MAF unstable - stock LS3 A6 Corvette

[Pekka_Perkeles]

This is totally stock 2008 A6 LS3 Corvette. The owner complaints there is bucking at low TPS situations. It is easily noticeable at coastdown situations.

Now, after googling a bit, it seems to be a common problem with LS3 engines. I took a two scans with just default parameters.

It is interesting to notice that MAF output seems to be very unstable. In the picture below I have changed MAF and RPM scale so it is a bit misleading, but anyway as an example there seems to be huge, +50% change from 5 g/s to 8 g/s, and at the same time RPM varies as well. All this with steady TPS, steady vacuum, steady advance.



Is this something that can be tuned out? Any pointers what to do (you can PM me, PayPal ok)?

[wstaab]

Read this:
http://store.gmpartshouse.com/partlo...catalogid=8669

You may have a "weak" MAF card.

[carlrx7]

disable the maf, run VE only, there is a known issue with weak maf's sold to GM, google it to find out a solution, (usually to purchase a new one).

-Carl

EDIT: what wstaab said... lol damn cookies

[Pekka_Perkeles]

Thanks for the comments! I will try to disable MAF next week.

Meanwhile, aren't these nice logs...:

[DSteck]

Wow.

[Pekka_Perkeles]

Wow.

But hey, MPH (km/h in european units) seems to be steady... :-)

Ugly, yes I agree. We'll try new MAF in coming weeks.

[DSteck]

Are you seeing misfires or anything?

[Pekka_Perkeles]

Are you seeing misfires or anything?

Didn't bother. Local dealer took care of usual stuff.

A private joke:

Well, ok, I requested this for just a sake of "unknown tuner":

http://www.youtube.com/watch?v=AdKjEHfHINQ

[Pekka_Perkeles]

Disabled MAF, car runs very smoothly, no problems whatsoever.

In the pic below, while MAF is disabled (in a sense that PCM does not care of it's input), I think HP Tuners can still log it.

Dynamic Airflow is smooth, and thus RPM's as well (note: the RPM range is very small):



Funny thing, MAF Hz at idle is around 1800 Hz, so it isn't inline with this:

"What we discovered was that the 'WEAK" meters were reading 1560-1670HZ at idle with the A/C off (OEM is 1800MHZ+)"

http://store.gmpartshouse.com/partlo...catalogid=8669

[DSteck]

Sure there's no vacuum leak?

[Pekka_Perkeles]

Sure there's no vacuum leak?

That's something I was thinking also by myself. At least dealer couldn't find anything.

Also, I guess erratic MAF readings could be because of some kind of turbulence, or some kind of resonance.

As far as resonance, the thing that bothers me is why it won't go away when MAF goes from 8 g/s (average) to about 21 g/s (average) as in the pics in post #4.

[DSteck]

I'd check to see if anything is messed up inside the air cleaner assembly or there isn't a hole in the filter.

[2xLS1]

I've run across this on a A6 Vette too. It had some stupid high rear ratio rear. I want to say it was a 2.56:1. What I found was with the converter locked up in 5th and 6th gears at those low rpms it was just lugging around. I cured some of it by delaying the converter lock up in the higher gears. Seemed like just a stupid gear ratio for a performance vehicle with so much overdrive ratio in the transmission. Driving around at 50 mph in 5th and 6th gear with the converter locked at 1200 rpm is just a bad combination for smoothness regardless of the engine tune.

[Ben Charles]

Is the Timing stock? I have seen higher timing values mess up the HZ reading from the MAF sensor before.

[Pekka_Perkeles]

Is the Timing stock?

Yes sir, very stock!

First I thought it's the bloody torque management that's causing the issue, as I saw up to -5 degrees of advance in some situations (coastdown mainly), but when tm disabled, it didn't help at all. Only after that I started zooming logs and found how erratic MAF and RPM's were.

[Ben Charles]

I have heard of bad card style MAF from the factory. What code is on your MAF, does it start with a K?

[Pekka_Perkeles]

Just an update. The car visited local tuner shop and a new MAF sensor was installed.

Still the same old behavior.

New MAF is not those "weak MAF"'s mentioned earlier in this thread.

It seems that MAF is actually reporting airflow correctly. So there must be some kind of turbulence or resonance ongoing at low throttle openings.

I googled Corvette Forum a bit, and found some interesting posts there:

"I have heard people complain of surging from just about every type of aftermarket unit out there. Even running the stock LS3/7 air intake and adding a Vette-Air scoop can cause turbulence (I had bad surging, and CEL, with this MOD and it took a lot of time to correct the problem by fabricating a diffuser to prevent the incoming air from being forced directly into the mouth of the air breather. I found that using the Attack Blue filter in the stock housing did not straighten the airflow nearly as well as the OEM Donaldson PowerCore element). GM has experienced customers complaining about surging (and CEL) on completely stock cars. The TBS instructs the tech to not mess with trying to diagnose the problem if the car has any type of aftermarket air breather modifications. If the car has no air breather MODs, then the tech is instructed to make sure there is no air leaks between the shroud and the bumper beam and even instructs the tech to tape over the holes in the bumper beam to prevent air intrusion directly to the mouth of the air cleaner housing."

Another post on CF:

"As only part of my engineering background dealt with air control I think I remember the part about turbulence in a closed system being a function of shape and directional speed of airflow. 70 MPH is about 1500 RPM and only requires about 165 CFM on a LS3. That's roughly 4 CFM per square inch of filter face area. The stock LS3 filter is about 3-4" thick with around 100 passages per square inch, which gives plenty of time to difuse turbulence on direct inlet airflow at most any speed and create nearly linear airflow which is then directed by the housing shape prior to passing the MAF.

The surging happens when that airflow is not evenly distributed and/or perpendicular to the face of the filter. Pre-filter turbulence causes differential pressures post-filter in the same manner as putting your hand out the window to experience the buffeting effect. GM resolves the problem by eliminating direct turbulent sources. Vararam takes in air and straightens it slightly, then dumps it into an expansion chamber prior to the filter to slow the speed, which effectively kills the turbulence. A propped shroud works kinda OK at low vehicle speeds opened wide and higher speeds with a narrower opening to reduce turbulence."

Rather interesting, isn't it?

[vette_c6]

post your file

[Pekka_Perkeles]

Here it is.