Cooling

[Huw]

Well....the zed has started to do something odd....

 

it is not overheating (I just drove it 200+ km tonight) but the temperature increases at idle (after the thermostat has opened).

I am thinking that the funny coupling (articulate, i know) for the original fan may have died and am pondering simply installing an electric unit this weekend.

 

Thoughts? objections to diagnosis?

 

Would like to be 100% on it before summer hits.

[jamo240]

Hi Huw

 

When you say it has 'started doing it', what new thing is it doing?

 

All cars will get hotter when they idle, as there is no cooling ram air (produced by vehicle movement) to allow the radiator to exchange the heat from the coolant to the ambient air. As you state, the only air movement is created by the fan, which is no where near as much air flow as ram air while you're moving. My 240Z always gets hotter when idling, and the ECU (it's injected) manages that the temperature by turning the fans on at 93C, and turning them off at 89C.

 

As for your mechanical fan (bolted to the water pump drive), the thermatic hub is designed to couple as it gets hotter, and therefore produce more flow, and decouple as the temperature drops and it does not need to pump....this characteristic is there to reduce engine load and noise at times when the fan is not required. Provided your fan is pumping at idle (you can tell by putting your hand in front of the radiator and you will feel air flowing over your hand), then that's working fine. The other thing to be sure of is that the thermostat is opening properly and allowing full coolant flow so that the radiator can remove the heat from the coolant via the airflow I just mentioned.

 

If you're getting worried because the gauge is going up at idle, I wouldn't worry too much, unless it's getting right up into the red. When I worked at Holden engineering, we did all we could to eliminate temp gauges from cars, as the movement of the gauge usually just caused people to worry, while adding no useful information that they could action....best just to have a light that goes on to say 'there's a problem, stop now!'

 

If it's none of the above, then you could have a more significant problem with the head gasket or cracked head etc, causing the cooling system to malfunction....I would also suggest you check the coolant and engine oil to ensure both are in the correct condition, and there is no evidence of mixing.

 

Cheers

 

Jamo

[madz]

Jamo beat me... 

 

If you can rule out the fan, and if it has only just started doing this, replace the thermostat, as it may not be opening soon enough, at the required temp. Its the cheapest and easiest thing to eliminate first... I think there are a few threads on here about installing fans etc, a search should come up with the goods

 

 

[Huw]

well it shouldn't be the thermostat.....as I replaced that 3 weeks ago....to vast improvement (as it had jammed and I only just caught it).

 

I am not concerned so much about it varying at idle, it is more the fact that previously it has not varied a noticeable amount (so the change).

Thanks for the posts, will check the fan and if it is as you say, shall relax and go for a drive!

[jamo240]

Sounds like a plan HUW...just to round out the topic, cooling system theory goes like this....

 

When the car is cruising and the cooling system has more capacity to reject heat than the engine is discharging to the cooling system, the engine is said to be under thermostat control...that is, the thermostat will restrict flow to the heat exchanger (radiator) to prevent the engine running too cold. So, if the thermostat is an 87C unit, it will maintain the discharge temp of the coolant from the engine as about 87C. As the coolant flow is restricted, the coolant entering the engine from the radiator will have rejected more heat than is necessary to balance the system, and hence will enter the engine colder than when in balance (when in balance, the coolant loses about 6 deg C from the hot side to the cold side of the radiator). Balance is the preferred condition, as the temperature difference across the engine is relatively low...this is why trucks sometimes put blinds across the radiator in cold climates to prevent very cold water entering the engine due to low coolant flow.

 

Now, when you come to idle, or on a hot day, when the ambient air flow is low, or the high ambient temperature means the thermal efficiency of the system is more challenging, the car may go out of thermostat control. That means that the cooling system does not have excess capacity, and the thermostat will fully open, allowing maximum flow to the heat exchanger. Under this condition, maximum heat is rejected, resulting in the 6 deg C temp difference I mentioned above. If the system is not in balance, the coolant temperature will rise until balance is achieved between the heat being input to the system vs that being rejected. You see this as your temperature gauge rising.

 

If the system cannot achieve balance (eg you are sitting idling on a 46C day in a strong tail wind), you will see the temperature rise, possibly up to the point where the engine will boil. This condition is known as Air-To-Boil (ATB)...OEM's have engineering specifications for this, and GM's is 55C...that means that if the ambient air is 55C, the car will boil...this allows some margin for sub-optimal conditions, such as tail winds, or radiators clogged with bugs etc, because a 55C day is a rare event.

 

Good luck!

 

Jamo