D40 Sudden Power loss/gain

[DRK]

Hi All,
Hopefully this particular issue hasn't been posted before. Read similar but not quite the same. We have a 2007 D40 STX manual with 100k on the clock and have just got back from a 5000k trip were it ran fine. After getting back at low revs under load (1500 rpm) the power would drop off for a second and then comeback with bang. More noticeable when cold. When it first happened I thought it was because I was being lazy and not shifting down gears but it has just got worse. Does not go into limp mode or produce any codes apparently.

Now we thought we might have got a batch of bad fuel so we took it to our mechanic (not Nissan) to get the fuel checked and the filter done. Still did it so we tried blocking off the EGR and still no good. Thoughts turned to the timing chain as this might throw out the cam sensor. The chain (single) was stretched and the hydraulic tensioner on the front was stuffed so we upgraded to the YD double row kit as we intended to keep the ute as it has had very easy life even if it has heaps of little niggles. That didn't fix the issue though.

These are the things that have been tested since;
Both ECUs/computers
Fuel rail sensor
Engine earths
Crank sensors
Cam sensors
They have continuity test the loom to the injectors
That's what I recall from our conversation
Thankfully one of the staff has let his D40 be a donour vehicle.

They have had the scan tool/computer hooked up with two staff in the vehicle whilst driving and even though the issue occurs nothing shows up.

Now I am by no means knocking our mechanic but he has just about exhausted all his ideas and contacts so I am reaching out for other options. We have invested in the timing chains with intention of keeping the vehicle.

Thanks

 

[Old.Tony]

I wonder if this is another symptom of a dirty SCV? It's on the fuel pump. It can be washed, but it should be examined for any scoring etc and replaced if suspect.

Before throwing money at it though, put the gauges back on and drive it again and pay specific attention to the boost levels in real time. If they're fluctuating almost in sync with the power from the engine, you might have a faulty vacuum solenoid. This device sits on the back of the pipe that forms the inlet to the intercooler. It should have vacuum hoses attached and one of those runs up to the turbocharger actuator.

At 1500rpm the turbocharger is just getting enough thrust from the exhaust stream to start being seriously effective. If the solenoid is acting up, it could be closing the vanes dropping the boost (and in the D40, that's going to show as a big drop in power) and then bringing it on again which will seem like a big surge of power.

Neither fault will provide codes. The SCV should cost about $240 if you have to replace it, I don't know what the vac solenoid is worth. It might pay to replace the vac lines as well (they'll be a little worn by now!) and that should set you back about $20 or so.

 

[DRK]

Cheers Old.Tony will pass it on and see what happens. I do recall now comments about fuel pressure and turbo boost appearing normal but they may not have been paying close attention.

 

[Old.Tony]

Problems with the SCV aren't reflected in the fuel rail pressure very clearly. You'd look at it and go "well, apart from this repeating pattern of peaks, it looks pretty normal".

That pattern of peaks is an essential part of the emission control system. The ECU adds a little pressure at regular intervals and measures the available oxygen in the exhaust which should drop in response to the increased fuel pressure. If it doesn't match up, the sensor is bad, and the car can't rely on knowing if there's enough oxygen in the exhaust for the catalytic converter to be charged (with a hydrocarbon, and in our Navaras that's just diesel, but in trucks, buses and some cars now they're using AdBlue).

I'd also wager the solenoid isn't going to be too obvious either, unless you watched a graph of the boost pressure measured against time, throttle and engine RPM.

 

[kernels]

Yep, certainly sounds like a 'sticky' SCV.

The SCV is the number 1 culprit in D40 problems by a country mile.

 

[Old.Tony]

Absolutely, Kernels. Even had an issue with mine - when cold, my engine would 'hunt' - it'd surge and back off, surge and back off, surge and back off, every second or so.

The way I fixed mine - and what you could try before pulling out your SCV - is to get the fuel tank down to about half, then drop a full bottle of Liqui-Moly Fuel System Cleaner from Supercheap Auto. Drive around for about 100km and you should notice a difference.

 

[Whosthomo]

Absolutely, Kernels. Even had an issue with mine - when cold, my engine would 'hunt' - it'd surge and back off, surge and back off, surge and back off, every second or so.

The way I fixed mine - and what you could try before pulling out your SCV - is to get the fuel tank down to about half, then drop a full bottle of Liqui-Moly Fuel System Cleaner from Supercheap Auto. Drive around for about 100km and you should notice a difference.

I have the same problem with my 550 I've noticed its at exact time intervals the RPM surges up approx 250RPM every time

 

[DRK]

Hi Guys, so far isn't the SCV, and haven't ruled out the solenoid yet. What are the risks of cracking the fuel rail if you were to replace that sensor as indicated in some other threads?

Thanks for the advice so far.

 

[Old.Tony]

The problem is in getting the fuel rail sealed again. It's designed to hold 30,000psi - that's an enormous pressure. I remember working with a guy doing cold cutting years ago slicing through concrete with water jets at 20,000psi ... concrete is cut like warm butter. Ain't nothing like it.

A mistake with the fuel rail can be costly. It's one of the latter things I'd try. If you suspect the sensor, it'd be the electrical connector that I'd be more concerned with at first. Pressure washing the engine bay is probably the worst culprit, and some mechanics/dealers will happily shove a Gerni in there to clean it up. It's not a good move, pressurised water can separate electrical contacts just long enough for a bit of debris to get in, or leave them just wet enough for corrosion to start.

The next most likely candidate of course is that solenoid. It, and the vac hoses, are another source of joy *cough* for D40 owners.

 

[DRK]

Waste of time

Well about to pick the ute up today with still the same issue. We have done fuel filter, SCV, timing chains (YD25), fuel rail. I have asked them to check the solenoid and vac lines which they say they have done but I am not convinced. This is something I will try and do myself I think.

The next options they are offering is fuel pump and/or injectors?

We just want the damn thing fixed. Does anyone have any recommendations as to good diesel mechanics in western Victoria or am I

 

[Old.Tony]

I wouldn't think it'd be injectors. Might be the harness, but if the engine is normally running smoothly with similar levels of smoke to its usual operation then injectors are probably fine. Fuel pump is probably good too. Combined, pump+injector replacement usually costs around $9K. That's a "last resort" thing to do.

There are several areas where electrical gremlins could sneak in, some causing unpredictable behaviour. Here's a short list:

* Both battery terminals. Clean 'em really well, reattach the leads and spray sealant over them.

* ECU earth. D40 has an earth point near the ECU (right hand side of the vehicle in the engine bay near the brake booster on the side wall.

* Engine earth. Just follow the negative battery lead. Remove from the side of the engine, clean, reinsert.

* Alternator connections. Mostly the positive terminal, because it earths through the engine block back to that other cable. It might be worth experimenting with a heavy black cable from the alternator mount to the battery negative just in case there's debris in the alternator mounts causing the problem.

I can't help but think that if they were doing a thorough job, they've already popped each of the injector leads and reinserted them. Same for the fuel pump. Still might be worth trying - just unplug and replug.

Finally, how's the battery voltage first thing in the morning? If it's below 12.4V you'd have to wonder about not only the battery, but the alternator or the belt.

 

[MattWij]

Hmm, don't mean to bring up an older thread, but I have the exact same issue.

Only difference is that it ONLY happens when the car isnt fully up to temp, the power delivery is incredibly stepped and as 'Whosthomo' points out the RPM's jump up about 250 revs every "step"

I first got this when accelerating entering an on ramp, engine continued to behave a little jumpy till it warmed up and the issue disappeared.

A little odd though that a STX-550 would get the same issue as the YD25 based model..

I'll go through the suggested steps above, but it will probably be a while before I isolate the issue by the looks. Not super concerned though, as the car drives incredibly well 99% of the time, If I find the silver bullet I'll report back for the future generations! haha.