I spent all of last weekend with Louie Ott installing a new timing belt and water pump on my '89. After I had my intake powder coated last summer I was having some water loss problems and didn't want to waste money on antifreeze since it would just run out anyway, so I ran the car without antifreeze. I solved the leak problem, but the water pump later began to leak. Guess I learned at least one valuable lesson.

When Louie and I got the pump out we found that the impeller shaft had migrated forward about 3/16" and was actually getting a nice polish rubbing on the plastic t-belt cover. Glad it moved in the proper direction! I also took the opportunity to do a few other things like change oil and filter, install new plugs and hood struts, tighten up the leaking hoses on the power steering reservoir, and repair some corroded wiring. We found that whoever had replaced the water pump before had used
some bolts of the incorrect length which made reassembly a little baffling for a while. I forgot to order a new belt tensioner boot and had to make an emergency run over to Dan Brindle's to get a serviceable used one from one of his six cars. My tensioner has probably been without oil for years, but now is happily lubricated again.

I made a wrench to tighten the four 32mm nuts on the hose fittings that attach to the reservoirs on the sides of the radiator. A Crescent wrench is just not the right tool for this. If anyone needs one I can make you one too.

My water temp has dropped, but not as much as I had hoped. It now runs about a needle width or so below the uppermost white mark. I have the cooler thermostat installed. Does this agree with the temps others are reading.


Tom Cloutier


I did a very similar exercise 2 weeks ago on my '88 928s4.
I put all the bolts in the new pump as I took them out, and put the cover-bolts in the holes as soon as I had the covers off. Makes it soo much easier to remember. I saw exactly the same phenomena at the WP cover. The impeller shaft had moved outward and clearly touched the cover. The pump did not feel as if the bearing was in a poor condition though, but I decided to put in a new one and not take any chances. The boot at the tensioner was is in a VERY bad shape. I replaced the tensioner roller, but left the guide rollers. They were ok.

What makes me wonder a bit: I hear a lot of talk about engines running hot: My '88 euro 928s4 always makes it to the first white marker, but never over it. Not even when I was on vacation in Italy where it was 38C (100F).
I can imagine that in warm Florida summers the 928 gets hot, but this only suggests that the cooling capacity (flow of air /flow of water/ size in the radiator) is not enough.

If the water temp is close to the open temp of the thermostat, the thermostat controls the flow of coolant in the radiator, it would be on an open/close cycle constantly when cooling manages to lower the temp below the close temp of the thermostat. Ie: you get enough cool water back from the radiator to coo the engine to thermostat temp. This is a
nominal level. I would think this is my first temp marker.

If the water temp exceeds the open point of the thermostat permanently, you only have a free flow of water, controlled by the pump.... This is when the switch at the radiator should engage to add some extra airflow to cool down the coolant. The radiator, and the fans need to be able to lower the temp sufficiently. If all is in max-cooling state and the temp rises even more, it should stabilize because of the increasing difference between air temp and water temp, and therefore the cooling becomes better (read: more efficient).
You can lower the engine temp with hot air...as long as there is still a substantial temp difference between coolant and air temperature.

If it does not stabilize and you go into over temp situation, you will stop the car before the coolant starts to boil. The pressure system will delay this point, but with insufficient cooling this eventually will happen.

Apparently cooling conditions are ok in Holland where I live, since I never have the indicator above first line.

Any comments?
(oh eh.. the 32mm nut at the oil cooler is a nasty one, I agree)
'88 928s4 cherry red


that's just what made me wonder. The thermostat is not the one
that is helping to solve this problem. I think it is like this:
1. make sure your engine temp is too high (maybe use a "Temperature Gun" instrument)
2. make sure you have enough clean airflow thru the front grill (visual, grill, louvre)
3. have enough coolant of good quality (replace if old, check level)
4. have a good working water pump (changed date, no leaks)
5. have a good working thermostat (check at hose, monitor opening of thermostat)
6. check if the fan (s) come on when triggered by the temp switch (watch)
7. check if there is enough flow thru the radiator. (hard.... it may be clogged. Temperature Gun could help a lot)
8. check for proper gasket sealing internally between pump and engine (have to remove pump)

9. check for proper sealing of the O-ring at the thermostat. (have to remove water pump)

10. ... ask for colder weather/climate... pray?

The thermostat should run on an open/close cycle if everything is stable.
Taking a lower temperature will only make the thermostat start cooling earlier. It will not make the cooling more effective, so you end up in the same sh%#tt that happens with a normal thermostat.
It is either a bigger water pump, more airflow, cooler air to cool with, or more efficiency from the radiator (square inches you call this?) if folks in the hot climate really need more cooling..



I agree with Theo's statements below.

1) If the car truly overheats, replacing the old thermostat with a colder one is just a futile attempt to mask the real problem, which is cooling system inefficiency.

2) Inaccurate gauges cause more headaches than those created by "real" problems. Get an infrared heat gun and measure the water temps going in and coming out of the radiator. If there is not a significant temperature drop, you just found your problem.

3) Typically, a stock 928 radiator will cool a stock motor just fine. In fact, Mark Anderson uses one on his racecar. Of course there's no A/C condenser in the way to heat up his radiator. On a seriously hot-rodded motor such as a supercharged or 6.5 liter, the bigger radiators are often required.

4) If the radiator is clogged or leaking, then it's time to replace it. You can get a stock radiator new or used from 928 Int'l, or, if nice, big and shiny is desired, we have, in stock, for immediate delivery, all types of HD Ron Davis aluminum radiators. Offered exclusively by us, is a 36 month warranty against defects in materials and workmanship. And they are relatively cheap!



There had been some discussion of engine temperatures recently, so I asked an engineer friend for some input on the subject. As one of the senior engineers in a large and respected consulting company, Scott has done extensive testing for major automotive companies in several areas, including cooling system performance. I asked what the expected coolant temps would be - here is his answer.

"Typical values are:
200-210F at the engine outlet.
190-200 at the water inlet
Most radiators are designed for about 10degF dT in worst case conditions.
More modern engines are designed to operate at higher temperatures for better efficiency."

In an earlier discussion, Scott had said that at steady-state cruise, the difference between radiator inlet and outlet temps would be VERY small - often only one or two degrees.



John wrote to the list:

> Since the car was running a little hot, I replaced the thermostat with  a 75 degree C one. There just does not seem to be much difference in operating temp. I note that there are also available lower temp  switches for the fan, but when I asked, they said not for the 93 GTS. I looked for restricted air flow (none) and everything is up to snuff.
> I know there have been "strings of dialogue" about what is a good tem, and mine is certainly staying at least 1/8" below the red. The consensus is that this is normal....yet is there anything I can do  to drop the "normal" op temp?

I remembered a post I did year or two or more ago about some basic cooling systems functions.

---< begin included text >---
Basic function of cooling system components has the thermostat setting the --minimum-- temperature the engine will run at after warm up. Unless it is sticking closed, whatever happens after it reaches its set temperature is a separate function. Once the thermostat is open, heat is transferred through the radiator to the air, subject to the capability of the water flow and the airflow. Water flow assumes that the thermostat is flowing adequately, so the finger goes to the water pump if flow is inadequate, or directly to restrictions in the radiator tubes where they might be corroded or plugged with scale. I normally vote for radiator problems first, if only because they are more common on under-maintained cars, plus they are easier and cheaper to fix generally.
---< end included text >---

I've since done a little more snooping, and can say definitively that changing the thermostat to 75C on a S4/GT/GTS car does little for the running temp except when you are going downhill on a very cold day with the AC on. It seems that the S4/GT have flaps in the nose that close to maintain heat at 85C, and the fan controller on S4/GT/GTS cars runs fans to maintain 85C. So even if you had enough radiator capacity to draw the temp down into the thermostat range, the fans on your car don't come on soon enough. So what controls the fans... there's a temp sensor element in the radiator that controls the fans and also the flaps on the earlier cars. As quickly as your new thermostats opens to improve coolant flow through the radiator, the fans are still off and therefore limiting air flow especially at lower speeds.

The temp element operates in the reverse of what you might expect, where resistance goes up as temperature goes up. It is certainly possible to add a resistor in series with the sensor element and the controller to 'fool' it into thinking the engine is warmer. I know where to install the resistor (inside the connector on the fan control module) but haven't bothered to verify what the value should be. I too bought the cooler thermostat thinking that a little cooler would be a little more comfortable, but discovered as you did that it made virtually no difference in the gauge reading.

Hope this helps! More than a few folks have tormented themselves of the gauge reading, tried some stuff, and still had the reading higher than what they think they want. Meanwhile, mine runs 'cool' now, a hair above half on the temp gauge non-scale and well below the red zone.

dr bob


As pointed out by Wally and others, the 928 engine is designed to run at or above the temperatures that cause the OEM thermostat to open and the fan switch to come on. Putting in cooler a thermostat and switch actually causes the engine to run less efficiently, at least until the engine reaches its "normal" running temperature. In Summer the cooler switch and thermostat only delay achieving optimum operating temps. In winter, they may actually never allow the engine to reach its optimum operating temperature.

If you fear the engine is geting too hot, then look for other reasons why that is the case. A sticking thermostat is a possibility, as are: a faulty radiator cap, poor coolant circulation, clogged radiator, poorly operating fan(s), etc.

~ Merry motoring ~