Positive air pressure in air box at idle

[Andyfive Member Find all posts View Profile Close Andyfive]

I was cleaning under the bonnet today with the engine running. I noticed that there was Positive air pressure in air box at idle. Ie air coming out the air box from the drain valve and air filter sighting hole. It disappears when you raise the revs.

Is this normal?

[Early Member Find all posts View Profile Close Early]

That's funny...Everything before the turbo impeller should be negative pressure because the engine is sucking in air. Unless there is a Venturi effect through the restricted opening of the airbox?

[Mule Member Find all posts View Profile Close Mule]

Cant help but think no. How could it possibly get to be above atmospheric. Curious to know if you solve the mystery.

[Rusty62 Advanced Member Find all posts View Profile Close Rusty62]

Most likely caused by the pulsing of af he 4 cylinder diesel. The catch cans tend to pop the relief valves at idle as well.
I am not an expert but wouldn't worry about it.

Rusty.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

The only things likely to cause pressure would be as above, pulsating of the air flow
due to the induction process, ie the cylinder draws in air as the valve shuts the moving
air just can't instantly stop so you get a small amount of pressure build up which exits
the valve though I haven't noticed this on mine 120 but could be normal, note a snorkel
would probably amplify the arrect.

Second blow back from the sump via the the breather but I would think there would be an
oily exhaust smell to it and it would most likley dirty the cleaner element.

[Andyfive Member Find all posts View Profile Close Andyfive]

I have a snorkel too. You can sense the air coming out at the snorkel too.

[Bushbasher Out of control poster! Find all posts View Profile Close Bushbasher]

Not wanting to be disbelieving but are you sure it is air coming out and not the sensation of air being sucked in? What device are you using to detect the flow of air? Air flowing away from your skin can also feel breezy. The venturi effect of air being drawn through the turbo won't allow any backflow to the airbox so I think the pulsing idea is out. The amount of air inside the airbox, intercooler and piping would be many times greater than air consumed as each cylinder can only consume 1/4 of 3 litres of air per cycle. Overlap of valve openings and closings would also dispel the pulsing theory IMO. That's my understanding of physics, I could be wrong.

[krypto Avid PP Poster! Find all posts View Profile Close krypto]

Agree with Bushbasher, how have you checked the flow direction? Suggest you use some light paper or tissue to double check as it doesn't make sense.

[Andyfive Member Find all posts View Profile Close Andyfive]

I checked it with my trusty hand. You could tell the difference from idle to rising the revs where it then drew air in. I was with a mate who used to be a mechanic. He couldn't explain it either.

[Bushbasher Out of control poster! Find all posts View Profile Close Bushbasher]

Make sure the positive ion diffuser/rust inhibitor is off when you check.

[krypto Avid PP Poster! Find all posts View Profile Close krypto]

Make sure the positive ion diffuser/rust inhibitor is off when you check.

Have you had the car from new? Maybe one of these was fitted http://www.speedcafe.com/2011/10/12/hdt-polarizer-set-to-return-to-australian-roads/

[Andyfive Member Find all posts View Profile Close Andyfive]

Yeah I have had it from new. I found the flux capacitor was playing up.