Dangerm

Whenever I turn on the park/headlights, the temperature gauge and fuel guage increase up a bit, and reduce when I turn thr lights off - any idea what would cause this to happen?

Also I have just replace the clutch fan, and the temperature gauge seems to sit just under the second white line, and doesn't go over - does this suggest that the car just runs hot (radiator replaced by previous owner) or perhpas that the guage isn't that accurate?

John Veninger

Sounds like your getting a voltage drop.
Have you checked the condition of the battery cable on the battery and also the ground strap from the engine to the body?

WallyP had a write-up for electrical maintenance. Included removing relays and cleaning grounds. May want to do a search for it.

Dangerm

Thanks - I'll check these out

Rich9928p

Is a reading just under the 2nd white line an indication that the engine is running too hot? Actually, any reading that is not in the red zone is within Porsche's acceptable operating range. Naturally, if you're running at high power levels (high speed, climbing hills, hard acceleration) you will expect to see the gauge in the higher range then when you're cruising at 55 mph in the flat lands. If the temperature reading falls when you ease up on power the cooling system is working OK.

Somewhere in the archives there reside long posts with pictures and such. The outcome was
- who says that the gauge must be in the lower 1/3 to show OK? Porsche may not have subscribed to that idea
- some cars read higher than others, for example my '87 S4 pretty much always is just below or above the 2nd white line, my earlier '83 and '84 often were in the lower 1/3 (older cars seem to read lower in the gauge than newer cars)
- the gauges aren't all that accurate and are subject to inaccuracte readings due to the temperature sensor, corroded contacts, degrading flex circuit behind the instrument pod or the gauge itself

Some cars may actually be running too hot due to clogged radiators (inside and outside), failing thermostats, or the gasket behind the thermostat hasn't been replaced [look at item 33 in the diagram, p/n 928.106.163.00] , etc. A more rare situation that does cause boil over is if the impeller blade of the water pump separates from the shaft - largly attributed to plastic impeller blades. Something as simple as a crack in the rubber on the radiator cap can cause a pressure leak that enables premature boiling.

It may be useful to use a heat gun to confirm the actual temperature of various underhood areas. If it isn't hot, then you're OK. If it is too hot, look for potential causes.

I'm more concerned with trend vs. actual position - if the gauge is rapidly rising then trouble is coming soon and I reduce power demands to see if that cures the issue. If the needle indicates consistently in one position, and if no problem has resulted in the past decade, then that position is probably OK for the car.

Brian B

My 79 does the same thing. I checked the engine with an infrared thermometer and everything was running well at 75 degrees (that's the upper white line on my guage).

One of these days I have to really clean up all of the ground points and clean the contacts inside the pod...

Good Luck! Let me know if you find a solution.

Doug Hillary

Hi,
I thought that these figures may be of some assistance in measuring your engine's "core" temperature. The core temperature is the engines operating "equilibrium" - this may vary from car to car but it appears to be mostly around 90-95C. This is excellent and is one reason why the 928 is so easy on its engine oil in normal use. My two new Benz engines and Z3 2.8 show oil temps around 100-120C (140C @ turbocharger outflow) - and core engine temperatures are all around 95-100C. Permissible coolant temp on the Benz engines is 120. All engine have thermostatically controlled oil coolers of course

The following MY89 928 S4's figures are the product of nearly 200 IR readings taken over some time - about two years. The parameters used were;
a) constant measuring points
b) a minimum of 30 minutes running time before any measurements taken
c) ambient (in Centigrade) always recorded

(First column=average, second column=maximum, last column=lowest)

1 - Ambient 25, 35, -2
2 - Gauge 85, 87, 81
3 - Thermostat housing 86, 92, 78
4 - Top radiator hose 73, 88, (55)
5 - Bottom radiator hose 76, 90, (61)
6 - Valve covers 89, 98, 85
7 - Vee near sender 88, 95, 80
8 - Oil cooler - top 75, 88, (54)
9 - Oil cooler - bottom 77, 90, (53)

Considering the averages, the figures in ( ) probably indicate no flow when readings were taken

Allowing for the great thermal "discharge" characteristics of the 928's engine/heads metallurgy and the S4's temperature warnings at 118C and 120C, in my viewpoint, if any of three readings from items 3, 4, 5, 8, 9 are not constantly above 105C, the engine is NOT overheating. Even then there is still a "margin" up to around 115C

Cooling systen faults such as coolant loss etc are another issue of course. As well, we all know an uninterrupted airflow is very important to these engines

I hope this helps

Regards
Doug Hillary

Rich9928p

To provide an example of correlation of water temp. with external gauges and also oil temp, I'll provide some data.

I have Autometer water and oil temperature gauges in my '79 track car ['85 5-liter 928 block, '83 Euro S heads/cams intake - CIS fuel injection - Devek radiator]. I have an electric pusher fan that I normally turn off when at speed, so 100% aero wind flow provides cooling air into the radiator. The Autometer water temp probe was placed in the rear of the left side head (as looking forward), in a plate that covers an unused water passage that is normally just blocked off [a fairly worst case position compared to Porsche's up front best case water temp. sensor position]. The Autometer oil temp. probe is drilled into the lower rear portion of the oil pan.

When running hot laps, my track car water temp. usually runs at 205 - 210F and the oil temp is 10 - 20F warmer. This corresponds to the upper 2nd line on the dash temp. guage, sometimes getting near the red zone. My goal is to keep the water temp below 220F and the oil temp below 240F during sessions. During tight twisties when there isn't as much airflow I can see a 5 degree rise and after long straights and when decelerating for corners the temp. drops 5 degrees. When I'm off the throttle in the cool down laps the temps. drop as much as 20F.