Another MAF failure?

[nzflash]

On my 99 Chim 400 rovergauge is showing - 100% long term trim for both banks.

Have tested MAF and spikes to just over 0.6 volts before settling down to about 0.4 after about 10 to 20 seconds.

Firstly, does this sound like a failed MAF?

Secondly, does anyone know of a reputable place to source a replacement?

Other than appearing to run a bit rich the car runs well.

As usual, thanks in advance for the support.

[nzflash]

blaze_away said:

Cheers Steve....yep here to help if you want it.

I will need a specifically run dataset. Will post how later when I can fire up my laptop..

Edited by blaze_away on Wednesday 1st May 08:15

Awesome, thanks.

[nzflash]

blaze_away said:

your profile does not permit me to email you so you'll have to msg me via ph

Can you try now? I've tried updating my profile.

[nzflash]

Thanks all. Frank is reviewing a log for me to determine if anything is amis. I'll provide an update when we have the results.

[nzflash]

Thanks Frank that's awesome.

Further detail on the car..

Running bpr6es plugs with plug extenders removed.
Full manifold vacuum
Relatively new stepper motor which improved running behaviour

Generally the car runs well and doesn't stall, returning to smooth idle after driving.

I'll try resetting the base idle again to try and lift idle bypass to 30%

Cheers.

[nzflash]

Latest update. Brought the car up to temperature to the point the fans came on and off.

Adjusted idle so bypass is now reading 27%.

Disconnected ecu to perform reset.

Started car and confirmed long term trim was zero.

Over a relatively short period the longterm readings went to - 100% on both banks.

If I clamp the idle bypass hose the revs drop significantly and the engine will stall at about 500rpm....

Would appreciate any other thoughts out there to explain the long term readings.

Thanks.

[nzflash]

That is my next plan. I've been in touch with a mate to try his on my car once he's confirmed his is reporting correctly.

If anyone has a healthy spare they're willing to part with I'd be interested in buying it. No harm having a few spares as I'm in New Zealand so spares are not readily available.

[nzflash]

blaze_away said:

Before you off chasing ghosts that is not how to set base idle.

Fully warmed engine clamp stepper feed and adjust revs to 650 700 ish. Unclamp stepper hose.

See what % you get. Is it at 25%ish ?
No you have a problem eg vac leak.

Setting it the way you did it will result in system being set incorrectly. What you've done is useful because it now does suggest you have vac leak albeit only a small one.

Hope that helps.

Edited by blaze_away on Saturday 4th May 12:41

Have reset base idle again and bypass is back to 20%.

Have gone over all pipes carefully and can't find any leaks.

Have checked plenum and all seems good.

Not sure if running full manifold vacuum affects achievable bypass values?

Trying a friend's MAF soon will help with diagnosis process...

[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?

[nzflash]

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

[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.

[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?