When I get home from driving around, I'll pull into my garage, pop open the hood (bonnet) and let the car ('94 V-12) cool off. I figure all that residual heat can't be too good for what's under there and best to just let it out.
Lately some OBC's (Owners of Boring Cars) have commented on how silly it is to do this. When I explain my reasons, all I get in return is "Pffft".
So, I'm wondering, is opening up the hood;

A) A good idea, and the OBC's will just never understand the intricacies of XJS ownership.

B) Kind of ****, but can't hurt.

C) Not a good idea. Jag engineers, realizing it's a close cowled engine, designed it to cool off slowly. Don't mess with the system.

Any thoughts or comments?

Thanks,
Bill

 

When I pull in the Garage, I always pop the hood/bonnet. I see no reason why this would not be a good idea...

 

Actually once I touched the intake manifold to see how it went cold.
When I left the hood open it cooled down really quickly, but when I left the hood closed it was hot even after an hour.
It can't be a bad idea to keep the hood open after engine shutdown. What is boring is their thought and attitude, not their cars :P

 

I vote "B"----kind of ****, but can't hurt

Personally I never did it when I owned a XJS V12 but, hey, if it feels good, do it !

Cheers
DD

 

I one up you, I have a large stand fan I put next to the fender and aim at the engine for an hour or so.

 

If the Jag engineers had their way, they'd insist that the engine be kept running 24/7. It's the heat/cool cycles that induce the most wear, not running at constant temp.

Allowing it to cool with the hood open might make an owner feel better, but the car couldn't care less.

 

Why do you care what OBCs think??
If they don't own a Jaguar their opinion is not worth consideration in these matters!
(';')

 

Based on some of the engineering in my 85 v12, the jag engineers would be wise not to comment. I do exactly the same. I would rather have my friend think I am silly than knacker an engine. Besides most of them don't have a clue!


Unless they own an XJS they will never understand one, I call it XJSealousy.

 

Correct me if Im wrong. By opening the hood you allow the block/heads to cool quicker. I think this would be good if the valve seats are still hot because they would be expanded by the heat while the heads have contracted by being cooler. This should keep the seats tight in the head.

 

Here's my take.

If you are sure that you have good coolant flow in-and-thru the cylinder heads and have reasonable coolant temps with the engine running then the coolant will absorb the post shut down heat without any problem and your valve seats will stay in place.

If you lack either of the above then you have a much greater chance of dropping a valve seat ....engine running or engine stopped....and ventilating the engine bay probably won't prevent a dropped seat.

<shrug>

Carry on


Cheers
DD

 

Is there a precedent of seats falling out or than in overheating events?

 

Doug has pretty much nailed it i think. Nevertheless the placebo effect soothes my nerves

 

I think that Doug is most probably correct about the valve seat drop prevention effect being minimal. But I have always popped the bonnet because there are loads of other things under the 'hood' that most certainly will be helped by not being cooked. A poster not so long ago actually measured the under hood temps after shutdown and they were very high indeed, I seem to recall. To my shame I cannot recall who it was.

On my list of things that are helped are: all the looms, the ignition amplifier, the diaphram in the adv/ret capsule, the aircon flexibles, the vac tubes, the many rubber items. Quite enough to make it worthwhile, for me, anyway. Then there are all the admiring glances you get too!

Greg

 

The underhood temps after shutdown are roughly equivalent to those present while driving. Opening the hood to cool things faster after shutting down the engine would be equivalent to stopping the car sooner.

Since there's no known issue with running these cars to very high mileage levels, what problems are being fixed?

 

I believe that these things are being helped
On my list of things that are helped are: all the looms, the ignition amplifier, the diaphram in the adv/ret capsule, the aircon flexibles, the vac tubes, the many rubber items. Quite enough to make it worthwhile, for me, anyway.

I do not believe that, on my car at least, the under bonnet temps are the same in motion as they are when the car is stopped. I think that heat soak into various 'soft parts' is a serious "life-ing" issue on the V12. I wish I could find the post that included the temp figures from the poster who actually took the trouble to measure all these things.
Anyone got an idea? it was (I think) about 6 months ago. USA poster.

Greg

 

Found it. Here is the graph, and these are engine coolant temps, it is reasonable to deduce that underbonnet air temps will also rise considerably, I believe.



Here is the thread; it was started by JCR from Indiana. Great stuff, and it does indicate post shutdown temps rise quite a bit, let alone in certain hotspots:

https://www.jaguarforums.com/forum/x...-graph-111523/

 

No offence but in all seriousness, where on earth are you getting your data???? Common sense would suggest that a car in motion has airflow though the engine compartment and underwood temperatures would vary based on OAT, speed driven, sun, road temp etc. To simply state "the underwood temps after shutdown are roughly equivalent to those present while driving" is no where near a fact. I'm just as stumped by " there's no known issue with running these cars to very high mileage levels". Says who??

 

The next logical question (well, for me, at least) is : "Is if 210ºF is significantly hot?" . At what temp do the hoses, wires, etc begin to suffer particularly badly? Do we know? Is 210ºF significantly more than most other cars, the ones that we don't feel compelled to ventilate? (in other words, any car except a Jag V12! )

If 210ºF is significant how long does it take to drop to a less damaging level with the hood open versus the hood closed? Thirty minutes versus fifteen minutes? Or....what?

If engine bay temps are 180-190ºF (if I'm reading the graph right) when running then is the increase 210ºF really problematic?

Is thirty minutes @ 210ºF during cool down particularly more damaging (and therefore worrisome) than, let's say, a two hour baking along the highway at 180-190ºF?

Curious minds want to know! Or, maybe not

I dunno, gents. It seems to me that the very best way to preserve your hoses and wires, and to ensure that the valve seats stay in place, is to park the car in the shed and avoid running the engine at all costs.

I think ventilating the engine bay is a feel good thing and I remain firmly in the "if it feels good, do it" camp.

OT: Did I mention mention my recently acquired love affair with Baldwin oil filters? I'm normally very casual about oil filters but I finally found one that I....you guessed it....'feel good' about !

Carry on.

Cheers
DD

 

What the graph indicates is COOLANT temps, not underhood AIR temps which is the area of concern. All cars experience this effect, called 'soakback', after shutdown. The effect is far less pronounced on these cars than others I'm familiar with. I'm surprised.

Engine stops producing heat as soon as the key is turned off. The heat that would normally be shed quickly and efficently into the air via the radiator and unfortunately into the engine compartment while the engine is running is now conducted exclusively through the engine exterior, also heating the engine compartment. The net effect is the same.

I went through the same discussion in my other car hobby where the belief was similar. Actual measured engine compartment temps after shutdown were no higher than when the car was running.

If these cars had remotely mounted radiators that did not dump heat into the engine compartment, the discussion would be very different. There again, there might be a side discussion on the effects of the engine assembly operating at too low a temperature.

 

Generally, oxidation and degradation go faster when hot. So some advantage must be there.
But I don't think it's significant... I believe many of the parts' lives won't last so long anyways, so Doug is right.
I still feel comfortable to unlock the bonnet to release the heat, though.