Idiot under bonnet asks ........

[Mr Cerbera]

..... what is the black, rubber tube that exits the rear of the left-hand (passenger) airbox please ?

Is it some type of air-pressure sensor?

Ta !

[Nickccc]

Yep, barometric pressure sensor hose.

[Mr Cerbera]

Thanks Nick.

What does the connection (or lack of) provide ?
Is it summat for the ECU ?

I presume it is measuring Negative pressure in the box. There is also a sensor at the front of the left-hand, air-filter-feed housing. Is there some differentiation between these two readings that is used to calculate summat ?

Edited by Mr Cerbera on Thursday 1st September 11:01

[Tanguero]

No - its measuring the atmospheric pressure. There is no negative pressure in the air box as one side of it is open to the atmosphere (via the filter). The other sensor is the air temperature sensor. The ECU uses the barometric pressure and temperature to calculate the air density (and therefore the amount of oxygen) in an induction charge and adjusts the fueling to suit.

[Mr Cerbera]

Pete, you're a bloody genius !!

(Just one little thing though) When you hit the pedal and the throttle valves open, isn't there a negative pressure in the manifold ?
Just asking to confirm my tiny piece of motoring knowledge, Ta !

Why does the ECU need barometric pressure from the airbox and not just pick up its own environmental pressure ?

Edited by Mr Cerbera on Thursday 1st September 11:17

[Tanguero]

The only way that the pressure will be lower in the air box, is if the engine is sucking out more air than can get into the air box via the filter. That can happen on some engines (Jag V12 in an XJS strangles like that) but I am fairly sure that the pipes to the airboxes on an AJP are big enough that there is little if any depression.

Of course a badly clogged air filter limits the incoming air and affects performance at WOT for exactly that reason. I suspect that is why the barometric pipe goes to the airbox rather than the open atmosphere.

[Mr Cerbera]

Tanguero said:

The only way that the pressure will be lower in the air box, is if the engine is sucking out more air than can get into the air box via the filter. That can happen on some engines (Jag V12 in an XJS strangles like that) but I am fairly sure that the pipes to the airboxes on an AJP are big enough that there is little if any depression.

Of course a badly clogged air filter limits the incoming air and affects performance at WOT for exactly that reason. I suspect that is why the barometric pipe goes to the airbox rather than the open atmosphere.

Genius !

Ta !

[vatanen]

Ah, I see what you did there!

- Lee

[Thom]

Tanguero said:

No - its measuring the atmospheric pressure. There is no negative pressure in the air box as one side of it is open to the atmosphere (via the filter). The other sensor is the air temperature sensor. The ECU uses the barometric pressure and temperature to calculate the air density (and therefore the amount of oxygen) in an induction charge and adjusts the fueling to suit.

So the MAP sensor gets its signal from before the throttle bodies?

[jamieduff1981]

Is this tube normally sealed in? Mine is just kinda loosely poked in to the hole.

[Tanguero]

Thom said:

Tanguero said:

No - its measuring the atmospheric pressure. There is no negative pressure in the air box as one side of it is open to the atmosphere (via the filter). The other sensor is the air temperature sensor. The ECU uses the barometric pressure and temperature to calculate the air density (and therefore the amount of oxygen) in an induction charge and adjusts the fueling to suit.

So the MAP sensor gets its signal from before the throttle bodies?

There is no MAP sensor. It is all calculated from throttle position.

[Tanguero]

jamieduff1981 said:

Is this tube normally sealed in? Mine is just kinda loosely poked in to the hole.

Yep - thats right.

[Mr Cerbera]

When I reassemble mine I usually put a dab of sylkaflex on the outside of the plastic insert - just to ensure that the reading does, in fact, come from inside the airbox.

[Thom]

Tanguero said:

There is no MAP sensor. It is all calculated from throttle position.

Thanks for that, so if the main load source is TPS, there's still got to be a MAP sensor if you say the ECU measures atmospheric pressure through the line going to the left inlet plenum, no?

Edited by Thom on Tuesday 20th August 17:41

[jamieduff1981]

Tanguero said:

jamieduff1981 said:

Is this tube normally sealed in? Mine is just kinda loosely poked in to the hole.

Yep - thats right.

Sorry I'm feeling thick... which is right? My car or that it should be sealed in to the airbox?

[Mr Cerbera]

jamieduff1981 said:

Sorry I'm feeling thick... which is right? My car or that it should be sealed in to the airbox?

Put it this way, in true, TVR style, you have any option you require

[Tanguero]

jamieduff1981 said:

Tanguero said:

jamieduff1981 said:

Is this tube normally sealed in? Mine is just kinda loosely poked in to the hole.

Yep - thats right.

Sorry I'm feeling thick... which is right? My car or that it should be sealed in to the airbox?

Just stuffed into the hole is the way mine has always been.

[jamieduff1981]

Thanks chaps

[Tanguero]

Thom said:

Tanguero said:

There is no MAP sensor. It is all calculated from throttle position.

Thanks for that, so if the main load source is TPS, there's still got to be a MAP sensor if you say the ECU measures atmospheric pressure through the line going to the left inlet plenum, no?

Edited by Thom on Tuesday 20th August 17:41

No, there is no MAP sensor. The barometric pressure sensor measures ambient pressure in the air box, which for the reasons above is not measuring manifold depression in the way a MAP sensor would.