Time2Kill

iTrader: (49)

I've got a Silverado SS that I just put a 414 into, and when I took it the dyno, the tuner cannot get the AFR to stay where he wants it (12.8~13.0). He will make one pull, and it will be dead on, but then make another pull without touching the tune and the AFR will lean out to 15 or 16. When scanning the MAF during the pull, it will just all over the place, as much as 40~45 g/s throughout the entire pull. He thinks that there is an electrical problem or interference with the MAF. Is it normal for the MAF to fluctuate that much during a pull?

We have tried several MAFs, moved the MAF farther away from the intake, tried a stock truck intake tube with baffles in it, and nothing seems to make a difference.

The only thing I can say, the tuner did not adjust the P0101 max delta airflow or the MAP max error tables. The MAF and MAP codes were being thrown on the dyno, but I have since fixed that on my own and the MAF still acts the same way on the street when I log it. The tuner also did not disable STFTs during openloop, so I'm not sure if those could of been throwing the tune off that much or not. I have since also disabled STFT during open loop to try and figure out whats going on.

When I do back to back runs on the street, the O2s can be at .930mv for one run, then I can stop, do another run, and they will only be at .880mv. Seems kind of odd to me that they flucuate that much from back to back runs during openloop with both LTFT and STFTs disabled.

If anyone has any ideas, please post as I need to get this truck running ASAP. I've been to the tuner twice already with no luck.

Two log files are attached along with the tune. For both logs, the truck was locked into 2nd gear for the WOT pull. One log is with the stock truck intake tube on, the other log has a 4" intake tube with a K&N air filter on the end with the MAF right next to the filter.

Engine mods: Engine is a 414 (Iron block, 4" stroke), 246/252 .630/.635 +4 112 lsa cam, Edelbrock Pro Flow XT intake (http://www.edelbrock.com/automotive_...f-xt_ls1.shtml), 42lb GM Marine Injectors, 4" Intake tube (90* off of the intake and runs over to the fender with a K&N cone filter with the MAF attached to the Filter), AFR Heads ported by TEA, flow right at 340 at .600 lift, 1 7/8 LT Headers.

98Camarod

iTrader: (43)

Is the MAF ported?

Time2Kill

iTrader: (49)

No, its a stock 85MM MAF for the truck. I have tried three different stock MAFs that I have here in the shop. They all do the same thing. I have also moved that MAF to different spots in the intake tube and get the same results.

Gary Z

If you can't find a problem with the MAF wiring or connector then try another PCM.

Time2Kill

iTrader: (49)

I have already tried another PCM. Forgot to mention that in the original post. I checked all the wires of the MAF, all are less than 1 ohm resistance. I also took the plug apart and pressed the pins down to ensure a good connection when connected to the MAF.

Does everyone agree though that there is a problem with the MAF bouncing, or is that normal with this large of an engine? I've had multiple 408s tuned and as far as I know, its never been a problem before.

Gary Z

I see nothing obviously wrong with your MAF signal. Raw MAF signals respond to turbulence in the airflow and are always somewhat erratic. The PCM filters the raw MAF signal when calculating dynamic airflow.

Time2Kill

iTrader: (49)

The tuner said that the maf signal should not very by more than 20 gram/sec. Mine is varying up to 45 gram/sec. The only reason I question it is because we can have back to back runs with the exact same tune that provide completely different Air/Fuel Ratios. The tuner is trying to blame it on the MAF and I don't know the first thing about tuning so I must take his word for it.

The tuner did forget to turn off STFTs during open loop, so I don't know if thats what as responsible for throwing the tune off or not, although I find it hard that it would swing from a perfect run at 12.8 to a terrible lean run at 15 or 16 AFR.

Gary Z

The difference between 12.8 AFR and 15 or 16 is much larger than the supposed MAF errors. AFR is not that sensitive to the MAF signal. I think replacing the MAF and PCM, and checking the wiring as you have done, shows that the MAF is not the cause of your AFR instability. You could have a fuel supply problem.

JonCR96Z

iTrader: (33)

How can you blame it on the MAF when you've tried 3 of them and tested the wiring? The dynamic air looks pretty smooth throughout the WOT pull.

I tuned an 03 SS that also has the OL STFTs enabled stock. Not being used to that being enabled I didn't even check it, but the tuned dialed in fine. And over countless miles of tuning and WOT pulls I never noticed any out of the ordinary until I replaced the O2 that I had been using the bung for my wideband and it was fucked up. Then the truck went to ****.

Though in my case the VE and MAF were pretty thoroughly tuned. Your MAF table is only modified in the upper airflow areas and VE only in the low RPMs range. And you're PE is commanding 11.0:1 or richer. So maybe the airflow model being so radically different from what it actual is, is allowing quite a bit of tweaking from the STFTs.

Time2Kill

iTrader: (49)

The truck has a fuel pump hot wire kit along with a inline Walbro 255 LPH pump. Fuel pressure is set to 58lbs and drops to about 54~55 at WOT throttle. Wouldn't a fueling issue show up more towards the upper RPM ranges? When the truck leans out, it will start right at the beginning of the pull and stay lean the entire pull.

Time2Kill

iTrader: (49)

This tuner has tuned this exact same motor (used to have an LS2 intake instead of the Edelbrock Pro Flow) in two other vehicles. Looking at previous tunes he has done, it looks like he tends to tune via the IFR instead of fine tuning the VE and MAF tables. Looking at the tune from when this motor was in a F-Body, it looks like he went the same route. The lower VE table had fuel added, and the MAF was slightly modified in the upper half.

I just can't believe he is having troubles with this truck, he has tuned plenty of other vehicles just fine, including this motor in two previous f-bodies.

Gary Z

By "fuel supply" I was thinking of everything from the tank to the injectors, not just the pump. You may have a loose injector ground connection. It may not be fuel. Has the wideband been checked?

Time2Kill

iTrader: (49)

He tunes several cars a day, so I can only assume that the wideband is reading correct. I've been to his dyno twice trying to figure this out, and there have been many cars between my trips.

Any way to diagnose a loose injector ground?

I did have to depin the injector connectors on the driver side because I had to reroute some wiring to get the Pro Flow intake to fit. I'll double check all the connections tomorrow when I get back to the shop and check the resistance to ground on them also.

My only other solution if this doesn't go away at the next dyno is to put a new engine harness on the truck. I have another set of injectors that I could swap in also. I currently have just a fuel pressure gauge on the regulator which is hard to read under WOT because the needle isnt steady, but I'll run a digital fuel pressure gauge inside the cab tomorrow and verify that fuel pressure is stable under WOT.

JonCR96Z

iTrader: (33)

IFR raping isn't good tuning. Do yourself a favor and invest in some tuning software and a decent wideband. Not likely to be your problem but it's definitely not the solution.

98Camarod

iTrader: (43)

That's the uninformed way of tuning. Are there any other tuners in your area?

If you buy your software: you should have the tune in SD first and get the VE table right, and then since you want the MAF, turn the MAF on and get that right. The IFR should be a constant in the tune, not something that you use to compensate for fueling.

Old Geezer

iTrader: (3)

"Looking at previous tunes he has done, it looks like he tends to tune via the IFR instead of fine tuning the VE and MAF tables."...

Time2Kill

iTrader: (49)

I understand that you should not tune via this way, but what are the benefits to tuning via MAF/VE Tables. I understand that the PCM will have the correct airflow models in it, but does it lead to driveability, more power etc?

turbolx

Tuning by tweaking the IFR or PEvsRPM tables ignores any temperature or actual airmass changes from run to run. As temp actually changes, you open the door to an incorrectly paired "corrected" IFR to go with what is also an incorrect VE or MAF value. Turning off the IAT/ECT compensation algorithm only makes this worse...

Eagle02

iTrader: (20)

Silly question, but with a cam that big... you're probably dealing with some reversion through the MAF? I'm going to take a look @ your scans here in a few.