Hi, I'm writing in hopes someone may be experienced with how a SDV8/TDV8 fuel return path works. I've just sent my injectors for testing and they came back within factory specs so no problems there. As I started tracing on how the return fuel line works I seem to have stumbled across some oddities on the design.

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Let me take a stab at answering this. Though someone more knowledgeable can probably do a better job.

As you have pointed out there are two possible Low Pressure lines the injector leak-back fuel can go.

1. Fuel return from High Pressure pump and Pressure Relief Valve. At less than 1 bar of pressure and sometimes even under slight negative pressure. Goes back to the tank through the fuel cooler.

2. Fuel feed to the High Pressure pump. At around 5 bars as you pointed out. Output of Low Pressure pump that goes through the filter.

There are two considerations when deciding where to dump the leak-back fuel.

First, if you think of it in terms of relative pressures and improving overall pumping efficiencies, you have 100s if not thousands of bars of pressure on the inlet side of the injector. So the injector has no problem dumping its excess fuel into a fuel rail that is at a measly 5 bars of pressure. In fact, you just saved some energy by not having to compress that leak-back fluid back up to 5 bars so that it can be ingested again by the High Pressure pump.

Second, and perhaps more important reason, is the piezo fuel injector has a small hydraulic (fuel filled) amplifier in it that couples the stack of piezoelectric ceramic disks to the valve needle in the nozzle. The piezo disk stack has a very small amount of movement (less than 0.1 mm) when energized. This amplifier, also known as the hydraulic ratio transformer, converts that small movement into a larger stroke which then moves the needle. If there is very low or negative pressure (vacuum) on the overflow side of the injector, then the fluid in this hydraulic amplifier drains away. And the amplifier stops working. Causing the whole injector to stop working. This is why having a little +ve pressure on the leak-back side of the injectors is important.

This video at the 4:10 minute mark shows what the amplifier module looks like and how it functions.

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What exactly is the problem with the fuel return?

Hi all, I managed to find out what is wrong with the vehicle. It was the ECU all along. So I updated the firmware using IID Tool and everything is fine now. Had to go over the entire fuel system including changing all low and high pressure fuel pumps, test injectors, and check all fuel lines of clogs to find out nothings is wrong with the fuel system. The symptoms I was getting is fuel pressure oscallation. At idle the fuel pressure creates a wave of high and low points. You can't feel it but if you use a IID tool it will show up in the logs. Problem appears when I bring the rpm up. It will hit P0088-17 high fuel rail pressure code at precisely 3.6k rpm as the oscallation waves grew between high and low points. It's a lot of work involved but I guess my fuel system will be trouble free for years to come.

interesting, I have a similar problem with the injection system I replaced the ECU and Last update but nothing to do the problem persists in my case it is not oscillation it is the ECU which controls the pressure at each deceleration 1000bar 600bar and 300bar this making a noise


https://www.fullfatrr.com/forum/topic72660.html

Are you referring to the ticking noise at the very end when revs drop back to idle? Mine used to hear that, infact it was also ticking on idle as it was cutting off pressure. So far I haven't noticed any of these problems after flashing the updates file. I'll test it out later today and see if it's audible when I'm outside of the vehicle.

Tested my 2019. Sounds pretty much the same to yours. I noticed the engine tends to be noisier at idle after EGR delete. Not sure if this is the case. Anyway I've checked the logs between ecu desired and actual, it's spot on so I doubt there are any problems there.

This is exactly the noise I'm talking about, god thank you I finally found a 4.4L tdv8 owner with an IDD.

Is it possible for us to cross-share our data to close this thread definitely ?

Sure. I can do that. Which data points do you need me to log as cross reference?

Thank you very much.

Desired fuel rail pressure PCM
Fuel rail pressure

All battery and alternator voltages

More specifically, can you maintain the engine speed at 800 rpm with the accelerator pedal and then lower it to 750 or 700 rpm, millimeter by millimeter,

around 700 or 750 rpm? Do you have an unstable idle?