fraggle

+1 use an IR gun. My gauge is really sensitive to system voltage, it will move up if the headlights lights are on or when the AC clutch is engauged. You may also want to simply reseat all the temp connections on the bridge.

dr bob

Ed, did you change the thermostat as part of your last service? Reason I ask -- I changed mine with the recent TB/etc maintenance, and saw that the temp gauge is two needle-widths higher now. Temp is actually a lot more stable with the new t-stat, and the fans run less, so I'm not really complaining. It's just a little higher than what it was reading previously.

linderpat

I did Dr Bob. Maybe that is the difference.

dr bob

The thermostat I got from Roger was not a Behr as I (think...) I had previously. The one he supplied seems to do a better job sealing, both at the rear for the bypass and the front for return circulation from the radiator. So it warms up faster, and is much more temperature-stable than it was with the previous thermostat. It's just 5-8º warmer (using the IR heat gun at the discharge end of the water bridge) that the previous one.

Roger, if you are following along, is there a slightly cooler version of the thermostat we just installed? Sounds like Ed and I are sharing a common symptom. I like the temperature stability with the one you sent me (a while ago BTW...), just used to it runnning with the gauge mid-span.

Bertrand Daoust

As per the PET, # 928 106 129 16 is a 75 degrees thermostat and was for cars up to '89.
# 928 106 129 17 is a 83 degree and was for others ('90 and up).

Don't know why they switched.

I changed mine few years ago and went from 83 to 75 degrees and notice that the car was running a bit cooler. That's on the gauge.

MainePorsche

I will bear that PN difference in mind for future reference.
Thank you again Bertrand.

Mrmerlin

Ed will you provide the info about your running temps please?
You will need an IR gun to get the running temps.

dr bob

That handily explains the difference I'm seeing too. Once we ge settled, I'll probably get the cooler one and put it in.


For others following along at home or work, virtually all car manufacturers have settled on 85ºC or hotter as a standard thermostat temperature. IIRC, GM has settled on 192ºF/89ºC for their passenger cars. My almost-new Honda runs between 195ºF and 205ºF in normal driving, and the temp gauge sits rock-steady at less than 30% of span. No actual temp values are displayed on the guage, just the needle and lines. I see the actual temps on my OBD datalogger. With everything "tuned" for that higher temp, HC emissions are reduced, and CO can be more accurately managed. There's less fuel condensation on intake walls, enough to make a difference apparently. For some of us white-hairs, running with the 928 temp gauge in the upper half of normal span can be a little upsetting in normal driving. There just doesn't seem to be much safety 'headroom'. But that's just the gauge and a perception issue I guess.

In the 928 with the slightly higher steady operating setpoint on the thermostat, the radiator is slightly better at heat transfer due to the higher approach temperature. Flow through the radiator is slightly reduced, enough that I notice the fans running less when AC is not being used.

On the opposite side, the system pressures are undoubtedly a bit higher, so marginal cooling system components might be 'discovered' while driving. There will be a larger temp difference across the radiator from inlet to outlet, so core thermal distortion and tank seal issues might be exacerbated.

Lots to think about!

linderpat

Stan, I do not have an IR gun. I will ask my mechanic to check it when it goes in next week for a replacement CV boot.

dr bob

Ed--

Harbor Freight sells a very serviceable IR gun for less than $30 on sale. Worth it to have one in the toolbox for lots of things IMHO. Be aware that theirs (and others) have a cone for a recognition area. The little laser spot ID's the center of that cone. If you need accurate numbers from a specific spot, get close to it or risk a reading from a larger area surrounding your target.


In my collection of test tools is a really interesting Fluke thermal camera and software. I used it to help ID heat leaks in steam generators and turbine casing insulation, but rent it out now to others. The thermal image is in colors that represent the temperatures it sees, overlayed over a 'standard' picture of the surveyed equipment. With the images on the screen, you can drag the cursor around and see the measured temps on that spot. Might be interesting to do a thermal survey of a running 928 engine. The condenser blocks the view of the radiator in the car, else you could instantly see the heat pattern and decide if there are any flow issues. Wouldn't that be a handy diagnostic tool? Only about $3k last time I looked. There are also some digital camera hacks that allow them to pretty much do the same thing, at least as far as generating the thermal image. You would still need to analyze the colors in the image manually to determine local temps, but it would still be valuable for looking at heat dissipation in radiators, intercoolers, AC condensers etc on the cars. FWIW, waste no time with the cheap or free current software apps for phone/tablet that purport to give you a thermal image; they don't.

linderpat

UPDATE - got an IR gun (Sear's gift card, father's day gift). Here's the data:
measured today, late afternoon. Ambient temp 75 F.

First measurement, run thru the burbs in moderate Saturday shopping traffic, no use of a/c, upper hose at bridge outlet, 189F, lower hose at bridge inlet 192F;

After high speed highway drive, then with a/c, upper hose 209F lower hose 213F. Guage reads just below the 2/3 mark. Still between the white lines, just now closer to the high line.
when the a/c gets turned off, temp drops down somewhat. The a/c condensor fan is having issues, and I'll start a separate thread about that.

Mrmerlin

Ed thanks for getting an IR gun,
wow those temps are higher than I expected.

Is there any possibility that you can install the old thermostat?
since you have an early car you wont have the rear thermostat seal to contend with.
I was expecting to see top hose temps at the 185 range and return hose at 170.

the temps your reporting at the return (lower hose) should be less than the top radiator hose

linderpat

no, old t-stat is gone. Wished I would have kept it.

linderpat

the fan in front of the rad is not working. Could that affect things too? That is just an a/c condensorr fan, and should not affect normal non a/c operation.

soontobered84

Ed,
Did you post that you did NOT burp the system after refilling?