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sdelasal · Joined: 29 Sep 2007 Posts: 20 Location: UK

On the road, the temperature generally sits just below the 3rd mark - the one halfway between the red zone and the half way mark. Temperature will occasionally wander down to the half way mark.

When the car is idling, temperature will creep up past the 3rd mark and the fans will kick on and the temperature drops to just below half way mark. This proves to me that the fan works and is able to pull enough air across the radiator to cool the engine - so both fan and radiator are okay and the thermostat is open enough for the cooled flow to get thru to the sensor.

Now for the puzzling bit - on an open road at about 70MPH, I knock gear into neutral and coast down to 30MPH. Now the temperature just stays under the 3rd mark. I would expect it to drop to the half way mark as the airflow across the radiator must be somewhat similar to that from the fans when i'm stopped.

Anyone any ideas why that happens?

steve

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Airflow through the toofah front end is not that great, and gets worse when people "delete" the belly pan. Is yours still in place?
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

sdelasal · Joined: 29 Sep 2007 Posts: 20 Location: UK

yep - belly pan still in place

edredas · Joined: 09 Dec 2004 Posts: 861 Location: Charlotte, NC

There is probably too much air in the cooling system. When bled correctly, it should take about 5 minutes of idling on an 80 degree day before the fans kick on. At freeway speeds, the needle should sit just below the second mark, and when driving slow it should sit just above it.

This should always be the case; belly pan or not. The purpose of the belly pan is to keep road debris out of your belts.
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'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

dvalmont136 · Joined: 18 Sep 2008 Posts: 98 Location: Columbus, OH

edredas · Joined: 09 Dec 2004 Posts: 861 Location: Charlotte, NC

I would definitely check the cooling fans.

http://www.clarks-garage.com/shop-manual/cool-01.htm

However, I once had my gauge showing that I was over heating. I pulled the car over and popped the hood only to find the engine still cold, (I'd only driven about a mile.) I got back in the car I hit the gauge and it started reading right again. When I got home I took it apart and cleaned the contacts and it's been fine since.
_________________
'84 944 -White, Brown interior
'84 944 -Red, Automatic
'86 944 -Garnet, Fully loaded, Koni suspension
'87 924S -Red, 300hp 951 swap
'87 924S -Red, Project Car
'88 924S -Red, Daily Driver

sdelasal · Joined: 29 Sep 2007 Posts: 20 Location: UK

Just to close this one out, I found that the bottom rows of the rad were not doing their job & so I replaced the rad. After numerous attempts at bleeding, the gauge now sits just above 1/2 way when on the road - creeping up to 3/4 at idle when the fans come on.
In hindsight, I suspect that more effort on bleeding would have helped the original situation. I ended up driving around with a 12mm wrench and pulling over when the gauge registered 'too hot' then venting any air via the bleed screw. Took a couple of days before it stopped.