gregrob

iTrader: (8)

I'll make a long story short here.

I picked up a 99 Z28 bone stock. It ran ok, but pretty lazy.

It was like something was holding it back and it didn't want to rev. Revved slow and kind of "stair stepped" through the RPMs.

I got EFI Live on it and started scanning. Scanner was reporting 46 degrees of timing at WOT I was like

All of this with NO Knock Retard

I started looking at the 02 mv at WOT and they would drop to 42-46. That's right FORTY, no fuel at all.

I started thinking fuel pump, etc.

However, I decided to unplug the MAF. Voila, car felt like it picked up 100hp. Timing back to 28 deg, normal, and 02 MV in the mid 900s at WOT.

The MAF was obviously bad and commanding NO fuel at WOT. Car kept adding timing to compensate, and wasn't even getting enough fuel to make it Knock lol.

Anyway, runs great on the speed density tune I brushed up for it, til I get a good MAF back on there.

Just thought I would share my experience, in case anyone runs into this. Severely high timing on a stock tune, and low 02 readings. Check the MAF.

Frost

iTrader: (45)

This is an example of MAF failure that does not code that I frequently refer to...

It still works but it reports LOW. The timing was high because the PCM was thinking that the car was being operated at a very light load condition (because of actual airflow being so far under-reported).

There are tuners that are even sponsors here that have missed this in the past and just jacked a pile of fuel into the tune to "fix" it.

Another sign is that the car will typically be carrying a high + LTFT at idle/cruise when this condition is present.

gregrob

iTrader: (8)

Yep, the LTFT was a lot higher! Good to know you have ran into this before. Had me scratching my head for a few minutes.

Slowhawk

iTrader: (12)

Pretty common problem with these cars getting older.

Vengeance Racing

YEP the older they get the more issues appear like this.

brettmc

could this be seen in the log as a low maf flow rate?

macca_779

More reason to ditch the MAF forever.

revtech101

Good to know.

One question, did the MAF voltage reading show up much lower during real time scanning? I understand you may have to drive the car with the laptop plugged in to move more air through the MAF or could you test this while parked and revving?
I'm assuming F-body MAF's can output up to ~5v max like most other cars.

The reason I ask is because this will eliminate having to wait for a "MAF code" to show up.

Frost

iTrader: (45)

They don't code for this mode of failure, so people generally never know about it. They also use a frequency output vs. voltage (these aren't Fords!).

revtech101

^Okay. Real time scanning and comparing to a good signal would track this issue in seconds regardless of what type of maf it is.

2TR

There is no compare good vs. bad "MAF signal" feature in the VCM scanner.

You would have to KNOW the MAF is doing something out of the ordinary. Like a mechanics diagnostic test of process of elimination until a non-obvious source of issue is located.

revtech101

I suppose I should have said that you need another car with a good maf signal to compare to.

firebird99

iTrader: (2)

Not the jack your thread but my car does that same thing even after buying a new maf it even has the super high timing but I get knock every third or fourth time I run the car not every time.

RamAirRocket

iTrader: (1)

It wouldn't be that hard to code for it. Would be easy to have a common sense table that compares what SD would be anticipating and if MAF readings seems off, send them to the dealer.... In other words, if you are at 4000 rpm and MAP value is around 90 kPa, and MAF reads 40 gm/sec, it doesn't pass common sense....

Probably a simple lookup table for it would do the trick.

My 3800 L67 MAF started to fail like this, I caught it after scratching my head for a couple datalogs and it didn't pass the common sense test. Pull MAF and it pulled good again. LS6 MAF now....

joecar

Log MAF g/s and DYNAIR g/s, if the MAF is good then those two would be close.