Fiat 131 Working Temperature Ranges

[Bird's Fury]

I know this is a basic question, and answered in-part in a number of other discussions - but I would like to get the full story in one place.

What are the optimum working water and oil temp ranges for a near standard Fiat 1600 tohc?

What temperature should the thermostat open and close?
What temperature should an electric fan switch in and out?
Over what temperature range would you expect the engine to run smoothly?
What temperature should you not let the engine exceed?
How should the oil and water temperature track each other under normal and hard driving?

Background:- my car is a Sylva Fury Kit (build 1993) fitted with a 131 1600 tohc (W reg donor) - only modification is twin 40mm Dellortos, and optical points.

The engine temperature management doesn't seem to work great - it runs under temperature in cold weather, and over temperature in warm weather. Oil temperature (measured at bottom of sump) is general 10 degrees below the water temperature unless the car is being driven very hard for sometime. Even then it does not tend to exceed the water temperature. Within a water temperature range of 50C to 95C the engine response varies wildly - with a lot of exhaust poping and banging at the hight temp and blowing back through the carbs at the lower end.

Regards Martin Bird - frustrated amateur engine tuner!

[Guy Croft]

Martin, hi

You asked some constructive questions - but I am going to give you some brief but important GC advice on this.
You asked: What temperature should you not let the engine exceed?

The answer (in my exp) for best power and reliability too is 75 deg C engine and 85 deg C oil. You can let the engine and oil go hotter, say 80 & 90 when idling but not under load.

This put a 'stake' firmly in the ground, those are you benchmark targets.

[What temperature should the thermostat open and close? What temperature should an electric fan switch in and out? Over what temperature range would you expect the engine to run smoothly? How should the oil and water temperature track each other under normal and hard driving?]

To achieve that engine temp you need to get rid of the external OE thermostat and fit an in-head one, eg: Unipart GTS 102 or 104 (cannot remember which) -a 74 deg C rating. The stat is fitted under the coolant outlet elbow at the front of the head, bonded down with silicon sealant (no gasket) and one side of the T piece is blanked off by welding it up. The stat must have an air bleed hole in it, which I think the Unipart one does, else you can drill a 3mm hole.

The OE fan switch if that is what you have cuts in at 90-94 deg C and out at 85-89 deg C. Better to have a bypass switch with warning light than rely on that. I don't like that but I suppose if you are not competent to alter the circuit it's OK, it lets the engine get rather hot really. This can lead to detonation and over-rich mixtures because the intake air becomes very hot and less dense. Driving along the fan should have cut out by about 20mph when the main airflow thru the rad has cooled the motor.

By 95 deg C engine temp you are going to run into serious over-enrichment problems and you're firmly in the detonation-risk zone. If the oil is allowed to climb over 90 deg C the oil pressure is going to start dropping significantly due to the high bleed-off rate from the bearings, not good!
Many things contribute to achievement of oil and coolant temp eg: radiator and oil cooler size and placement, use of water-wetting agent (or antifreeze) in the cooling system (Fiat Paraflu is all I recommend for the TC unit), venting from the underhood region etc etc. The golden rule with cooling systems is that if the air cannot vent from the heat exchanger it cannot get in either so the transfer of heat to the surrounding air cannot take place effectively.

I want to make the point that good instrumentation, is something many, many people place far_too_little emphasis on. A well set-up car will have oil low pressure warning light, 0-100pis oil pressure gauge, 0-110 deg C oil temp gauge, 0-110 deg C engine temp gauge. Without good gauges you cannot analyse the engine behaviour in this context at all, far less set up the installation to make it robust.

There is no relationship between oil and coolant temp (I mean coolant - not engine), but the coolant exit flow temp and the temperature of the rear cylinder head (hottest part) should be about the same, moreover the oil temp will always tend to be higher than the coolant temp (and should be).

I hope this is helpful,

GC

[NOSferatu]

Hi Guy,

I join to this conversation, as I have been having problems with in head thermostat.
It is rated to 74c, but always on hotter days temperature rises to almost 90 degrees.
I have 16 rows oil cooler too.
I have tried several Fiat radiators with no help to the problem.
Should the original radiator's capacity be enough?

Temperature seems to be unstable, and it "saws" between 70 and 90.
One trick I tried was to drill a bit larger bypass hole into the thermostat, but it didn't help either.

[Guy Croft]

This is quite common.

Let's assume the engine is well-calibrated - in other words if fitted to a bench dyno it would run under load in stable manner all day long. Because incorrect (I don't like the word but there we are..) fuelling and ignition settings can cause periodic overheating and are a primary cause of detonation and overheating in competition cars.

So, in-car, if the engine will not 'cool down' even with a low temperature stat, or the temperature fluctuates wildly under load as described, the problem lies either lies with slipping water pump belt, worn out impeller (rare but have seen it) or external to the engine.

Some common causes - assuming of course that the gauge is accurate (which if Fiat/Lancia OE - it's not very..):

1. Hose from radiator to pump being sucked flat
2. Heat just not getting away from the radiator - serious underhood venting problem and generally not one simply solved by cutting holes here and there or running with hood tilted up an inch or so.. At low load the engine runs right at optimum temp, but under heavy load it overheats (or gets 'too hot' - according to whether you go along with my recommended temps or not..)

The converse to (2) above is when the rad is way too big and overly effective (so to speak) - the engine will never run hot enough.

The oil cooler's contribution to engine temp is relatively small compared with the rad.

GC

[Bird's Fury]

Thanks - GC

The engine definitely runs better at 75C so your advice looks good.

I will take your suggestion and fit a 74C thermostat in the head. I will also take the opportunity to pumb-out the T housing as I need to improve the hose runs anyway.

Unfortunately a wally in a drive-in tuning centre told me the car should run fine at 95C and proceeded to attempt to tune it at this temperature. No wonder he did such a rotten job!

May also explain why I have blown the head gasket twice over about 20,000 mile (abeit over 10 years).

The low oil temperature is still a bit of a mystery, but I will put it down to the large original sump (with extra protection plates welded on) which hangs down below the floor line and the position of the sensor at the very bottom of the sump.

Thanks Again - Martin B

[Graham Stewart]

The Unipart GTS102 thermostat is the 74C part (listed as the 'hot climate' option). The GTS104 is the 82C part ('temperate climate').

[Guy Croft]

Well done Graham!

GC