Greetings! First post here...

Looking for help on a 2016 L405 3.0L TDv6 single turbo.

For about the first 2 minutes after starting the idle keeps hunting/surging. Inspecting things under the bonnet I can hear fluid gushing sound from the Pressure Control Valve (PCV) fuel return line. Touching that return line I can feel it swell up a bit by pressure. Twice every second the PCV is being commanded to release almost all pressure from the high pressure fuel rail.

Have confirmed both by observing the PIDs on a scan tool and by putting a scope on the PCV control wires. The ECU starts out with about 38% duty cycle on the PCV and keeps reducing it rapidly until it hits around 9 ~ 10%. At which point the engine begins to stall and the ECU immediately increases the duty cycle back to 40%. The engine recovers and the cycle repeats again. Happens almost twice every second.

This happens no matter if the engine is cold or is at operating temp. This hunting/surging starts happening from the very moment the engine is started.

If I attempts to drive the car when this surging is happening the whole car turns into a bucking bronco. The ECU continues to ramp down and then step back up the duty cycle of the PCV in a saw tooth manner even with light application of throttle.

During all this the duty cycle of the metering valve on the CP4 pump is being maintained at around 30%. The rail pressure sensor correctly reports the pressure in the rail. I can even see what I think are the spikes of the injectors opening when looking at the output of the pressure sensor on the oscilloscope. Not sure if it is the sensor that is picking the pressure change spikes or its just getting induced into the probes from the injector wires. When the overall pressure in rail drops significantly, those spikes disappear and then show up again when the pressure increases above a certain threshold.

Around the 2 minute mark after starting the engine the idle smooths out. The ECU appears to go into a different operating mode. The duty cycle of the PCV changes to ~37% and stays steady. The fuel metering valve changes to 40%. Once this happens I can then drive the car and all appears to be ok. Power seems fine. Overall mpg is low and the exhaust smells like that of a diesel truck from the 70s.

At this point with engine running smoothly if I shut it off and start is up again right away then it goes back into this hunting/surging idle mode. Give it around 2 minutes and it smooths out again.

Here's the kicker. If I loosen the PCV just the right amount and allow just a small amount of pressure to leak from the high pressure side of the fuel rail to the low pressure return line the the engine does not have this surging idle problem. The ECU starts out the PCV at around 37% and very gradually brings it down to ~30%. The metering valve is kept steady at around 32% duty cycle. The engine idles smoothly. Then around the 2 minute mark, right about the same time when the car would go from bucking idle to smooth idle when the PCV is torqued down to spec, the duty cycle of of both the PCV and metering valve is switch to ~ 37%. Engine continues to idle smoothly.

The only reason I even know that I can get it to idle smoothly with the PCV not torqued down to spec is because the car came to me in this state. Whoever was working on it before me somehow discovered it.

The issue with leaving the PCV not fully torqued down besides the obvious safety concerns is that the fuel rail tops out at 56 Mpa. When the commanded pressure and actual pressure start to drift apart, the ECU shuts the engine off and throws a P0087.

Speaking of codes, with the PCV properly torqued down the ECU throws absolutely no codes. None. So far have consumed about 5 gallons of diesel in driving and idling and there are no codes what-so-ever.

Now the history of the car that I've managed to put together based on some paperwork that was in the car at the time of auction and what is installed on the car. The fueling system on it was replaced. As best as I can tell everything except for the tank and the lines from the tank to the fuel filter was replaced. CP4 appears to be remanufactured unit. The fuel rails and injectors are new and genuine Bosch. The reason for the repair stated on the paperwork is Vandalism - Sand introduced to fuel tank. I took lift pump out of the tank. Found a couple of dead insects floating around but no sand. Took apart the lift pump as much as I could and its screens were clean. No tell tale signs of sand in there.

Stuff I have checked:

1. Powers and grounds. Wiring appears to be fine to/from the ECU.

2. Visual inspection other than signs of frustration from whoever was working on it before, I dont see anything wrong in there.

3. Replaced the PCV with a used one I had sitting on the shelf. No change. Bought a new PCV and installed it on the left side rail I had sitting on the shelf. Installed it in the car. No change. With all 3 PCVs I can reproduce the same behaviour of smoothing out the idle if I intentionally leave the PCV loosened just the right amount.

4. Replaced the metering valve with a genuine Bosch. No change. The one on there was a Chinese clone.

5. Replaced the rail pressure sensor no change. Though with the pressure sensor disconnected the engine starts bucking/surging almost 3 to 4 times every second instead of twice every second and a bit more violently.

6. Checked low pressure side. It is a little low reading at ~ 6 psi. I plumbed in a pump in series which boosted the pressure on the low side to 12 psi. No change.

7. Checked the internal Pressure Relief Valve (aka overflow valve) on the CP4 by removing it. Its got a very small amount of rust pitting on it but appears to be working.

8. Checked the IQA codes stored in the ECU. They are correct.

9. Have run the "Powertrain set-up - Diesel fueling adaption re-initialization" and "Powertrain - control module adaption clear" service functions, no change.

10. Have left the battery disconnected for a week. No change. Been working on this lump for 3 months now (weekends only.)

Things I have not done yet:

1. Checked the timing of the CP4 pump. Mind you after the first 2 minutes and when the idle smooths out I can drive the car no problem.

2. Replaced the CP4 pump. Hmm... Dont want to do that if I dont have to. All the lines and rails I have taken apart so far have had no signs that I've found of the internals grinding away.

So if you have made it this far, thank you! Would love to hear any suggestions you may have. I have some videos of the scope captures. Will post them once I upload them.

Shahid