austin9111

I have a 1988 924S with 87K miles.

A couple of years ago, it started overheating.

I first replaced the temp sensor (the single pole one on top of the engine), then the thermostat with a lower temp (170 degree?) version.

The car largely runs cool now (at or below halfway point).

But occasionally, it will still peg the temp gauge to the far end of the red before settling back down. And other times the gauge will act erratically, moving high then low quickly (faster than thermal intertia in the the system would allow).

Not sure it is a fan issue as it happens both when stopped and moving (anywhere from 50-100mph), and at different temps (from 60-100+ ambient). Both fans run when I have the opportunity to check them (like after stopping completely and parking). though during the most recent temp gauge pegging event at a long multi-cycle stop light, the gauge quickly pegged for several seconds before the fans audio-ally kicked in (awful late it seemed) and brought the temp back down to just above the midpoint.

Anyone have an idea of what is going on? Do I need to replace the temperature switch on the radiator itself? Or do I have some other electrical issue (maybe a bad ground?)

sp1990

Have exact same issue atm. 99.99% grounding or gauge issue. Clean ground in back by firewall

austin9111

Thanks. So not that temp switch for the fans? (Figured not because it jumps around even when car is at speed and fans have little effect on airflow thru the radiator).

Can you point me a little more to this ground point. Which side of the firewall: cabin or engine, driver or passenger, what else in the car can I use as a guidepost to locate it?

Zirconocene

I don't know which one, exactly, that circuit is grounded to but you won't go wrong cleaning every ground you can find. On my car, there are two grounds in close proximity that are pretty easy to check: one on the bellhousing and one on the engine block very near the BH ground (this may be very different on the 924 or non-16V engines, I don't know). If you dig around a little bit, there are schematics for where all the ground points are in these cars. If I had to bet a nickel, it's one of the engine bay guys that's causing you issues.

Good luck

16again2009

I have a 1988 924s. My gauges were also bouncing around. I ran an extra ground wire straight from the neg battery cable to the engine block and they calmed down. You might give it a try.

rdonahe

I know the OP mentions the gauge needle moves up and down faster than "thermal inertia in the system would allow". But, just in case, do you know if your thermostat is good? Could it be failing closed and then re-opening and failing closed again?

Reason I raise this question is I have a 924S and chased what I thought was a gauge issue since my high speed fans were not kicking in. Ended up being a failing thermostat sticking closed and fan thermoswitch was never getting the full dose of hot coolant to turn them on as it was just recirculating in the engine. I understand this is not likely your issue but figured I'd toss it out there.

PaulD_944S2

My initial guess is that you are low on coolant.

Here's why:
1. Temp rapidly fluctuates = inadequate flow of coolant into radiator and past temp sensor(s).
2. Engine can't change temperature that quickly.
3. If there was a cooling system issue (thermostat, water pump, radiator, etc.) then why does it suddenly cool down at times?
4. Fan sensor is not immersed in coolant and therefore can't get hot enough to turn on fan(s) or make accurate gauge readings.

What's happening is that when engine RPM is increased, or decreased, the coolant surges through the system bathing the sensors in hot coolant, enough to change gauge readings and fan sensors.

The 944/924S engine is a bit of a challenge to get ALL of the air out of the system, especially the 944S2, but the system will settle down when it is full.
Do a search on "air in cooling system" for further enlightenment..

Best of luck!