mfa mpg adjustment

[pj027235 10]

Ok, I have a 93 2.0l 16v 9a ke motronic. The mfa mpg shows around 5 mpg less than the car is actually doing over a long run. The engine is standard and all gizmos such as lambda, tps, mfa, timing, isv are set up and working. I am fairly sure the mfa mpg read out is ecu generated as there is no vacuum line to the back of the clocks. My question is; can this be corrected or adjusted?

Edited June 2, 2017 by pj027235

[seanl82 23]

Unfortunately not. They usually read over rather than under, but there is no adjustment

[VAG-hag 0]

since i went to 4" MAF I get 50+ MPG..... its ace :lol:

[pj027235 10]

So yours isn't reading right either...

[dr_mat 0]

There is a fudge factor in the firmware that is supposed to translate average injector timing to fuel use, then divide by miles driven. This self calibrates to some extent as the ECU is busy balancing the readings from the maf against the reported fuel/air ratio from the lambda. But as individual engines flow better or worse than others it's never exact.

 

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[pj027235 10]

I don't think my engine has injector timing, it's a bosch CIS, but thanks anyway.

[dr_mat 0]

It's fuel injected, therefore it has injector timing...

[pj027235 10]

When you say injector timing do you mean they 'switch' on and off?

[dr_mat 0]

Correct. So if the injector is on for, say, 5 ms, then in theory it delivers X amount of fuel.

 

Though a quick bout of Wikipedia reading shows that the CIS system uses variable pressure fuel distribution instead of individual switched fuel injectors - but the principal of computing the fuel delivery is the same. High pressure for X ms means Y amount of fuel delivered via the injectors.

 

I was just trying to say that the ECU does have some sort of notion of how much fuel is delivered, so the maths can work, but I would be incredibly surprised if there was a user-adjustable correction factor.

 

Obviously the assumptions inside the ECU only work if nothing major changes on the engine - so if you install high flow injectors, or radically alter the diameter of the maf without also recalibration of the ECU then although it will still get fuelling right (if it has a lambda) it will no longer be able to figure out the true mpg.

Edited June 3, 2017 by dr_mat

[pj027235 10]

Yup...seems logical to me. Today I am going to change the tps as I have a spare one and see if it makes any difference...

[dr_mat 0]

I would be surprised if 5mpg out is anything other than "to specifications". It's a relatively old design so I really wouldn't expect it to be accurate. Even bang up to date systems are rarely reliable within 10% or so.

 

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[Jim Bowen 1]

No one cared about mpg in the 90s. Not like it is now.

[fla 9]

Since installing obd2 in mine the mpg has gone up by about 5! Great for a psychological boost when you look down and its showinv 37mpg. In reality it has improved but literally by 1-2 mpg

[dr_mat 0]

Impressive what you can get out of a VR6 actually. Even on OBD1 I managed to get 37 mpg one time. Only once. More recently I'm seeing a good number off 33s and 34s.

 

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[VAG-hag 0]

So yours isn't reading right either...

Correct

[pj027235 10]

TPS has been changed but the car won't be on the road for a week or so due to other work. I will update this thread when I can...

[pj027235 10]

**Update**

I am not sure whether or not the change of TPS has improved things as I haven't covered enough miles yet. But I have noticed a rather interesting anomaly between speedo and mfa. Whilst doing a constant 60 mph on the speedo I reset mfa 2 and found that the average (mfa 2 MPH) was only showing 56 :scratch:. I believe the speedo and mfa are both driven from the electronic sender on the gearbox, what on earth is going on :shrug:.

So it's possible the mfa is computing a fuel used over a shorter (than actual) distance, hence a worse mpg (than actual).

[dr_mat 0]

More likely it's more accurate than the speedo... ;)

 

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