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The good old - Tech A says... Tech B says... Who is right?
Application - 2005 Nissan Altima with the L4-2.5L.
Customer Complaint - vehicle has an erratic idle (i.e. idle flucuates quite quickly between 400 - 1200 rpm; 400 - 1200 rpm; 400 - 1200 rpm; and the up and down rpm cycle continues)
Background Info - I was speaking with a tech/buddy at another shop before he started the diag on this 2005 Altima with the erratic idle. My buddy never got any numbers from the car (i.e. no fuel trim data or MAF voltage / MAF Hz reading). The shop that he worked for simply had a spare MAF for this vehicle - so he simply put the other MAF on it and all was fixed (so he says).
So, the good old Tech A vs Tech B question:
Tech A says - idle fluctuation is due unmeasured air after the mass air flow sensor (i.e. vacuum leak)
Tech B says - idle fluctuation is due to the mass air flow sensor providing bad info to the computer.
Who is right?
I say Tech A is correct. If the MAF was bad, you could have a bad idle, but it would not keep fluctuating up and down; up and down; up and down. If you ask me, the RE and RE of the MAF cleaned up the vacuum leak.
Technician C:
I find that erratic idle, IF CAUSED by a bad MAF is due to sensor dropout and an under reporting or over reporting MAF tend to at the least stay somewhat steady (more like they’re out of calibration.)
Making Pressure Differential Sensors (PDA Sensors) for pressure pulse diagnostics.
Currently servicing Central Texas.
I'll take B. Nissan relies heavily on the MAF reading for just about everything. A skewed reading could easily cause an idle issue.
I'm not sure how replacing the MAF would have fixed a vacuum leak? Unmetered air, maybe, IF the intake boot was found to be torn and replaced along with the MAF.
Anyhow, I go with Tech B for the following reasoning:
Nisaan/Infinity engine controls of that vintage - and some still do up to data - feature a quite peculiar strategy:
When registering a lean condition ( which well might have been a false lean due to a bad MAF ) the pcm will ramp up the short term fuel trim in an intent to correct the issue. However, once the stft reaches the 20% mark and NO change in the O2 signal is observed the pcm reverts to open loop and after a couple of seconds again enters closed loop, increases the stft up to the 20% level and this cycle will repeat over and over.....So a badly skewed MAF may heve been the root cause.
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