Negative Boost (Vacuum)

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Black_Outlaw

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Learned forum members.

I am, under acceleration, often seeing NEGATIVE boost. It will quite literally go from 18psi to -0.86. At this point i can lift slightly and gain POSITIVE boost, or I can keep going (and feeling the loss of power) and then it will power up again.

Things to rule out:
Replaced EVERY SINGLE VACUUM LINE - and cable tied them on
Replaced Boost Control Solenoid
Cleaned inter-cooler and MAP sensor
Re-clamped ALL boost lines to ensure no leak
Cleaned intake
Liqui Moly the crap out of every system.
Lubricated the Actuator Arm on the turbo.

I can confirm actuator is "bobbing" at idle.

So - thoughts or "possibilities". My guess is the VNT system - but it makes no sense it going from Boost to Vacuum and back..
 
Usually Both.. The car "moans" (like the engine is being retarded in timing to reduce torque) and loses power - then comes back again.

My thoughts are the VNT is sticking and boost production stalls, at which point the engine sucks the intake system dry - and the MAP sensor see's the -'ve pressure.
 
I think the next thing to try is an examination of the turbocharger itself an the impacting sensors. If the actuator is bobbing at idle it means you not only have vacuum, you have a working solenoid and therefore a functional control system. Barring poor data from the MAFS, TPS and NPS, the turbocharger SHOULD be commanded correctly. If it's commanded correctly, then it must be the turbocharger's primary shaft stalling. My own turbo did this same thing and replacing it with a Forefront Industries job fixed it.

However, there are other things at play. The MAFS in the air intake which will tell the engine both the temperature and velocity of the incoming air basically tells the ECU how much air it has to play with.

The TPS obviously tells the ECU how much demand you're placing on the car. If that signal is erratic (possible bad connector on the throttle pedal) the ECU might do this.

The NPS tells the ECU whether or not the car is in gear and will not allow high engine demands in neutral. Uncommon to hear of NPS trouble with D40s but not unheard of.
 
I have a log of this mornings drive to work, which I have dumped into Excel.

MAFS seems consistent and backs up similar numbers for Boost and Revs
I am getting 20.8psi at just under 1600rpm with a stomp on the pedal from Idle.
Fuel Rail Pressure seems good and consistent. From 4750 at idle to 23000 at full tilt
AIT is low and slowly rises as both AT or slow-crawl / stop acts on the incoming charge
TPS ranges from 0 - 100. While it shows I am doing XX km/h and YYpsi of boost, that only tells me i've throttled off but still traveling and the turbo winding down. And most all 0 points it has a low Fuel Pressure, which tells me that the engine is not loaded.
No sign of Negative boost this morning. I need a log with one to compare whats actually going on.

At this point Im throwing it in the Stuff-It bucket and calling it #d40life.. There is nothing standing out like the proverbial Dogs Bollocks that would suggest any of the above.
 
Maybe boost reading too high? Possibly boost is reading out of range and off scale so shows as negative and ecu restricting power to return to range. Saw negative boost on scangauge when too much Liqui Moly used in fuel (my bad) and it was boosting its nuts off with readings >25psi. It would shoot up then go negative. Fresh fuel and all good again. Maybe boost sensor issue?
 
I have same issues as BlackOutlaw...
ive been trying to insert a.pic of a screen shot of my TorquePro Turbo Boost dial when it hits 21psi Boost and then the needle winds backwards to the -26 Vaccum.

why is this hapening ??
 
Vacuum is easy enough to generate if you want to prove it's actual vacuum - get the revs to about 3500-4000 on a downhill (low gear is good) and back off the throttle. Immediately, the turbocharger will spool down and create a restriction in the airway, boost pressure will fall into the negative.

If it doesn't react specifically like that, it might be a sensor malfunction - for instance, if you're decelerating as described and it flickers to -26 and back to a low vacuum. -26 vacuum is close to 2 atmospheres below. Your Torque must be set correctly if it's reading 21psi (which is a fraction high for normal driving but possible under full throttle and high load). The sensor gets a 5V input and based on the pressure adjusts the return voltage (variable resistor based on pressure). -26psi might be its bottom limit (zero volts returned). To test this, unplug the boost sensor. What does Torque read? If unplugging it works, clean contacts.
 
I get it accelerating. The other night it was boosting to 22psi and then -11psi and the smoke would POUR OUT the exhaust. you could feel the car drop power - Back of the pedal, reset and away it went... until 22-23psi and then -11 psi..

Next day - would not fault. 22psi all day..

Im seriously over this Navara!
 
**UPDATE**

As some would know, my D40 went in for cold start stalling (basically start / stop - unusable) and it was found to be the pump - insufficient pressure at idle. Pump removed, new seal kit installed, replaced - problem solved..

HOWEVER - it has *also* solved my issue of NEGATIVE BOOST. Every condition where I could get the car to go into the -VE, it now just pulls hard. Boost comes a strong as too the power delivery.

So, having had nothing else done on the car, except that of the pump, I am going to assume that the pump was under-supplying fuel for the load requested, which in turn was spinning the turbo slower and the ECU trying to correct for lack of boost?

Take what you will from that.
 
Sounds like a sensible conclusion to me. I'm having issues with mine negative-boosting at the moment, the gearbox is throwing a TPS error on me and my mechanic has said he's highly suspicious of my TPS - a replacement goes in on Wednesday.

If that doesn't work, a fuel pump rebuild will be on the cards.

Just to be specific about the symptoms with mine: upon light-to-moderate acceleration (around 2500rpm), my boost level reaches 22.1psi then flips to -13 or -14psi for a few seconds then rises to 15-16psi as the engine is hauled back to 2,000rpm. I have applied full throttle at this time and it's done very little. I've had this effect happen when the cruise control is on (no throttle application) so it's not specifically the throttle itself, but the command as perceived by the gearbox. Release the throttle/CC and reapply and it goes hard again for about 10-15 seconds then repeats. Was a bit exciting hauling the caravan back from Grafton the other week!
 
Tony. I spoke with my mechanic today on an unrelated matter and we got talking about my car, and the MASSIVE difference since the new pump seals.

He informed me that also carried out a manifold clean with some Eurpoean Spec cleansing machine they had purchased a few months back. He tells me he has seen some *very positive* outcomes from cleaning the intake manifold. He was sceptical and "loaned" the unit for a few months. Every car they cleaned, had positive gains with both fuel economy, power delivery and performance. He since purchased it and uses it daily.

Now, whilst I am not suggesting it was this alone, it might be an interim option for you. My car now smokes a whole lot less on both take off, and acceleration - and the negative boost has gone. Also, my turbo used to flutter A LOT on part throttle back off. This has also now ceased. It's as if the engine can now "consume" the excess.

upon light-to-moderate acceleration (around 2500rpm), my boost level reaches 22.1psi then flips to -13 or -14psi for a few seconds then rises to 15-16psi as the engine is hauled back to 2,000rpm. I have applied full throttle at this time and it's done very little.

When it did this, did the engine appear to "groan" as if it was being retarded? Also PLUMES of black smoke out the exhaut. This was my symptom especially when the car was under load, going up a hill.

Just feeding back my experiences, as this community has helped me plenty.

If you want more info, happy to chat anytime - or even pass on the details of my mechanic.
 
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The engine did sound like it was being held back. Didn't notice a whole lot of smoke though and I looked for it on numerous occasions - had plenty of time to do that hauling the caravan back from Grafton to the Hunter Valley!

I'll ask my guy tomorrow about the intake manifold.
 
My problem was solved - interesting repair. Throttle body, TPS and the vacuum solenoid. Willing to bet the throttle body wasn't a problem, or the TPS ... another one I'm putting down to experience. If the turbocharger starts doing odd things, look at its control ... not elsewhere.
 
The vac solenoid is the boost control solenoid, same horse, different name. Has 3 vac hoses connected to it and mounts on the back of the intercooler's intake pipe.
 

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