O7 d40 zero boost pressure.

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Daughter's D40 again ....

Before shift running fine ... getting off shift after midnight and zero boost pressure reading on aftermarket gauge and car won't go over 25k.

She tells me the airbag light has been flashing for a couple of weeks, but no engine check light on ...

All hoses to the bcv look intact, although aged.

Is sudden complete failure of the bcv likely? Or some other issue that we should be ruling out before replacement? Actuator failure?

Cheers.
 
The small black boost control solenoid (mounted on the intercooler hard intake pipe) is a common failure point, but so are the vacuum hoses. There's a simple test: if the hoses are in good order (you could change yours - 3 metres of 3mm hose from Supercheap will do the job) then connect the vacuum feed across from the top of the motor directly to the actuator and see if boost is produced.

If there's no boost: two possibilities. Either the brake booster has a fault (it is vacuum-driven as well, but not a common fault) - and you'll know it, because the brakes will not be as effective - or the turbocharger has a fault.

If there IS boost - most likely culprit is the BCS. Nissan flog 'em for over $200. You can pick them up on eBay for about $35. I'd normally add a suggestion to look at a part from a wrecking yard but given the price of the eBay ones (and that there's been very few complaints about these above and beyond the ones Nissan supply) there's no point, just go new, or replace with a Tillix or Dawes valve.

The turbocharger fault might be stuck vanes (rebuild/replacement) but it might just be a leak in the actuator's diaphragm too. Test this by getting a new piece of hose, connect one end to the actuator and suck on the other end (yes with your mouth, not with a vacuum cleaner/pool cleaner or a Chery A1 - which sucks more than almost anything else) and watch the arm underneath the actuator, which should move as you create vacuum. If you can continually draw through the hose, it's a leaky diaphragm (has happened, but not often) and replacing the actuator will fix it. If you can't, then the vanes may have siezed (this is quite rare).

Tillix/Dawes valves: personally I have a Tillix valve on mine, it's reaction-driven, no input from the ECU, and you need to adjust it properly to prevent overboosting which the Garrett 2056V can easily do (this WILL engage the check engine light and throw a boost sensor high error - I think it's P0238). If it's WAY too high and the ECU hasn't triggered, it's quite possible (yes I've done it) to pop the high pressure hose off (mine slipped off the intake manifold as we were climbing a hill on the Putty Rd towing the caravan late one night).

Now, with that endorsement and warning out of the way, I have purchased a cheap BCS from eBay (it should be here during the week) because I want to see for myself if they're any good.
 
Thank you for the extensive and detailed reply. Have been playing a bit today and have a few revised observations:

The zero boost reading may have been due to an overtightened cable tie on the aftermarket gauge's hose - now reading boost again after playing with hoses and things in the engine bay.

The wiring for the temp gauge that is paired with the boost gauge had dropped onto the exhaust and the earth wire was earthing on the exhaust ... so don't think that should have been having any effect.

Picked up a Goss bcv and put that in, which initially made no difference, but suddenly came good and throttle/boost response was normal again. Test drive made it about 3 minutes down the road before returning back to very sluggish, although I was able to cruise along at 40km/h whereas the daughter reported a 25km/h top speed on the flat.

Will check the actuator operation as you suggest, while I am replacing the vacuum hoses, in case my fiddling temporarily closed a partial leak, and will see what happens.

If that has no effect, will swap my obd reader out of mine into the daughter's and see if there are any error codes stored.

Couldn't identify the bcs in the hard intake that you mention, but will have another look.
 
Ahh ...ok. bcs=bcv. Have played around with NA petrol engines for years, but turbo and diesel is a bit of a new beast for me.

The bcs in the daught's is on a bracket mounted off the chassis rail underneath the airbox to turbo pipe ... nestles in a rubber block that pushes onto a prong on the bracket.

Actuator appeared to be functioning fine, btw ... so replaced all 5 lengths of vacuum tube across the top of the engine bay, and so far, so good. One of the old pieces was getting extremely powdery at one end in particular, so it is possible that this was leaking partially, and my fiddling while replacing the bcv had temporarily sealed the leak. A couple of short test runs and it seems fine to me; I hadn't driven it before today, so the real test will be when the daught gets off shift I suppose.

I am sure some detailer at some stage has contributed to the degradation of perishables in the engine bay - several areas where exposed wires show evidence of insulation cracking, and the top of the rad overflow and washer bottle have gone all powdery as well.
 

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