D40 Turbo Overboost Issue

Apologies if this ends up a bit of a tome, but I’ll attempt to fill in all the relevant (?) history.

The vehicle is a 2006 D40 Navara, with the 2.5 turbo diesel engine, and it’s done around 250000km The problem first arose in early 2022, and is triggered when the vehicle is under sustained load for a period. The vehicle is often used to tow a loaded horse trailer, and the problem typically happens with long, uphill, open road sections where there is high power demand for maybe 10 seconds or longer. Now that we know what triggers it, the fault can be created on demand. When the problem occurs, the vehicle has an instant loss of power, similar to limp mode, but power can be restored by lifting and reapplying the throttle. It is possible to generate the fault without a load, but is a bit harder, and doesn’t happen in normal use. Minimal faults were being logged, but I think I now understand why.

The vehicle was away from home when the first occurred, so was not handled by our normal garage. When they first looked at the ECU, they noted that there were MAP sensor and boost control solenoid faults present, but have no other details. They tested the supply and MAP sensor earth and all OK, but problem persisted. They replaced what was invoiced as a “41-540 Turbo Dies.pressure valve” (I suspect there is a typo there, as don’t know what this is other than it wasn’t cheap!), and installed a new boost solenoid sensor. I recollect that they also replaced something expensive, but were somehow able to return this after verifying it made no difference, but don’t know what this was. When the vehicle was returned, the problem was supposedly resolved.

But No! About 4 weeks later, it was given to our normal garage to do a full service, as a precursor to try to get to the bottom of this. They noted there was a boost sensor P0238 code present, but were not certain if this was historical. They were supposed to check/replace the fuel filters, but missed the pre-filter. They road tested the vehicle with live data running, and found around 200KPa of boost, which is at the upper limit, and possibly indicated overboost, but other than this they couldn’t fault it.

After another 4 weeks it was back again. By this stage we had figured out how to create the fault on demand, so expected there would be mass of faults logged, but they only found a single P0234 (another overboost code). They removed and checked the DPF. This time I was able to demonstrate the fault to them, and their suspicion turned to the fuel supply. They then “found” the pre-filter they had missed earlier, it was full of muck, so replaced it. At this point the problem appeared to have been resolved. That was June 2022, and the problem did not reappear for 2 years.

Mid way through this year, the problem reoccurred, and as before, it’s triggerable on demand, but really only evident under the load conditions mentioned earlier. It’s been back with a third garage, and after rechecking everything he can, he is convinced that the problem is the turbo itself. I admit to being a bit sceptical, particularly as replacing this without a smoking gun is an expensive hope.

To try and understand this better myself, I’ve got an ODB scanner running, and have been playing with generating the problem, and seeing what is logged. Even time I trigger the fault, I have a pending P0234 code logged, which will disappear after a period (supposedly after a restart, but in the Navara it takes longer than that – just seems to be based on a period without the fault recurring). These all indicate an overboost issue. I didn’t understand about pending codes earlier, but this presumably explains the earlier scenarios with no apparent faults being logged.

I feel comfortable that the ECU is detecting an overboost situation and is taking remedial action, but the question is why. I’d appreciate any thoughts, ideas, things that should be checked, whatever. I’m not averse to replacing a turbo, but would like something definitive to pinpoint this as the issue. I do have some questions though!

• Can this be caused by the turbo itself? I’ve read on this forum about potentially stuck vanes on these turbos which could presumably do this.
• I couldn’t find anything in the OBD sensors that gave me a direct turbo boost reading. Without this, how does the ECU come to an overboost conclusion? (there is a “Calculated Boost” sensor (or pseudo-sensor), but this seems to indicate the boost the ECU has requested, rather than what it has. This tops out around 1.5bar (150 odd Kpa), which seems about right for this engine.
• The apparent resolution two years ago by replacing a dirty fuel filter has me puzzled, as seems to indicate a totally different issue, whereas everything feels identical. I am wondering if there was already a marginal issue around the turbo, that fixing fuel supply issues simply masked for a period.
• If we do go the replacement turbo route, Nissan replacements are eye-wateringly expensive, and I’d probably just (sadly!) replace the vehicle. There are a number of aftermarket turbo’s around for these, many originating in China. I gather quality varies from rubbish to good! Anyone had any experiences with these?

Thanks for any help or pointers anyone can give!

Andrew

Should also have been clear that with the last lot of garage checks, all the fuel filters were checked again (for obvious reasons!)

Based on my limited experience, it might be worth checking the turbo vanes and possibly cleaning them. They do get sticky and without any specific knowledge I've read that the earlier boost control is more prone to problems than the 140kW version.

There are certainly good, cheap turbos out there. I have an AU$250 turbo on my Holden Jackaroo/Isuzu Trooper that has done 200,000km with no issues. Identical to the OEM IHI turbo for a fraction of the cost.
My mechanic has put a $300 turbo on a Navara and it's been going well for 2 years.
Unfortunately I don't know how to pick the good ones...

If overboosting is your problem, you may be able to resolve it by adjusting the actuator on the turbo. Certainly I would start there before buying a whole new turbo. See @Scott52 's helpful post here.

Incidentally I recently experimented a little and did not bother with the extra turn of the stop screw after contacting the lever arm (with actuator arm under vacuum at max lift and vanes closed). I did that maybe 2 months ago and so far have had zero issues. If anything my economy has improved marginally.

Have a play and see how you go.

I also did not use a feeler guage but rather monitored with a cheap endoscope camera I bought on ebay.

My understanding is dirty vanes are more likely to be stuck in the open position than the closed position, which would cause underboosting not overboosting. Nonetheless if you do decide you want to de-carbonise your turbo to free the vanes up a little, oven cleaner does a terrific job. Bugger of a job though. If you remove the actuator rod from the control arm - as I recommend you do so you can easily manipulate the vanes open and close while cleaning - try not to lose the circlip.

Thanks for the info. Definitely understanding more and more how this actually works, rather than my prior assumptions.
I'd always assumed that the ECU was involved in controlling turbo boost on variable vane turbos, but if I understand you and the info you linked to correctly, the turbo boost is purely vacuum driven (I assume intake vacuum?), and the ECU only chimes in if it gets too high - ie. It's a mechanical control with the ECU acting as a safety valve. Is this a valid comment?
If so, it would seem that boost is totally determined by vacuum and the length of the actuator arm, and your suggestion of simply tweaking the arm to reduce the boost for a given vacuum would make a lot of sense. Why would one of the garages not have done this?
Can you think of any other reason why a turbo would overboost - is there any failure mode on a turbo that would lead to this?
Also, any idea where the ECU derives the current boost levels from - I couldn't see any sensor that seemed to provide this.
Thanks again for the help - it's an education!!
Andrew

Vacuum pump on the driver side of the engine provides vacuum to the brake booster and to the boost control solenoid (BCS) over on the passenger side, located under the airbox near the alternator.

BCS then apportions vacuum either to the turbo actuator (where it closes the vanes under vacuum) OR it bleeds it to air intake. BCS is easy to recognise with 3 vac lines coming off it (1 input, 2 output).

ECU tells the BCS how much vac the turbo is to receive. ECU is also reading boost (amongst a gazillion other things) and making mega fast micro adjustments via the BCS as required. Its a very hard working, precise little piece of equipment.

I have never heard of a failing BCS causing overboosting. Underboosting, absolutely, but not overboosting. BCSs do fail but they are designed to fail in the open position with vacuum bleeding to atmosphere (intake). Meaning an unresponsive turbo.

But they are not expensive (if you buy an ebay knock-off), so you could try that if you wanted.

I am not an expert but I would think overboosting is more likely due to the actuator arm on the turbo not being properly calibrated (as discussed above). It might be getting the right amount of vacuum but is overboosting due to the vanes being in the wrong position (i.e. too closed for the rev range). In my case I had the opposite: the actuator had failed and stopped lifting the arm at about 12 in Hg of vacuum leading to chronic underboosting.

You are correct the ECU not only brings on boost by regulating vac to the turbo actuator but also acts as a safeguard against overboosting. I have never got mine to go into limp mode from overboosting or throwing a code like you have, but I understand ~22 psi or thereabouts is where the ECU gets upset and shuts the show down. Have you recorded and charted your boost while creating the fault on demand? Be interesting to see what the number is.

I am not sure where the ECU measures boost but I expect its from the Manifold Absolute Pressure (MAP) sensor. This is on driver side front, bolted into the intercooler. It's plausible that if there is a fault here then that could also lead to overboosting due to bad data going to the ECU which in turn affects BCS calculations. Or just the MAP sensor sending a high number to ECU and it in turn throws the toys out of the cot.

I periodically pull my MAP sensor off and give it a clean with contact cleaner. Likewise the Mass Air Flow (MAF) sensor at the air box. I figure the better data these two sensors are providing to my ECU then the better instructions ECU will in turn provide the turbo via the BCS.

Thanks for all the detail - I'm finding it enlightening!
Maybe a dumb question, but if the ECU is ultimately controlling the turbo boost, why doesn't it simply back it off rather than create an overboost situation?
Are there any implications I should be aware of tweaking the actuator arm adjustment? Could this just be masking another issue?
And on the boost detection, there is an inlet manifold pressure sensor, which is where I guess the ECU is getting it's info from. I'll get the rig out again in the next couple of days and see what that is telling me when it fails.
Thanks again for all the insights. Andrew

As I understand it controlling boost is both an active and reactive process for the ECU. I say reactive because there are a bunch of variables the ECU does not control, most notably what you're doing with your right foot. And as you know turbos can spool up pretty quickly. In reality it is regularly backing off the boost to keep it within its happy range. But it is also foreseeable that it would have a programmed upper figure in mind where it considers boost is now potentially damaging to the turbo or engine and when that figure is hit - perhaps because you buried the boot - it does not just wind back boost via the BCS but also throws a code and/or goes into limp mode. Its a pre-programmed fail safe.

Haven't touched anything yet, but planning to get out all loaded up to get a baseline turbo trading before I tweak anything.
Did have the vehicle out over the weekend for a decent drive, so had a live view of the inlet manifold pressure running (I wasn't driving as well!). As expected, this started around 100kpa atmospheric pressure, so the body was everything above that. Turbo behaved exactly as I expected - would boost every time demand when up, drop pretty much instantaneously when the throttle came off.
What wasn't normal was the degree of boost. I saw a maximum reading of close to 280, which equates to around 180kpa of boost, or around 26psi. This was very brief, and didn't trigger the overboost fault, but from my understanding, this was well within the ECUs overboost territory. Without a load, it was never going to sustain the need for this, but I imagine running at that level for any appreciable time would have triggered the overboost condition. Interestingly, this was close to the 200kpa one of the earlier garages mentioned - I believe this was with an independent boost gauge, so independent to the manifold pressure sensor.
The plan now is to take a live reading with a repeatable situation that will generate a fault (load and road), then once I have this baseline, extend the actuator arm fractionally to get a feel for how sensitive this adjustment is.
I'll keep you posted!

mclarenaml said:

Haven't touched anything yet, but planning to get out all loaded up to get a baseline turbo trading before I tweak anything.
Did have the vehicle out over the weekend for a decent drive, so had a live view of the inlet manifold pressure running (I wasn't driving as well!). As expected, this started around 100kpa atmospheric pressure, so the body was everything above that. Turbo behaved exactly as I expected - would boost every time demand when up, drop pretty much instantaneously when the throttle came off.
What wasn't normal was the degree of boost. I saw a maximum reading of close to 280, which equates to around 180kpa of boost, or around 26psi. This was very brief, and didn't trigger the overboost fault, but from my understanding, this was well within the ECUs overboost territory. Without a load, it was never going to sustain the need for this, but I imagine running at that level for any appreciable time would have triggered the overboost condition. Interestingly, this was close to the 200kpa one of the earlier garages mentioned - I believe this was with an independent boost gauge, so independent to the manifold pressure sensor.
The plan now is to take a live reading with a repeatable situation that will generate a fault (load and road), then once I have this baseline, extend the actuator arm fractionally to get a feel for how sensitive this adjustment is.
I'll keep you posted!

Click to expand...

Couple of typos there, but I'm sure you get the gist...

280 kpa seems high given the MAP sensor's range of measurement (per the manual) tops out at 253.3 kpa (36.73 psi) absolute pressure.

Deducting atmospheric pressure (~14.7 psi) you get a maximum (measured) boost of 22.03 psi which I'm guessing is also the magic number for throwing an overboost code.

I believe the Garrett 2056V is capable of the 26psi you are apparently hitting but doing so without upsetting the ECU seems odd. Be curious to know just high it needs to get before it does.

Perhaps your ECU has had a tune/remap.

I doubt the ECU has been mucked with. We are the second owners and have had it from 3 years old. The original owners ran a pre-school, and nothing indicates that they used it for anything serious.
I'll let you know what I find when we throw a bit of a load on it.

Why not on a simpler note, try a Tillix Valve.
Old Tony on here recommends them a bit instead of messing with replacing the ever dying BCS.. I lost a couple of them in 3 years & the cheapo online ones last about a month or two if they even work on arrival.

Ii bought the kit & the what sounded essential restriction add on to control the boost spool up speed. I rigged this Tillix up myself slowly & was amazed when i had manual control over my boost. It runs damn sweet !

I set it as follows,
max boost on the actual Tillix valve,
i have it teed off the boost sensor off the d/s intercooler boost sensor, and also run a guage in the cabin off this tee.

The unit by itself i got massive full overboost, but with the essential restrictor coming from the vacuum that comes across the back of the engine cover & goes into the spool speed/amount restrictor piece, then into the feed back in vacuum to the Air Filter piping,
i set the screw on this restriction Tillix addon til when i go for few drives......... the boost maxes at 21-22 psi.

Set this way, it basically boosts sort of like stock, but probably a bit faster, in a manual way, then hits great boost amounts to move my damn truck up those hills and those roads with minimal smoke from high load & avoids loading the motor & any running like crap , while also totally avoids overboost.

Im also running an i-drive & the more i turn it up the more it loves it.

I have a new scv & new Frp sensor & needed them both & neg battery discon pedal ecu/fuel reset both done.

no codes, no limp,
no overboost,
no underboost!

I would take note the tuning of the Tillix Grey anodised restrictor part is fiddly to tune ! As is the Tillix itself.

You turn the Tillix right up, u use the top dial on the Spool Speed/Boost Level Restrictor,
Its a round 1cm dial with a lock off round nut on a thread. You may need to roll up the street a few times to tune the restrictor, you need a boost guage 100 % !
Soon as its tuned u know but as youll boost like a rocket up huge hills at 21-22psi & blow nearly no smoke or have no engine or turbo underboost under load issues, and most importantly, youll get no overboost issues !

If you do overboost, simply carefully adjust the grey restriction piece top screw slightly left, which turns the boost/spool speed down.

It does not need to be turned much at all !
Very finnicky. Sometimes just 2-5 millimetres makes huge difference to boost.!! use caution!

Once u get it just right tighten the top lock off round nut onto the thread & give the top screw a good hand turn to the right ensuring nothing moves, and if the top screw does move a millimetre or two to the right, its a good thing as it properly locks it off from moving and rattling loose over use,
though it may very slightly increase boost... it is finnicky.. hand tighten lock off round nut, and then road test again. If it didnt move much you'll still be at that sweet boost psi still, if your even luckier maybe half or 1psi higher.. .. which is what u want !! u can also hold the top screw plus the round nut & nip up the round nut slightly with some pliars to ensure it never rattles loose and gives huge overboost one day Do the nipping it up when totally sure & even then road test to see the boost once more, and pray it didnt go lower or bit high.

If your steady handed and careful itll nip up just right and you'll reach that 20-22 and go no further. I had to do it again today as i had scv & frp issues before & i had mine overboosting from playing with the restrictor. Its a easy to set, but hard to keep set & locked off right, hard to master type part !! Now mines just right, im going to texter mark the top screw dials brass thread for reference

Finally got all the bits in play to run a fairly repeatable test to develop a baseline. Had the horse trailer on, and attacked a section of road not too far away that we know triggers the problem. I'll attach a screen dump of the inlet manifold (MAP) sensor leading up to the 0234 being generated, and immediately afterwards.

First up, the critical boost appears to be at a MAP reading of low 250's (252, 253) kPa, which pretty much equates to the earlier comment of arround 22 psi boost. In the screen dump, you'll see that the boost drops 4 times - the first three are brief, don't trigger any fault, and are imperceptible to the driver. Without the ECU data you would be unaware they were happening, but the pattern is repeatable, and I have previously seen an instance of this without triggering a fault or loss of power (I thought it was some sort of data glitch with the "extremely" sophisticated ODB dongle I have....)

When the fault is finally triggered (the 4th and longer slump), I could definitely register the slump on the screen prior to feeling it in the vehicle - might only be a fraction of a second, but it appears boost can be dropped before it is perceptible to the driver. I'm guessing that what is happening with the initial drops of boost, is that the ECU is attempting to allievate the problem by a brief reduction in boost, and when this doesn't work it finally drops it off and logs the error. Someone else may have more insight into this?

The other interesting thing is the climb in pressure following the fault. When the fault occurred the throttle was deliberately not released, and there was no indication that the turbo came back on song. But apparently it did to some degree, but again was not preceptible to the driver (or passenger). Again I'm guessing that the ECU is attempting to recover.

Next stage is to lengthen the actuator arm and see what happens. It's a bit of a pig to access, so I'm thinking of taking a full turn out, which should give me something that should definitely make a difference. Sounds like this may overdo it, but can always back it off a bit once I have a feel with sensitivity. Anyone see any issues doing this?

BTW - with the previous readings on 180kPa boost, I'm beginning to wonder if I was seeing things. I don't have anything captured, so can't go back and check, but the data today seems much more what is expected!

Success!!

Finally got to play with the actuator arm today, and extended it by a full turn. The image is the identical scenario as the previous one, and peaked around 235 kpa, which is around 19 psi of boost. This is a tad over the normal max of 18 psi, but only happens under extreme provocation, so not at all concerned about this.

Just for posterity, the actuator arm was a bit of a pain to access with the turbo in the vehicle. We ended up jacking up the corner of the car - once the suspension was extended, it could be accessed above the wheel and through the slot in the inner wing where the suspension arm comes through. It was all pretty frozen, so suspect it hadn't been touched from new.

Thanks to everyone for the help and suggestions. As well as solving the problem, I've also learnt a heap about how this all works, which is a great bonus.

Andrew

Well done mate, glad you were able to sort it out