In order to fix some fuel leaks, I had the upper intake manifold (and everything before it) off my car. I put it all back and start up the car, and let it idle for a while while I clean up. I go to the gas station to squeegy the windshield and as I leave, I notice that the temp gauge is right at halfway (s4 gxl, but probably has an s5 temp sender). As I watch it, it twitches upwards a bit so, I turn around to go back home.

All of the sudden, it falls to about 1/4 or 1/3 (in a second or two). I drive past my driveway so I can see what's going on, and it climbs up past the Hot Zone in the matter of a couple of seconds as I'm heading back to my driveway. I pull in and immediately shut it off.

After it cools for about half an hour to forty minutes, I turn it back on. The temp gauge reads a needle's width above the Cool Zone. As it idles, it goes up to a quarter (after about 30-45 seconds), then falls immediately to almost cold again. No smoke was present in the exhaust.

Before I did this job, the car would run at about 1/4-1/3, so I'm skeptical that something caused it to actually overheat, but I really have no idea. Just looking for a place to start troubleshooting. I will begin by replacing the thermostat just for the hell of it.

 

check your Temp Sender(cooling system) wire ..the wire is in the area of the Oil filter..if that Wire is corroded,then it will give you Crappy readings..

 

That might be it! The reason I redid the fuel lines is because the one headed towards the engine (not the return line, whatever it's called...) popped off at the t-connector. The temp sending unit would likely have been soaked in gasoline as I cranked the engine (before discovering the problem), as the place it would've come out of it pointed in that direction.

I will look at that first...it's my first clue as to what could have damaged the sending unit and/or connection, which was puzzling to me, since I made it a point to be careful as I re-did everything.

 

definately check the wire, it might be shorting out on something - which would cause it to read full hot.

 

Gas won't corrode metal. In fact it cleans it nicely, as can be seen when you're changing out old fuel hoses. But a loose connection, or a wire that was corroded in the first place could be at fault. You might have bumped it, and now it has a poor contact surface.

 

I will check this as well.

I wasn't thinking that the gas would corrode it, but that it would leave a conductive residue and short it out, as slpin mentioned. I'm just basing this idea on the fact that oil that collects in spark plug wells on 4g63s can cause the spark plug wire to arc to the head, rather than arcing through the plug gap. I might be drawing the wrong conclusion, though.

If it turns out that the connection is physically solid, but that there is oily residue all over the thing, what would be a good cleaning agent?

BTW, thanks for the replies so far everyone- I'm trying to restore the car's DD-ability after an engine rebuild and it's going slowly...

 

just clean the connectors until you see metal, nice and shiney. You can buy the electrolube spray which not only cleans but helps to keep that electrical contact. Dont discount the gauge but if your car was overheating it would not drop off like it was and its all the symptoms of a bad connector or sensor/gauge problem. The FC gauges along with the FD are not particulary accurate and hence the multitude of aftermarket gauges most owners opt to fit. Mine sits just slightly below half way on my TII and never twitches or goes above it even when sitting in long traffic.

 

Thanks for confirming what I really hoped was the case! I figured that the engine couldn't cool down to near startup temps after only 30-40 minutes of sitting after being off-the-scale HOT, but the possibility of overheating sort of sits in the back of your mind regardless of the evidence.

 

Alright guys! RX7club night owls/insomniacs come through again! Thanks a lot.

Sure enough, the water temp sensor was disconnected. Immediately after the episode in the first post happened, I went to look for this, but like a n00b, I mistook the oil pressure sending unit (which was all wired up nice and tight) for the water temp sensor.

The contacts were shiny, but greasy, so I wiped them down, plugged it back in and fired it up. It warmed up to normal op temp and stayed solid. I even tried my A/C (nice day) for the first time since hooking up the compressor. It blew ice cold, but tried to kill the engine at idle. I reduced the fuel correction at idle to -20% and it settled in at a nice 750rpm. This is the happiest I've been with this car in a while!

This is what I'm guessing was happening (somebody please correct me if this isn't a plausible interpretation, electricity isn't my strong suit):

The temp sensor connector was touching the block (just sort of by chance and the way it was hanging), so I'm guessing that it was being grounded on the metal and oil. The FSM seems to show that the lower the resistance going through this wire, the higher the temp gauge reads. So, as I saw it climb and made a u-turn for my driveway, the acceleration caused the wire to shift and cease to touch the block, causing the gauge to fall.

I interpreted this as cooling caused by increased water pump speed and some more air through the radiator. Now, I realize that the effect of these two would never cause the gauge to fall like it did, but the though of "Overheating!!!" didn't really help my ability to calmly reason through this. As soon as I stopped accelerating (or hit my brakes to turn in), I imagine this shifted the wire back to the block, grounding it fully and causing the gauge to read off-the-scale hot.

If anyone was able to make it through all that crap, does that seem like a fair interpretation? I know that it doesn't really matter at this point, I'm just trying to use this as a way to help me understand the way the electrical system works.



Cliff's Notes: The temp sensor connector was unplugged, it was all in my head, the car never even came close to overheating.


One last question: Is the difference in s4/s5 gauge displays at normal operating temps caused by differences in the gauges themselves or in the sensor unit? I have an s5 sensor unit in an s4 car, so I'm wondering if the actual gauge is registering hotter or colder than actual (normally between 1/4-1/3, occasionally creeps up to a needle's width below 1/2 when going into the mountains)?