D40 Fuel System

[irie]

Good day,
Posting this question seeking clarification on the operation of the fuelling system in the D40s

Some time ago my friends Nav was blowing a bit of black smoke on acceleration. It ended up being the boost control solenoid that was faulty. We swapped out the part and all was well afterwards.
After thinking about it some more, I was left confused. Correct me where I am wrong here. During the black smoke issues, I understand that the engine was expecting a higher air fuel ratio and as such was burning “rich” But then I remembered the Nav has a MAF.
Wouldn’t computer see less air coming in and adjust fuel to suit? Wouldn’t this effectively stop the black smoke from being produced?
I then realised that my understanding of the fuel system is flawed. Could anyone with more knowledge clarify my misunderstanding.
Thanks in advance!

 

[Old.Tony]

Hi irie,

I think the MAFS is used to check the flow rate and the temperature in preparation for fuel calculations, but they don't check the flow rate against the fuelling+boost achieved, they just use the flow rate to work out what fuel they can inject (so it's a system that assumes all is working properly, rather than checking that it's working properly).

Ordinarily, knowing the grams-per-second rate of air flow and its temperature, you can calculate how hot the air is going to be coming out of the turbocharger for a given boost level, and make an assumption about how much the intercooler reduces the airstream's temperature, arriving at the boost pressure detected on the outlet of the intercooler. I'm not sure the ECU does a check against the boost pressure, I think it just assumes that any boost it commands (via the BCS) will be achieved.

There's some assumption involved in that, but it makes sense to me.

 

[irie]

That’s interesting.
Essentially it is used too avoid detrimental situations.
I wonder why checking values and adjusting on the fly were never a thing.