CR Injector Failure: Common Causes & Solutions

I hope the answers to my questions aren't already available somewhere, as I haven't been able to find them. I would be grateful for any suggestions or pointers.
I would like to understand what fails in the CR injector, or what can potentially fail. The background is that I have a BMW 530d that runs very unevenly at idle, produces black smoke, misfires, and emits blue smoke after starting. The idle speed control system identifies an injector as the culprit by indicating that it is at its positive control limit.
I've read quite a bit about cleaning while the system is disassembled, and I'd like to try that approach. On the other hand, there's also the reverse flow rate test, and I don't really understand what that's supposed to achieve.
A possible symptom of a clogged injector nozzle is that the fuel metering may no longer be accurate when injecting small amounts at idle. But does this then increase the return fuel volume?
Or could the increase in the return flow be due to a different issue? Perhaps a fault in the injector's control unit, which would then also affect the accuracy of the fuel injection quantity?
So, is it true that if the return flow volume is too high, cleaning the injector won't help because the problem lies in the control unit? Or does a carbon-fouled injector cause an increased return flow volume?

I'm not an expert in this field, but I can explain it this way: When an injector is clogged, less fuel reaches the cylinder, and it's also atomized suboptimally. This means that the fuel that does enter the cylinder, especially at idle due to the poor atomization, isn't completely burned and therefore not fully utilized. As a result, the engine control unit detects this and tries to compensate for the reduced torque in that cylinder with an increased amount of fuel, which you can see in the idle speed control. However, this isn't enough, and you're hitting the control limit.

Okay, and now we get to the part where the return fuel volume comes into play. A carbon-fouled injector simply means it's clogged. I've explained all of this above, at least if my assumptions are correct. A clogged injector means less fuel is entering the cylinder and instead needs to be diverted through the return line. When you add the extra fuel that the engine control unit is trying to add to compensate, it results in a significant deviation in the return fuel volume compared to the cylinders that are working as intended.

I hope my thoughts are correct, and that I haven't written anything nonsensical here.

Hi,
It is necessary to distinguish between a fault on the injection side of the injector and a fault on the control side.
The first case, for example, due to poor atomization, results in a reduced power output from the corresponding cylinder. The resulting uneven engine running is detected by the engine control unit (ECU) and compensated for within the control range by the long-term fuel trim (LLT) by adding more fuel. Due to the higher effective manifold pressure (EM) and the associated longer injection duration, the return fuel volume of the injector is also slightly higher.
However, the injector doesn't necessarily have to be the cause; poor cylinder compression can also lead to the same symptoms.
In the second case, the return flow of the injector is too high due to a fault on the control valve side, for example, due to a control valve that is not opening or closing properly. The consequences can be more widespread, potentially leading to poor performance of individual cylinders, but also to lower rail pressure.
The error presented here is more likely related to case 1 - exceeding the LLR limit, resulting in poor idling and smoke.
The affected injector definitely needs to be removed. Either replace it or have it inspected and refurbished by Bosch. Some Bosch service centers can do this.
The amount of fuel returned depends on the injection system. With piezo injectors, the return flow has a common working pressure of 10 bar, so you can't simply open the system and measure the return flow.
With magnetically controlled injectors, the return flow is pressureless, which means the return lines of individual injectors can be disconnected.
The tool can be built yourself: for each cylinder, you'll need a disposable syringe (e.g., 20 ml, 50 ml) connected to the injector nozzle with a piece of return hose (sold by the meter). Of course, all the connecting pieces must be the same length. The syringes remain open at the top and are all hung together in a small frame.
The actual return lines will be closed. In idle or partial load conditions, the behavior of the return flow for each injector can be observed, allowing for the identification of a faulty injector.
You're already familiar with that, aren't you?
hg
Herbert.

Nice explanation, Herbert!

Although from Ford, it is well-described and illustrated .

http://www.mondeomk3.de/3397-rücklaufmengenmessung-selbst-erledigen.html

Thank you for the detailed answers.
As I understand it, a positive quantity correction typically indicates a defect in the injector or engine. However, if the correction is negative, it suggests that the upper part of the injector needle has less pressure, causing the injector to inject too much fuel. This is precisely why starting problems occur, or why the rail pressure is too low. I haven't experienced this, although two injectors showed a slightly negative correction.
This weekend, I tried to remove the (positive) injector. Unfortunately, I wasn't successful. Now I understand why the engine is running roughly: The nuts on the bracket were loose, which is probably why the injector is leaking, and it hasn't heard my taps yet. I test-removed a near-injector, and it came out with my setup after just two gentle taps, which means it would be fine for a properly seated injector. It's quite awkwardly installed on BMWs; you can't remove the bracket if the injector is still in place. Therefore, I'll have to unscrew the coil and attach an adapter there. First, I need to find a M27 x 1 thread tap. However, the coil itself isn't easy to remove either, because you can't access the hexagon.
Why can't the injector be reused after the upper part has been opened/disassembled? I don't see any reason for this based on the diagrams I've seen online. Are the individual parts available from Bosch?

Hi there.
The injector must be completely removable, even in BMW vehicles. That's all the workshop does. An injector that has been disassembled from the engine should not be installed without cleaning, possibly readjusting, and reassembling before installation.
Perhaps you could tell us the exact vehicle and engine type, and also provide some pictures. The books by HH Etzold and Korp can also be quite helpful sometimes.
hg
Herbert.

Regarding the vehicle, I apologize for not mentioning it earlier: it's a BMW 530d, model year 2002, which is the facelift version. This is relevant because the injector has an O-ring in the upper area to seal the injector bore. Therefore, in my opinion, spraying and soaking with various products will not be effective.
I believe the injector can be completely removed, but it's very difficult to access from the side, and the claw tool available for purchase wouldn't fit. For me, it would be easier to fabricate an adapter with threads. It's not necessary to assemble the injector there; once it's removed, you can assemble it outside. I need to remove it to seal the injector seat.
You mention that it would then need to be calibrated. What parameters are affected by disassembly? Where can adjustments be made? I think it's more likely that they are measured on a test bench for classification purposes. In the engine control unit, you can actually enter the injector identification number, precisely because there are different flow rates among the injectors.