86 951 Cooling Problems

[mridgeway]

I've replaced the thermostat, and don't seem to be leaking any coolant. Fans are working, but the car will get hot in about 5 minutes.
Options?
I'm thinking about doing the radiator and water pump together to eliminate these as potential problem spots - and of course do the belts and hoses while I'm in there.
Any other suggestions?
Thanks

[Lawrence Coppari]

How hot is it getting? What does the temperature gauge say? Have you bled the system? Is the thermostat in backwards? Did you test thermostat in a pan of water on the stove before you installed it? Do you have something simple like a collapsed coolant hose? Are the fans blowing hot air from the radiator at all levels from top to bottom of the radiator? Is you water pump turning?

Just asking questions; not trying to insult you.

[WinfieldKS]

The first thing to do is to determine if the engine is really getting hot or is this just a malfunction in the gauge. When the gauge starts to show hot then open the hood and put your hand on the engine -- you'll quickly know if it is actually hot or just warm. If the engine only feels warm but the gauge says overheating then the problem is in the gauge -- most likely just the sending unit.

If the engine really is hot then I'd wonder about the water pump. Maybe the impeller fell off the shaft (I saw that happen on a Dodge Dart slant 6 - it drove everyone nuts trying to figure out the problem) or maybe the shaft bushing is binding and the pump is stuck or barely turning with the belt slipping on the smooth pulley? If so then the belt should destroy itself very soon, bending the exhaust valves in the process.

The above is possible but unlikely. Most likely it is a thermostat stuck closed. The thermostat is located inside the large inlet tube of the water pump itself and it is a bit nasty to get to. You'll need a good set of snap ring pliers to replace it but fortunately there is a way to test it without getting into anything mechanical.

Do this with two people. With the engine dead cold, start the engine and have one person inside watching the temp gauge. Have the other outside with the hood open and their hand on the upper radiator hose. Allow the engine to idle until the temp gauge starts to climb as you've described. If the thermostat is operating correctly then the outside person should feel a sudden, rapid rise in temperature in the upper radiator hose; If the gauge says hot and a hand on the engine confirms hot but the hose is still cool then the problem is either the thermostat or the water pump itself.

If the hose does get the rapid rise in temperature but remains only warm and the engine seems only warm but the gauge says hot then replace the temp sensor -- it is the one furthest to the rear on the front LH side (Driver's side) of the cylinder head nestled under the intake manifold.

[Kirby2]

Same problem, 3 different 944s, all solved by a new radiator.
Fine fins + Marginal original effectiveness => easily crudded radiator