2002 firebird 3.8l mystery bucking

[camaroguy28]

I am working on my wifes 2002 firebird with the 3.8l v6. it drives well at part throttle and idles well too, the problem comes when you go WOT. what it does is it will pull ok up to about 4,xxx rpms and then it will buck very hard and the timing and fueling goes nuts and I get a lot of knock retard. I have been trying to figure this out for a while now and cant seem to get anywhere with it. sometimes it bucks at 4,000-4500, sometimes its up around 5,000-5400. On occasion it wont buck at all and will run straight up to 5800 or so where the shift point is set. If I let off the throttle while it is bucking it seems to smooth out, and it also would run up to about 5200 in open loop with pe set to 14.7. I only did that to hit the 7500hz like the guide russ k has in the stickies as I didn't want to hurt the motor. the car has a slp lid with a k&n filter, and port matched upper and lower intake manifolds. I don't think anything else is done to the car on the intake side. the exhaust is the factory v6 dual muffler out with a gutted cat that has the shell of the cat still in place. if anyone can look at the log and tune and see if they can figure it out I would greatly appreciate it. I will say that I have put a new MAF sensor on it about 3 years ago, and a new fuel pump and new injectors. the fuel pressure is constant even during the bucking.
WOT MAF run1.hpl
WOT MAF 1.hpt

[5FDP]

Do you have a spare MAF to try and see if it helps with the issue?

Even when you were at full throttle with increasing RPM the MAF did have a few random large drop outs and that could make it feel like it's cutting out.

What about misfires, have you logged that yet just to make sure it's not another cause to this?

[camaroguy28]

I have a spare maf off a parts motor I can try tomorrow, and as far as misfires, I was getting them on 2 cylinders so I changed the coil pack for those two cylinders and that cleared up the misfires. It bucks extremely hard during the "cutouts", and it seems like if I drop the throttle position just a little before it starts, I can make it avoid doing it at the expense of the extra power. the Maf error graph had some very high numbers for the hz that were affected, but it may be a result of the bucking doing that.

[coachcrow56]

Have you checked the fuel filter???? I had similar problem with my GTP----car would run smooth, but any time I pushed it, it would buck and spit. Filter replacement solved the problem!!

[ZeroBoostBuick]

You need to start scanning every little thing possible on the main table (on the left)... I always scan everything weather it matters or not. Just add all possible channels and let the scanner sort it self.

Since your car is N/A and your tune could use a little fine tuning... Take a look at what I had on a 98' Regal LS.
I did lots of mods to it and after learning how to tune properly I ran 14.3 @ 97mph down the 1320 tarmac on 87 octane. Before I tuned it, it ran 15.3
Warning,.. The WOT Timing is much higher than stock. Besides the timing, all other parts of the tune are safe to copy.

98_Regal-LS-read-sept11-2013-.hpt

[camaroguy28]

I installed the Maf off of the parts motor this morning, and she revved all the way to 6,000 rpms. I tried it twice and both times it revved all the way up there. I am now working towards fine tuning the part throttle and then will be working towards the WOT which is entirely too rich as in FI too rich lol. The part throttle trims are coming in nicely now, so we will see how it acts during WOT tuning. as for the fuel filter I put a wix filter on it about 6 months ago and that didn't really show any real changes, but I know what you mean about that fixing issues. my 98 z28 was bucking hard around 4k rpms and wouldn't go any higher and I changed that and it revved back up where it was supposed to be. Good news on that car was that was part of the reason I got it so cheap was it wouldn't rev up all the way. $7 or so and it was all better in that regard.

I do have a question however about tuning WOT. I understand the write up that Russ K made and it has helped a lot so far, but I am wondering if after I am done do I just change it back to closed loop enabled and let it ride with the new Maf table, or do I need to change anything so that in closed loop it commands stoich instead of the commanded afr jumping all over the place?

[ZeroBoostBuick]

For wot best is to set the PE around 13.0 and tune with the maf to match the wideband afr error% to the commanded afr vs maf hz.

[camaroguy28]

ok, will do. Afterwards I just convert it back to the original settings for closed loop and keep my pe stuff set where I want it, is that correct? of course I keep the new Maf settings as well. By the way, thank yall for your help so far. this has been an ongoing thing with her car, she didn't care but it drove me nuts lol. I had installed a new Maf but its a Delphi part, and I meant to pick up and acdelco part. I think it may have been junk out of the box, but who knows.

[ZeroBoostBuick]

Yes that's correct.

Just search on how to do MAF tuning. Setting up the histograms is the key.

[camaroguy28]

Alright, I logged a bit of WOT today, and good news. My first log with wideband on the car about a week ago, the afr was commanded 12.95 and the actual was like 10ish. After logging today and making some much much needed changes I have a commanded afr of 12.95 with an actual afr of 12.93. it trickled to about 12.00 in one spot, but I am definitely getting much closer than before. Again, I want to thank everyone for their help on this issue. I don't drive my z28 much, so I try to use the bird as a bit of a toy when I get to drive it(usually I drive our suburban, not much power for extra fun in that heffer). The car feels better than I ever remember it being, and I actually enjoy driving it now.

[ZeroBoostBuick]

Good to see you got it sorted. The 3800 cars are ideal to learn basic tuning.

[camaroguy28]

it hasn't been bad, I have had a bit easier time with the ls Camaro I have, but I think that is because I wasn't fighting the issue with the maf sensor.