ZV

First off, '86 951.

I'm curious whether anyone knows what the normal difference is between the engine block temperature and the actual coolant temperature.

My coolant temp gauge has been reading high after long idle periods (around the 100 degree mark) but the car does not behave as though it's getting that hot so I started poking around.

Testing with a potentiometer indicates that the gauge itself is spot on - at 69.1 ohms the gauge reads 80 degrees and at 38.5 ohms it reads 100 degrees, which leaves the temp sensor as the only possible failure point. I set up my thermocouple and the probe friction-fit perfectly in a small crevice where the coolant hose exits the block near the bleed screw (see image below, thermocouple probe was positioned in the crevice indicated by the arrow).



With the probe in that position, it reads 8 to 10 degrees lower than the dash gauge.

The rest of the system seems top operate in correspondence with the thermocouple readings, not the gauge readings. For example, if I start the car from cold, the radiator stays cold until the thermocouple indicates 87 degrees, at which point the radiator begins to heat up. This is consistent with the Behr thermostat I'm using. Similarly, the low-speed fans come on at an indicated 91 degrees from the thermocouple, which is close enough to spec for the car's stock 92/102 fan temp switch.

When the thermocouple is reading a steady 89 with the fans running, the gauge in the car reads just a tick under the 100 degree mark.

Taken together, this all seems to me to mean that my gauge temp sender is failing, causing the gauge to read high, but I wanted to run it by everyone else just in case the block temp was somehow expected to be much lower than the coolant temp.

If a am correct, how much of a pain is replacing the gauge temp sender? It's easy enough to get at if I pull the airbox and the intercooler pipe but I'm not sure whether I'd need to drain the block or not to replace it.

mahoney944

That sensor is easy to change. The car has a temp sensor for the DME and one just for the gauge. Both are on the head under the intake manifold. The DME one can be changed without removing the intake manifold(at the front of the head), the gauge sensor I'm not so sure as it sits between cylinder 2 and 3. They typically are never to hard to crack loose if you get a wrench on it. Also if the issue is just at idle you may just need to vent the system more.

ZV

If the temps I'm getting from the thermocouple are correct, there's no problem at all, the system is behaving as expected.

I know I can get to the gauge sensor without pulling the manifold because I've been accessing it to get at the wire leads when I was checking the resistance. My worry is that if I pull it without draining the coolant from the block I'll end up with coolant everywhere. On the other hand, draining and filling the block is a royal pain in the ***, so I'd love to avoid that if at all possible.

blade7

With the engine cold you wont lose much coolant removing those sensors.

ZV

Thanks. And yeah, I definitely wasn't going to do it with the engine hot!

spanky

You can remove the sensor without losing coolant by leaving the expansion tank cap on tight as you do this.

VirginiaF1

To OP, your original question still stands..
I've spent the past hour researching various threads here and elsewhere.

Neither concensus nor authoritative assertion was found.
Remains a great question, especially cars like ours that employ oil coolers.

ZV

Yeah, I went looking for it thinking someone had to have an authoritative answer, but nope.

It seems to me that the temps should be pretty close if you're on a part of the block that's very close to a coolant passage, with the block being slightly hotter since the coolant is pulling heat out of the engine but I haven't been able to verify that.

I'll have better data tomorrow when the new sender comes in though.

FRporscheman

I don't have the answer but I can say that infrared thermometers are really cheap on amazon these days. It would also be good to know the temperature differential at the inlet and outlet of the radiator.