dprantl

So a while back I posted about my car running hotter than I would like. My description for that is running above the last white line. IMO those two white lines are there for a purpose, and if the sender is working correctly under normal conditions the temp gauge should never exceed that last white line. In my case it seemed to happen only at speed on the highway when it was more than 90 deg F outside. I should say that I was double-checking my temp gauge reading with an IR temp gun on the two radiator hoses and also with the TempII readout of the SharkTuner. The IR gun was not too reliable since I had to pull over to take the readings and by that time, the car would cool down a little. When the gauge would cross the last white line, ST2 would read sometimes up to 98 deg C after 20 or more minutes at highway speeds.

So, the car's radiator was only 3 years old. I took it out anyway and pressure washed it from the outside and cleaned out all the crap by the condenser. My car is a '91 so it doesn't have the cooling flaps. I also checked to make sure the plastic side covers were properly in place, they were. I then changed the thermostat, o-ring and seal with a low-temp thermostat and did the timing belt/water pump change with a new updated Laso. My car is supercharged, so it had two chargecoolers right in the middle of the radiator, so I relocated them to the bottom left/right sides laying right on the bumper grille. These chargecoolers also prevented the inner plastic shroud from lifting up at speed and blocking airflow as reported by some on here. I also changed the coolant reservoir and cap (with the proper cap), some coolant lines and the coolant (always use 40% Zerex G-05 and 60% distilled water). I even put in a bottle of water-wetter. At this point, I pressure-tested the coolant system to ~15psi and it held just fine. But, same problem still there.

At this point, I just said forget it and lived with it, figuring many other people complained about this and it was just how the later 928's run, until just last week. The little coolant pressure sensor right by the coolant reservoir blew up into two pieces creating a nice spray pattern under the hood and dropping ~3 quarts of coolant on the ground. I bought a new hose and deleted the pressure sensor, and magically the running hot problem is gone. A couple of days ago it was 100 deg F outside and I was going down the highway for 30 min with the A/C blasting and even did some 5th gear 8psi blasts and ST2 reported the TempII temp at a rock solid at 92 deg F, never higher.

So... my hypothesis is that when I pressure-tested the coolant system, it was cold and that's why the pressure sensor held the pressure fine, but once it got hot enough under the hood and at constant 3k RPM running, the sensor would start to give and the system would lose pressure, causing temps to rise.

I would recommend anyone having hot running issues while at speed take a look at this sensor, it is very quick and easy to get to. Better yet, delete it with some longer hose because it has very little purpose, but can cause annoying issues.

*EDIT: For the sake of completeness, I should have also noted that my TempII sensor is only 2 years old.

Dan
'91 928GT S/C 475hp/460lb.ft

AO

Camboinc (Brian) had the same sensor fail on his car ('87) just before SITM. What a mess. Later I found out that sensor is a "High Pressure" sensor and would trigger the "Coolant" light. I remarked that if that thing were to cause the Coolant light to come on in my car, I would have no clue to even look at it. I would think I was low on coolant and when I see it's full, I'd ignore the light thinking it was a false alarm. I agree it should be deleted - on my list of ToDo's!

Glad to hear your cooling is better. My car also tends to run a bit warm. I did find, tucking the plastic flap helped when running the AC. I will report back when i delete the sensor to see if it improves.

JPTL

Now that I've gotten my leaking heater valve taken care of, I'd like to lower my engine temp if possible...for both the '88 and the '91.
Since I've had the '91 GT, the temp gauge has always settled between the white line and the red mark on the gauge in the summertime. It's never gotten into the red, but low coolant or blocked airflow would certainly get it there quickly I'm guessing. I've never done an accurate measurement using proper temp. gauges, but if these dash temp gauges are somewhat consistent, I assume that I'm running too hot (pic is a quick photoshop hack to show where my needle sits)
Although the system has been flushed and fresh coolant has been put in per schedule, I've never had the radiator pressure cleaned/flushed.
I'm interested in this pressure sensor delete if it could be causing hotter than normal operating temps. Is it the thing that I've circled in the 2nd pic?

dprantl

Yup, that looks like it.

Dan
'91 928GT S/C 475hp/460lb.ft

JPTL

..so I'll wait to hear back from you, Andrew to see if the delete gets you a cooler running engine.
But in the mean time, Dan you say "deleted the pressure sensor", I assume that the two wires simply get disconnected and taped off (coolant warning is off on an open circuit; trips on a closed circuit). Is that right?

jsherid1

I just replaced mine yesterday in an effort to get the temp warning light to stop flashing at me (it is still doing it)--it takes about 10-15 minutes to do. The part is about $80.00 from Roger.

dprantl

Yup, I just left the plug disconnected and taped the pins off.

Dan
'91 928GT S/C 475hp/460lb.ft

jpitman2

So how do you figure this sensor was losing pressure without losing coolant - didnt see any mention of coolant loss? Does it trigger on excessive pressure, or low pressure?
Loss of pressure wont INCREASE the temperature of the coolant per se, but it will LOWER the boiling temperature, which MAY allow steam pockets to form in parts of the engine that exceed 212F, which will reduce the conduction of heat to the coolant, and lift the temperature of the whole system....
jp 83 Euro S AT 52k

dprantl

The location of the sensor itself and its connection point is right at the top of the expansion tank, so a leak there would not necessarily cause coolant to be lost unless the leak is very large. If some coolant was lost there, I think it would be very slight unless the sensor blows apart.

Dan
'91 928GT S/C 475hp/460lb.ft

jpitman2

I guess the proof is in the eating, so if deleteion fixes the problem, thats definitive. Would be interesting to see actual pressures in a system under load, and be able to lower them and see the effect. Using a lower pressure rad cap and watching temps would be interesting.....
jp 83 S

dprantl

Well, the extreme test for this would be to run with the cap loose or off. That way the pressure in the system could never exceed ambient, and I would expect the car to overheat in hot ambient conditions.

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

losing pressure wont increase temps, but losing water will. So as was said, you can lose water, temps go up, pockets of water, boil and create voids, and then you dont have circulation through the system, and you get overheating.

ptuomov

Since the boiling point is reduced by lower pressure and air/steam bubbles form easier, doesn't it then follow that losing pressure increases temps?

dprantl

I was wondering if the water could boil, turn to steam, then condense back when it reaches the expansion tank if it doesn't go via the pressure sensor. And I wonder how fast the coolant level would go down if coolant was being lost via steam. Thoughts?

Dan
'91 928GT S/C 475hp/460lb.ft

JPTL