Blow Off Valve (BOV/BPV or maybe Relief Valve)

[Bonsai]

A BOV would typically release after building boost, when the system is releasing excess air, not when you are building boost or trying to accelerate. I had what sounds like a similar problem in a GTR. I parked it in the driveway, fiddled with the gas cable to wind the engine up and tracked the leak that way. I found that there was a loose hose on the intercooler that was making noise.

[Airgonzo]

I don't think my problem is the PRV because it's not a hissing sound at all, it's a hooting sound like an owl and it only does it at low boost and RPM.

[jessef]

exhaust leak?

[pajerry]

I've made a couple threads over the years on both delica.ca and the UK pajero forum hunting down the same sound and have never found a reasonable answer or solution.

Same noise though, sort of a hooting, only under light load going under 60km around 2000 rpm. Used to happen more when it was a colder humid morning. After EGR blank and boost up it only happens time to time. It seems really inefficient when it is happening and is probably robbing some mileage as you mentioned. I've tried rebuilding the intercooler, checking all the turbo lines for leaks, tightening etc. with no luck.

Giving it more thought.. after having the muffler and resonator removed it does not do it anymore, or not that we've noticed.. I only hear turbo spooling whistle and a good throaty rumble..

Japanoid tried to tell me it was the turbo needing a rebuild and charged me to drive it around the block.. not sure if that is the case, but two years later and it actually did it less than it used to. (they also told me the oil from my leaking valve cover gasket was a leaky turbo and the sound was all related and would cost big $$$ to fix... )

[FalcoColumbarius]

What Wikipedia has to say:

Blowoff valve
From Wikipedia, the free encyclopedia

A blowoff valve (BOV) or dump valve is a pressure release system present in most turbocharged engines.

Its purpose is to prevent compressor surge, and reduce wear on the turbocharger and engine. Blowoff valves relieve the damaging effects of compressor "surge loading" by allowing the compressed air to vent to atmosphere, making a distinct hissing sound, or recirculated into the intake upstream of the compressor inlet.

Definitions

A compressor bypass valve (CBV), also known as a pressure relief valve or diverter valve, is a manifold vacuum-actuated valve designed to release pressure in the intake system of a turbocharged vehicle when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.

A blowoff valve, (sometimes "hooter valve", not to be confused with a bypass valve) performs the same task but releases the air into the atmosphere instead of recirculating it. The blowoff action produces a range of distinctive hissing sounds, depending on the exit design. Some blowoff valves are sold with a trumpet-shaped exit that intentionally amplifies the sound. Some turbocharged vehicle owners may purchase a blowoff valve solely for the auditory effect even when the function is not required by normal engine operation. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere.[citation needed]

Blowoff valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. When the throttle plate on a turbocharged engine closes, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor. The compressor wheel slows rapidly and may even stall, and the driver will notice a fluttering air sound. The rapid slowing or stalling stresses the turbo and imparts severe turbo lag if the driver accelerates immediately after the surge event.

Disadvantages

In the case where a mass airflow sensor (MAF) is used and is located upstream from the blowoff valve, the engine control unit (ECU) will meter out excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. The engine then briefly operates with a fuel-rich mixture after each valve actuation.

The rich mixing can lead to hesitation or even stalling of the engine when the throttle is closed, a situation that worsens with higher boost pressures. Occasional events of this type may be only a nuisance, but frequent events can eventually foul the spark plugs and destroy the catalytic converter, as the inefficiently combusted fuel produces soot (excess carbon) and unburned fuel in the exhaust flow can produce soot in the converter and drive the converter beyond its normal operating temperature range.

One approach used to mitigate the problem has been to reduce the boost pressure, which reduces the required venting volume and yields less charge over-calculation by the ECU. The air can also be recirculated back into the intake, a typical stock setup for cars with an upstream MAF sensor. The situation can also be corrected by switching the fuel metering system over to a manifold absolute pressure sensor, a conversion that usually requires a compatible aftermarket ECU or piggy-back fuel controller. The MAP sensor monitors the absolute pressure in the manifold at all times and will correctly detect the change that occurs when the valve vents, allowing the ECU to reduce fuel metering accordingly.

Operation

A blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, the relative manifold pressure drops to below zero bar and the resulting pressure differential operates the blowoff valve's piston. The excess pressure from the turbocharger is then vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.

http://en.wikipedia.org/wiki/Blowoff_valve

[psilosin]

The L400 does not have a blow off valve, unless a previous owner added one, and it would be next to useless on an automatic, so the probability of that is very very low unless it was for bling bling purpose only. Even on manual a BOV would be useless since the L400 has no throttle in the usual sense of the word.

Some diesels like my Hilux Surf have a butterfly valve in the intake that simulates the effect and is used mainly to make shutdown smooth and soft but can be exploited on the manual to make a BOV function. The shutdown on the L400 is anything but gentle so it doesn't have that...

[Airgonzo]

Jesse, that could be a possibility. It had a new exhaust put on it right before I bought it. I'll look into it but it's hard to find an opportunity when I have someone around to work the pedal while I poke around looking for source. Made even more difficult cause I have to catch it when it's acting up and symptoms are intermittent.

[jessef]

exhaust pinholes, gasket tears and splits can make all kinds of interesting sounds.

The pressure release valve facing backwards on the intake manifold will only pop open at high boost so it does not apply to neither of you guys.

If you're hearing a 'howling' type of noise before boost building/low rpm, then I can't think of any other place to look except for the exhaust system.

Armchair mechanic guessing, I would say 99.9% look there.

[pajerry]

You might be on to something there. It seems to be from that side of the engine, but its hard to test while parked as it only happened under load. Im just picturing driving down the road at 60 with the hood up and another guy half in with just legs sticking out...

Not sure how the resonator and muffler could have caused that though.