ilikebig928butts

TGIFF Y'all!

I have a puzzle for you thermodynamics sleuthing bunch to start your weekend...

My 89 auto always ran a bit hot, I know I have an inop passenger side radiator fan that I'm going to fix sometime soon.
On the freeway, she runs with temp needle kinda fluctuating between the mid point and the last "acceptable" white line. (Not the orange "oh ****" one)
This the weird thing I notice:
When I go fast enough during a freeway interchange loop, usually clockwise, the temperature needle then drops all the way to the left. Almost even touching the 1st white line. Then it'll slowly get back up to the right hand side over the next 2 - 3 miles...

What y'all think?

V2Rocket

G forces pulling the needle over?

FredR

What temeprature rating thermostat do you have fitted? The car should have a 83C item but some folks fit 75C thermostats and in my experience that does not work logically with the stock fan programing producing some unexpected behaviours.

That being said if you have a fan down that might well induce some overheating compared to stock operation.

At speed the wind blast is controlling matters and so will use of the a/c system. The operating point you describe is where I would expect the needle to sit. If the system is under control it should be pretty much stable. If the a/c is running the fans will both be operational [if they work] but if the a/c is not running the fans will kick in and out as needed.

Finally, if your cooling slats are still operational that could also make a difference to the behaviour. That Porsche eventually dropped them on the GTS says something. In cooler weather the things may be opening and closing more. That may explain the phenomena you describe.

ilikebig928butts

RTFLOL... It's a 28 years old stock 928 S4, not a GT3. I don't think she can pull that much G!

ilikebig928butts

She's been de-louvre'd (permanently opened) and yeah, I did the A/C test bit. The right hand fan is no-bueno. I'll check on the rating of the thermostat.

huskeric

could be an air gap in the system... meaning, the temp sensor may be 'dry' when making the right-handed turns...

hacker-pschorr

Before getting too complicated, need to verify the gauge in the dash is accurate.

In one of my cars, if I hit the right bump my needles goes into the red then over the next few miles settles back down to normal.

928NOOBIE

What condition is the thermostat in? I ask this because I recently replaced my thermostat and the rear seal. I had temp gauge needle fluctuations...sudden flickers..very strange...

When I saw the condition of the thermostat I knew what was happening...the lower spring that acts on the bottom flat part ...instead of that spring being a nice concentric cone...my spring had maybe 1 or 2 coils max and the rest of the spring was flat against the bottom part....basically, the thermostat was not properly opening and closing..and the sensor was not seeing a continuous even amount of coolant...the car would run at very high temperatures...and it would take forever to heat up in the winter, if ever....my belief is that it was essentially stuck in a position and it had very little movement.

If you are not sure of the age of the thermostat then replace it...also DO NOT FORGET to replace the main seal...it can be a little fiddly to get out of there..most of them are in a fairly sad state by the time most of us get to them because unfortunately it's not sold as part of the thermostat, when it is very much a required replacement item when the thermostat is replaced.

Also, as the temp gauge was doing this weird jerking around...it was driving my voltage needle nuts..voltage from the needle perspective was terrible...the signal fluctuations from the sensor was making lots of things unhappy. When the new thermo and rear seal went in (I also flushed and put new coolant at the same time) ...no more jerky temp needle...solid as a rock.. The voltage needle also now hangs between 12 and 14 where it should..solid and steady..

GregBBRD

You should fix the obvious before trying to figure out a mystery.

Speedtoys

...93 posts for a bad Thermostat again?

ROG100

Change the thermostat, seal and rear seal and replace the fan - I have them all in stock - easy peasy.

928NOOBIE

If you don't know the type / condition of the coolant flush that out and replace with Zerex G-05 or equivalent formulated for our engines. Zerex is what I used..and also Water Wetter...I used that also..not expensive and helps regain some of the heat transfer capacity lost to the coolant...my car so far has been running in almost 80 degree weather yesterday...and run hard...temp gauge never moved from it's normal range of travel.

Make sure if you change coolant you access the block plugs and drain out the block also...be careful, the coolant comes out of the block plugs quite strongly, even after I drained the rad I got a nice shower when the block plug came out of the passenger side.

Mongo

It seems that any weirdness going on with temperature readings should point to a thermostat issue. Yes, per Jeff, I created the 93-post thread about strange overheating conditions that led to a thermostat replacement solving the issue.

Also, there is no issue with fan programming when switching from an 83c thermostat to a 75c. The fans come on sometime after 83c, which means that the 75c opens the flow of coolant to the block earlier until the fans come on at the temperature programmed.