How does the ECU know how to control the injectors?

[Mervyn44]

Basically, what I'm trying to discover is where the information comes from which ECU uses to control the injection adjustment value, mixture etc. I know Volkswagon have a pressure sensor built into the glow plug and feedback from this is used to control combustion conditions within individual cylinders... but how does the Citroen/Peugeot system work? How does it know when to make adjustments, or does it just work from a "look up" table with settings based on various sensor readings?

- Mervyn.

[GiveMeABreak]

Without wanting to state the obvious, but the injectors will work or they won't and are calibrated within extremely fine tolerances. The engine ECU will get a stack of information from many other ECUs and systems to determine operation. I.e. the induction air temp sensor, the EGR, the ESP system, the particle filter differential pressure sensor, diesel high pressure sensor etc., all of which feed into the engine ECU. The operation will be governed by the engine ECU software after taking account of the state of all the inputs - only a few of which I have mentioned.

[Mervyn44]

Hey thanks for replying.

I understand all that but it doesn't really answer my question. I realise there are many sensors which influence the the signals sent to the injectors but unless I am wrong none of these are cylinder specific, they just adjust the signals sent to all the injectors, not any particular one... so how does the ECU know how to adjust the injectors individually? (other than the actual code assigned to the injector in the factory, which never changes.

This question links into discussions I've been reading about injector performance and them wearing out. In a lot of places they tell you that individual injector adjustments shown in the live diagnostic data should be within certain ranges (lets say around 2%) and values approaching 4 are a sign that the injector is past its best and should be replaced. This suggests that the ECU must have some method of checking the performance of an individual injector and make adjustments to compensate, or alternatively evaluate the combustion characteristics in a specific cylinder... how is this done? What gathers this information?

Like I say, Volkwagon have a pressure sensor built into the glowplug which evaluates the combustion quality in each individual cylinder, but what is the story in my 1.6 Hdi Citroen?

Many thanks again - Mervyn.

[GiveMeABreak]

The ECU measures the throttle position, engine temp., engine speed to determine the correct air/fuel mixture needed by the engine and the timing of the injectors. All the other engine mounted sensors measure these and other factors to decide the metering.

The system should use this info along with data that is already programmed into the ECU at factory to work out how much fuel to deliver, by signalling the injector valve to open / close within fractions of a second. As you have already said - injectors are coded to a tolerance. Some injectors don't need coding to the ECU, others do - it depends on the engine and injection system used. And of course, the injection ECU knows which electrical connectors are hooked up to which specific injector number.

Beyond this, I'm not sure what else you want to know...

[RichardW]

I don't think there are any 'knock' sensors on this engine. I suspect control of pre-ignition is via a combination of programming the injector (which presumably gives the ECU some sort of map to work with) and checking the rotational speed of the engine. Our C4 had to have a replacement injector - a re-con unit was fitted. Since then it's had a much more pronounced diesel knock at low rpm than it had before (or indeed, the 307 has). Initially it gave the flat line -6% correction you are seeing, I reprogrammed it, but I think it still fails to communicate with the ECU, and gives the -6%; I'd need t check again.

[Mervyn44]

Thanks for your input chaps, it really is appreciated as I'm desperate to get this issue with my car sorted.

GiveMeABreak, I still think you misunderstand my question, that's probably due to me being unclear about the information I'm looking for, sorry... I know there are many sensors which provide information that influences the overall mixture control, my question concerns the source of information, if any, which influences mixture adjustment in individual cylinders.

RichardW, I'm really glad you chipped in, your post was very interesting (particularly the -6 point... I think this is probably the limit of adjustment possible which indicates an electrical problem or an injector coding issue). I have been researching and, as you say, the BSI is mapped to adjust individual injector firing based only upon the injector codes in my opinion. This would mean that the system really has no idea about injector performance in a particular cylinder, it simply adjusts all cylinders to compensate for feedback from the various sensors. This means that when wear and tear on one injector causes the sensor readings to go out of whack the BSI tries to compensate by over or under fuelling all four injectors rather than re-adjusting the one at fault, or flagging up an error that a specific injector is misbehaving.

I'm going to update my original thread "mysterious injector issue". I hope you will keep an eye on it as I would value any feedback you can offer.

On another note, your C4 surely can't be running all that well if the refitted injector is giving adjustment values at max tolerance. Based on my newbie steep learning curve It suggests that the new injector is is over supplying and causing the O2 sensor to report a rich mixture. This causes the BSI to lean out injection to all cylinders, which would contribute to the increased knock.

[Mervyn44]

One other thing I probably should have mentioned is that when I removed the injectors the car had not been running for a day or twobut they were soaking with diesel! When the car runs the exhaust fumes are white and stink of diesel which suggests an over rich mixture... or maybe this is just because the engine is not warmed up? I have had compression tests done and all came back healthy.