Hood Vents

ehall said:

If your cooling system works right, hood vents will not change engine temp at all

They will change hood temp

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Somewhat correct, An engine can still overheat from being worked to hard and not having enough air flow.

XJ platform is a prime example, the hot air under the hood has no path to escape at slow speeds. Highway speeds push fresh air in, but low speed the fans cant do the same thing. If you give the heat a place to escape you can fill the engine bay with cooler air from the fans and continue the flow out the top.

So instead of overheating you will stay at operating temp.

csailors said:

Somewhat correct, An engine can still overheat from being worked to hard and not having enough air flow.

XJ platform is a prime example, the hot air under the hood has no path to escape at slow speeds. Highway speeds push fresh air in, but low speed the fans cant do the same thing. If you give the heat a place to escape you can fill the engine bay with cooler air from the fans and continue the flow out the top.

So instead of overheating you will stay at operating temp.

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No sure why you single out XJ's.

The scenario you describe applies to 99% of the vehicles out there.

csailors said:

Somewhat correct, An engine can still overheat from being worked to hard and not having enough air flow.

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But that's heat transfer from the engine via coolant through the radiator. Ambient temperature of the engine bay has very little effect on the effectiveness of the radiator's ability to transfer heat from the coolant to the incoming cold air. Ideally, engine temperatures should not change at all.

TRCM said:

No sure why you single out XJ's.

The scenario you describe applies to 99% of the vehicles out there.

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I do just be use that is the platform we are talking about, compared to an old 79 k30 the engine bay allows for more heat exchange because of the amount of open area.

ehall said:

But that's heat transfer from the engine via coolant through the radiator. Ambient temperature of the engine bay has very little effect on the effectiveness of the radiator's ability to transfer heat from the coolant to the incoming cold air. Ideally, engine temperatures should not change at all.

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I agree with this statement to an extent. However to pull cool air through the radiator, cooling the engine coolant to cool the engine the air being pulled in needs somewhere to travel out also. The XJ engine bay has no good outlet do to its compact packaging (at slow speeds). So even a proper cooling system after time will still get warmer than most people think is ok. Hood vents give the cooling air traveling past the radiator an outlet, also drawing out heat buildup under the hood.

This helps with heat soak, this also helps with keeping a more consistent operating temp and even from the e fan cycling on.

A correctly functioning cooling system is key. But hood vents have there purpose that I believe is needed in the XJ platform.

When I installed hood vents on my 2000, I immediately noticed that the dashboard temp gauge normal operating temp moved down about 2 times the width of the needle, not that much, but enough to see. Also, when the e-fan turns on, it runs for a shorter amount of time. The 00-01 e-fan only runs based on coolant temperatures, it does not cycle ON/OFF when the A/C is engaged.

On the 4x4 trails, maximum observed temperatures are lower than without hood vents.

So when the AC/defrost is engaged the e-fan does not run continuously on the 00-01?