bureau13

(86.5 5-speed)

I put my new gauge temp sensor in this afternoon, sure enough that fixed my non-functional temp gauge issue. Unfortunately, while idling, the temp just kept going up, until it was just past the upper hash mark and the red overheat light came on. I shut it off at that point.

Previously, with the non-functional gauge, I would "randomly" get an intermittent overheat light. Temp gun readings at the metal where the lower radiator hose enters the block is about 140F. Another place buried inside the engine measured 183F. I don't know how accurate this gun is, but there have been no symptoms of overheating like you might usually expect. That's why I thought they were falses before. Now...I wonder.

Some other data points...my belt fan seems OK, but with those viscous cluches, I don't know how to test that. It doesn't free spin when the engine is off, but I can turn it.

The aux fan never turns on. The connection to the temp sensor on the radiator is loose, also I have no working AC (there isn't even a belt on the compressor right now). If I'm really getting that hot though, I think the aux fan should be coming on regardless of AC.

It's not a hot day...low 80s. Should it get hot enough in five minutes of idling to need the aux fan at all?

Any thoughts or hints are appreciated!

jds

bureau13

Just went for a leisurely drive around the area, a few stop signs, mostly 30-50mph, not hitting it hard. I had the overheat light on after about ten minutes, tops. So that's not right, clearly, but if it's really overheating it sure is doing so with minimal drama.

ROG100

Looking at the front of the car measure the temp at the top left corner of the radiator and the bottom right corner. Temp at the top left should be 180/190 and bottom right about 10/15F less. Are both the upper and lower radiator hose hot?? Is water passing through them. Wondering if the thermostat is not closing.

bureau13

They both feel hot. If the thermostat weren't opening would they be about the same? The upper left of the radiator is measuring around 200F once the light comes on, and the bottom right was like 160F. Actually, if the thermostat wasn't opening, the radiator wouldn't be that hot at all, would it?

jpitman2

I have a temp sensor for the fans in the top hose. it switches in at 208F, and out at 199F, and keeps the temp just under the second white line. I am surprised you get the red light at 200F....An S4 thing ?
jp 83 Euro S AT 55k

bureau13

It's almost behaving as if the gauge/warning light were calibrated so that overheat means "normal operating temps." The car is behaving just fine, and apparently I have been driving it around at that point, it's just the gauge that gets all excited. Unfortunately, now that I see it and it's not obviously some electrical gremlin with the gauge/sensor like before, I'm also all excited. It stresses me out seeing that light, even if the car seems to act fine.

ROG100

No something is wrong - the gauge should read correctly in line with the temperature of the coolant. We seem to have established that the thermostat is closing allowing water to circulate through the radiator and that car seems to be running at normal temperature.
To be sure, we are talking about the sensor with two spade connectors on the drivers side of the thermostat housing? Not normally a sensor that causes any issues.

bureau13

Right...the old one would result in the gauge always reading at the bottom (i.e. never moving) but the overheat warning would occasionally come on, then go off, etc. Now with the new replacement sensor, the gauge and light are acting normally other than that they're telling me I'm running hot but I don't really seem to be.

joejoe

Is there a thermostat in there?

bureau13

Lol I assume there is! For the sake of argument though, if there weren't, wouldn't that result in water always flowing through the radiator, making warm-up take longer? I don't see how that would make it get hotter?

ROG100

What is the temperature at the sensor base when the engine is hot?
If the thermostat is only partially retracting flow from the heads could be restricted leaving hotter coolant in the engine. See what the sensor base temperature is.

bureau13

Well, I tried to get that reading, and was getting something like 140 at the sensor base when overheat light was on, but then I was getting low readings that made no sense other places too. It's dark, so it was harder to find where to aim the thing. However I was just idling, with the hood up, until the light came on, and the lower radiator hose felt barely warm. Maybe I thought it was hotter earlier due to engine bay heatsoak after driving? Could a sketchy thermostat cause temps to basically steady out at a higher than specified temp? It really doesn't seem to keep getting hotter...if what I'm seeing has been happening all along, then I was driving with the temp in the "overheat" range, but it seems to get there and then stop climbing.

bureau13

OK, with better light, I repeated the test. Started the car, let it warm up. First, I don't think I was getting any flow through the upper radiator hose until the temperature gauge showed at or beyond the third hash. It did seem to be flowing at that point, and soon thereafter I got the overheat lamp. Temperature at the base of the sensor and on the aluminum housing next to it was around 205. I let it run a little bit past that, it really didn't seem to be getting any hotter.

This has to be thermostat related somehow, doesn't it?

WallyP

You need to be aware that the thermostat on a 928 does not work like the one on an American car. If you remove the 928 thermostat, little or no coolant will flow thru the radiator, and the engine will overheat.

Hopefully, if there is a problem, it is a faulty thermostat, or a missing seal at the back of the thermostat opening (I think that the '86.5 has the seal - I'm sure that I will be corrected if necessary!)

I said "hopefully" it is thermostat related, because other possible causes include a slipping or rusted water pump impeller, plugged radiator, etc.

bureau13

Interesting. I always thought they all worked the same. So I won't remove it and drive around to see what happens then

Barring some other ideas to try, I probably will replace it though. It's fairly inexpensive and easy to do as a test (I hope. It usually is, but...)