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3.9 V8 runs hotter with Air-con ON !?


evo828
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As the temps are going around 36degC outside I realised that when running with the A/C on the engine temp needle rises above the half and goes into the upper part of the range (not into red). This all was observed while the car is cruising at around 50-90 kph - with no extra loads (no uphill etc).

After connecting to Megasquirt - I've seen that the temps are going up to around and over 100degC which is frightening me. Switching the A/C off the temps settle down to around 93-95degC (even when the car is left idling for some time). Occasionally they come down to 89-90 degC.

I would expect that the electric fans that run while the A/C on will actually help the water to cool down better than if not turned on. It seems that the effect is opposite.

No coolant loss, no pressurinsing of the hoses. I had a failed thermostat 8 months ago, was changed for a new one including new coolant fluid (yellow stuff - yellow was there before) - observed that after the change the engine was running approximatelly 5degC warmer than before (before 83degC -after 88degC) under the same/similar conditions. Tried another thermostat - the same story. Tried a thermostat with "lower" opening temp (I thing from RRC) - the engine run too cold (77degC). But all these was done in winter - so could not compare to the ambient temps that are outside now.

1. are the temps around 100degC already dangerous?

2. if yes - what actions do you suggest to investigate the reason

3. any idea why A/C might have such an impact on the engine running temp?

thank you in advance for ideas.

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As the temps are going around 36degC outside I realised that when running with the A/C on the engine temp needle rises above the half and goes into the upper part of the range (not into red). This all was observed while the car is cruising at around 50-90 kph - with no extra loads (no uphill etc).

After connecting to Megasquirt - I've seen that the temps are going up to around and over 100degC which is frightening me. Switching the A/C off the temps settle down to around 93-95degC (even when the car is left idling for some time). Occasionally they come down to 89-90 degC.

I would expect that the electric fans that run while the A/C on will actually help the water to cool down better than if not turned on. It seems that the effect is opposite.

No coolant loss, no pressurinsing of the hoses. I had a failed thermostat 8 months ago, was changed for a new one including new coolant fluid (yellow stuff - yellow was there before) - observed that after the change the engine was running approximatelly 5degC warmer than before (before 83degC -after 88degC) under the same/similar conditions. Tried another thermostat - the same story. Tried a thermostat with "lower" opening temp (I thing from RRC) - the engine run too cold (77degC). But all these was done in winter - so could not compare to the ambient temps that are outside now.

1. are the temps around 100degC already dangerous?

2. if yes - what actions do you suggest to investigate the reason

3. any idea why A/C might have such an impact on the engine running temp?

thank you in advance for ideas.

1 - Its a pressurised system so it wont boil until about 116 degrees

2 - none

3 - Because the engine is working that much harder, it takes a lot of power to run A/c, the MPG will also drop.

Dave

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I'm guessing you don't know how the AC system works then :ph34r:

It takes heat from in the car and pumps it to the outside, to the AC radiator which is in front of your normal radiator - so you are heating the air in front of your radiator. Plus you are then using extra engine power to run the compressor to do the work, plus you are running the electric blower to circulate air in the vehicle and probably running an engine cooling fan (RRC have extra cooling fans which come on with the AC or as the temp reaches 100). All in all, the engine is doing more work on a hot day and you're paying it back by blowing extra hot air at it.

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Dave, Fridge - thanks for the clarification. :rolleyes:

I knew that works harder (a/c pump, alternator generating more apms to feed blower and those 2 fans in front of the radiator) - but expected the two electric fans to be able to cool it sufficiently down to the same level as before switching a/c on. Also at cruising speed (with cca 1900rpm); the airflow through the radiator should be sufficient not to allow the temps to rise to these 100degC levels.

OK - I will not panic - will just keep my eye on the temp needle :)

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Dave, Fridge - thanks for the clarification. :rolleyes:

I knew that works harder (a/c pump, alternator generating more apms to feed blower and those 2 fans in front of the radiator) - but expected the two electric fans to be able to cool it sufficiently down to the same level as before switching a/c on. Also at cruising speed (with cca 1900rpm); the airflow through the radiator should be sufficient not to allow the temps to rise to these 100degC levels.

OK - I will not panic - will just keep my eye on the temp needle :)

Check that your viscous fan clutch is working properly. Its normal that if your engine is working harder that more heat will need to be dissipated. The viscous fan is designed to add cooling power when the engine is hot. If your fan does not feel solid when the car is very hot there is a good chance that its defective and the car will run hotter because of that.

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I suspect your radiator is past it, and i'd also expect it would be unable to keep the engine cool under long periods of high load.

Yes, the AC is dumping additional heat, but the rad should be able to keep the engine cool when running at full throttle, and cruising with AC on will be generating nowhere near the same amount of heat as the engine at full throttle.

You might find a good flush out will help, or a clean externally, but you may also find the rad has no fins left, or is properly blocked up internally.

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Dave, Fridge - thanks for the clarification. :rolleyes:

I knew that works harder (a/c pump, alternator generating more apms to feed blower and those 2 fans in front of the radiator) - but expected the two electric fans to be able to cool it sufficiently down to the same level as before switching a/c on. Also at cruising speed (with cca 1900rpm); the airflow through the radiator should be sufficient not to allow the temps to rise to these 100degC levels.

OK - I will not panic - will just keep my eye on the temp needle :)

I would up bolt the top of the radiator and move it back towards the motor a little. Then look into the gap between the radiator and the aircon condenser. Quite often a lot of carp gets trapped in there that stops air going through the radiator.

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  • 3 weeks later...

Update - I cleaned up the rad from outside (was relatively clean anyway); removed some dirt which was trapped between air-con condenser. Today (22degC) - was not showing overheating when AC ON - however - I went for a small vineyard up-hill section - and after cca 2 km up hill the needle started to climb again - so I stopped and went down :) Not happy about it. :(

Will check if the radiator gets warm/hot equally all over the place after the thermostat opens.

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  • 2 weeks later...

radiator is OK - no cold spots, nice temp decrease from inlet towards the outlet.

But I seem to have found the culprit - I had approx 70% (or more) anti-freeze there - so took 5 liters out, put 5 liters of distilled water - and no signs of overheating anymore. the manual says that going over 60% greatly impact the efficiency of the cooling systems - and it really seems to be the case. :rolleyes:

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