Another MAF failure?

[nzflash]

No I didn't check with smoke. Tried carefully spraying brake cleaner around base of plenum, pipes etc. And didn't notice any change in engine rpm. Not sure how effective this approach is.

I have a fibre washer on the stepper motor which is very thin... <1mm

Out of curiosity what is a reasonable price for a healthy used MAF should I choose to buy one?

[Belle427]

Have you made sure the small vacuum pipe that goes from stepper housing to fuel pressure regulator is connected?
Often overlooked as it's hard to see.

[nzflash]

Thanks for the suggestion. Yes I have. All pipes are connected and where appropriate are secured with either jubilee clips or cable ties.

[blaze_away]

Just to add for clarity the idle control valve being a but low is a side show to the "it's running rich" issue. It won't have any impact on that.

[spitfire4v8]

Just as a sanity check pull the vac pipe off the dizzy. The massive extra advance the manifold vacuum trick gives will mean there will be much less airflow required to sustain the target idle speed. This will do 2 things .. firstly it will mean the stepper runs in a more closed position as you have found already, but secondly the fuel numbers being used will be further up the fuel map ..
Pulling off the vac pipe to the dizzy will mean that the stepper will need to open more, there will be more airflow required to sustain the target idle, and a lower portion of the fuel map will be read. It might be that with the full manifold vacuum advance being used that the low airflow part of the map around idle is actually too rich, hence the very large negative long term trim applied.
Maybe.

Edited by spitfire4v8 on Tuesday 7th May 09:40

[nzflash]

spitfire4v8 said:

Just as a sanity check pull the vac pipe off the dizzy. The massive extra advance this trick gives will mean there will be much less airflow required to sustain the target idle speed. This will do 2 things .. firstly it will mean the stepper runs in a more closed position as you have found already, but secondly the fuel numbers being used will be further up the fuel map ..
Pulling off the vac pipe to the dizzy will mean that the stepper will need to open more, there will be more airflow required to sustain the target idle, and a lower portion of the fuel map will be read. It might be that with the full manifold vacuum advance being used that the low airflow part of the map around idle is actually too rich, hence the very large negative long term trim applied.
Maybe.

Thanks for the suggestion. Tried removing the vacuum pipe and reset the ecu to zero the LT values. Other than increasing the bypass percent the LT eventually returned to 100%

Pretty sure the chip is original. Have also checked tune resistor which is white. This seems correct for a 400 Cat.

[spitfire4v8]

nzflash said:

Thanks for the suggestion. Tried removing the vacuum pipe and reset the ecu to zero the LT values. Other than increasing the bypass percent the LT eventually returned to 100%

Pretty sure the chip is original. Have also checked tune resistor which is white. This seems correct for a 400 Cat.

Oh well, worth a quick try !

[nzflash]

Latest update. Took a MAF off a mates 450 and trims behaved the same. So I think the next step is to test fuel pressure at the rail.

Also, for my understanding, am I correct in thinking a vacuum leak would show up as a positive fuel trim rather than negative?

[blaze_away]

nzflash said:

Latest update. Took a MAF off a mates 450 and trims behaved the same. So I think the next step is to test fuel pressure at the rail.

Also, for my understanding, am I correct in thinking a vacuum leak would show up as a positive fuel trim rather than negative?

That's logically a yes but the very nature of vacuum leaks is they are not entirely consistent. From what I saw in your logfiles and now this "maf test" I'd be looking at fuel pressure and smoke test for vacuum leaks.

[adavy]

You need to do a proper smoke test. You can make a smoke generator for less than £5

[pmc_3]

Might be worth checking the return to the tank from the fuel rail, if this is blocked it will cause the fuel pressure to rise. Also check your injector wiring, especially where it passes the coil bracket, it's been known for it to rub through and cause a short.