot!!)

Regards

Nicholbry

This is terrific advice guys. I'm going to suggest Water Weter to my mechanic since the climate here is pretty hot and traffic in Baton Rouge or New Orleans can be brutal on a car if an accident has traffic backed up for miles. Furthermore, I will test the system when I get the car back for any concerns and report back my findings. I'll go through the system with you guys component by component until the dilemma is resolved. I'm hoping a new thermo, radiator, temp. sensor, and water weter will do the trick.

RoadHunter

Had these same concerns with my 1990 GT.
Installed Devek oversized radiator.
Cleaned area and reseated thermostat.
Considered, but did not use Water Wetter because of corrosion and lubrication concerns (not to mention changing coolant for cold weather if use it without glycol).
Summary: no further issues