Is this a MAF or fuel issue causing accleration stumble? Both?

[Brendang2000]

This is a standalone from a 2003 Yukon 6.0. It had a 4l60, but I installed and segment swapped to a 2wd 4l80. The engine internals are all stock. Exhaust manifolds are Camaro that I cut off the flange and welded 3? exhaust to, air intake tube is 4? to stock (new) Delco MAF, two 90?s with 14? total straight between them and 10? straight before MAF. I don?t have a wide band connected currently, working on it, but have a non-log able fuel pressure gauge that I can view while driving. At idle, fuel pressure is at 52* and under acceleration pressure rises to around 58-60*. The stock truck intake/fuel rails are used and there is a vacuum line connected to the stock regulator; there is a return line on this fuel rail. No cats. Car idles great and as long as you don?t give it much throttle, drives fine. Under throttle, engine stumbles bad, and it seems it may stumble less with harder throttle. Log ?3rd drive? is partial throttle acceleration, while ?4th drive? has harder throttle acceleration and didn?t stumble as much. I did test drive this engine prior to the standalone swap, and engine had no noticeable issues. I made sure wires to the MAF are connected to the ECM correctly and tested that it has power and ground (at least when tested, it did). I checked for vacuum leaks using carb cleaner, and didn?t find any; stock (new) truck PCV is used. One DTC set is P0102.

What I have noticed in the logs is, timing is being pulled dramatically in some cases, also in ?4th drive? log, at time 2:53, the MAF jumps high to low multiple times, but throttle, RPM and MAP don?t change. I?m thinking I have a MAF issue as it reads very low sometimes and figure that?s why it set the P0102, but not sure if that?s the only issue. When timing is pulled, the engine appears to be running lean sometimes according to the stock O2s. Fuel pressure seems low to me, I?m thinking is should be 58* at idle, but maybe the vacuum regulator adjusts for that. When I disconnect and plug the vacuum line, pressure stays at 59* at idle; didn?t test drive without the vacuum line. I had the injectors refurbished, entire fuel system including tank, is new, 3/8 feed and return.

I?ve been using HP Tuners for years now, but mostly just for standalone operation, disable VATS, adjust fans, disable unused DTCs, etc. I can log any info that?s needed (usually), but don?t know how to interpret the information that I gather. Should I force this engine into SD mode and see if I still get the stumble, drive without the vacuum line connect to the fuel regulator, both? I?ll keep poking away at this and researching, but any help identifying what?s going on is greatly appreciated. Thanks!!! B

[Brendang2000]

So I disable the MAF by setting "MAF Frequency Fail High" to zero and got P0103. Drove vehicle around the block with a few acceleration attempts and didn't stumble at all. So, pretty sure it's my MAF causing the issue, but now why is this MAF not reading correctly? I'll pull a MAF from one of my other cars, re-enable the MAF and see if I still have the issue.

[Brendang2000]

Here's a run with the other MAF, (actually I swapped the entire intake tube with MAF) and no stumble. So either it's my intake tube, or my brand new MAF is bad. I'll try and do some more testing and post back what I find.

[5FDP]

Put the new MAF in the old intake, which I assume you are going to try.

While you're in there do these few changes, it should help.

Set the engine abuse settings to 7,000rpm and 0mph, so that isn't a cause for a power loss.

Then change the power enrichment settings to these. Raise the MAP KPA enable to 80kpa, lower the enable torque to 80%. Lower the HOT/COLD throttle TPS down to 65%. Raise the enrichment ramp in rate to 2.0000. Remove the 5,500rpm delay, set it to 1,500ish. That should help wake the engine up a little bit.

Also being a swap vehicle with the rear o2 sensors disabled, don't forget to disable cat over temp protection.

Add a few more things to log as well. Like knock retard, injector pulse width avg for both banks, cylinder airmass and commanded air fuel ratio.

[Brendang2000]

Put the new MAF in the old intake, which I assume you are going to try.

While you're in there do these few changes, it should help.

Set the engine abuse settings to 7,000rpm and 0mph, so that isn't a cause for a power loss.

Then change the power enrichment settings to these. Raise the MAP KPA enable to 80kpa, lower the enable torque to 80%. Lower the HOT/COLD throttle TPS down to 65%. Raise the enrichment ramp in rate to 2.0000. Remove the 5,500rpm delay, set it to 1,500ish. That should help wake the engine up a little bit.

Also being a swap vehicle with the rear o2 sensors disabled, don't forget to disable cat over temp protection.

Add a few more things to log as well. Like knock retard, injector pulse width avg for both banks, cylinder airmass and commanded air fuel ratio.

Thank you for the reply 5FDP! I have to run out for the evening, but will make the changes you suggest in the morning, give a try and repost a log. Yeah I was going to try swapping both MAFs, new MAF into other tube and old MAF into this tube, just to be thorough; the other tube is only 3.5". Thanks again!!

[Brendang2000]

Morning!

So I made the changes to the tune, logged the additional PIDs, put the new MAF into the old air tube and fired it up. The MAF didn't read anything, stuck at .21 lb/min. At first I thought I forgot to plug it in, but that wasn't the case, it just failed to read. After that, I swapped old MAF into new air tube and it ran great, no stumbles at all. So a brand new Delphi MAF was bad right out of the box; wish I bought this more recently as now I can't return it. But hey, at least the car runs good and I can move on to getting all the other bugs that show up fixed! Thanks for your help and tune recommendations 5FDP!!

[5FDP]

Good that you got that figured out.

One more thing in that log needs to be looked at, just based upon what I see. When you give it enough throttle and it dips into power enrichment the o2 sensors go lean. I wonder if under enough load you are dropping fuel pressure here. Without a wideband sensor in the exhaust it's hard to really know how lean it's going but usually you can tell by the o2 sensors dropping out to 50mv when they should be in the upper 800's to low 900's.

[Brendang2000]

Good that you got that figured out.

One more thing in that log needs to be looked at, just based upon what I see. When you give it enough throttle and it dips into power enrichment the o2 sensors go lean. I wonder if under enough load you are dropping fuel pressure here. Without a wideband sensor in the exhaust it's hard to really know how lean it's going but usually you can tell by the o2 sensors dropping out to 50mv when they should be in the upper 800's to low 900's.

Thanks for the heads up 5FDP!! I should be able to get my wideband in this week, although I'll only be reading one side as it's a dual exhaust with no X pipe. Until then though, I'll not be doing any hard pulls. I'll post a log with the wideband as soon as possible. Thanks again!!!

[Brendang2000]

So it does appear that I'm running lean, as long as I have my wideband connected and setup properly, the Innovate LC-2 is connected to input 1 on my MPVI Pro and I've added the channels config file. It's going to almost 17 AFR under acceleration and higher after I let off throttle. I currently don't have a way to log fuel pressure, besides staring at a gauge while driving, so I'll look into purchasing a log able sender. Also, I'll try disconnecting the vacuum line to the regulator, and going for a drive to see if AFR improves. When I disconnected it with my fuel pressure tester connected, it went from 52* at idle to 58-59*.

[Brendang2000]

Attached is a quick run around the block with the vacuum line going to the factory fuel regulator plugged. While not perfect, it seems to be better, it didn't hit 17 the few times I was on the throttle; it did hit 17 after I was off throttle. I'll change out that regulator and see if things improve, since I can't swing buying a log able fuel pressure gauge right now. If anyone has any other ideas I can try, please let me know.

[5FDP]

Try the regulator. If that doesn't help you'll need to tape a pressure gauge to the windshield or something so you can watch the pressure. Maybe your pump just can't keep up.

[Brendang2000]

Thanks 5FDP. While waiting for the regulator to arrive, I'll run it with my fuel pressure tester attached to the schrader valve on the fuel rail. Should be able to get some runs that way tomorrow.

[Brendang2000]

New MAF arrived and I installed that, and I was able to get a good run in this morning with my fuel pressure tester attached to the fuel rail, and pressure appears to be good. Off throttle, it stays at ~52#, but under any good throttle it raises to ~58#, even under prolonged acceleration it doesn't drop. Now this is pretty vague, my gauge is at an angle so can't see exact numbers and it's hard to watch a gauge while accelerating on the street, but it's very close. The major odd thing is though, the AFR numbers are no longer lean. Under acceleration I even saw a 10.xx AFR (11:22), with some knock detected. It goes way lean after I'm off the throttle, is that normal? I saw a 22.xx AFR after one pull (18:10). One thing I will try is, running the new air tube with the new MAF as this log is the old 3.5" air tube with the new MAF. I wonder if the 4" tube is somehow letting more air in than the MAF is seeing? The new regulator should be here this week, I'll still put that in and recheck pressures. Intermittent issues.....the worst

[5FDP]

The lean after you let off is normal. Watch the injectors and o2 sensors both go down to nothing. The computer shuts off the injectors for decel fuel cut off and that's why the widebands reads way lean.

Being richer than commanded under load is pretty common on a stock tune. It just needs a little work to the MAF/VE to bring the fueling in line with what it's commanding. You don't need anything larger than the stock intake tube size, you aren't outflowing that.

[Brendang2000]

The lean after you let off is normal. Watch the injectors and o2 sensors both go down to nothing. The computer shuts off the injectors for decel fuel cut off and that's why the widebands reads way lean.

Being richer than commanded under load is pretty common on a stock tune. It just needs a little work to the MAF/VE to bring the fueling in line with what it's commanding. You don't need anything larger than the stock intake tube size, you aren't outflowing that.

Thanks for all your help 5FDP!! I'm going to keep an eye on the AFR still and wouldn't mind logging fuel pressure eventually, but for now I think my original issue has been answered. If it starts to run lean again, or something else shows up, I'll start a new post. Again, thanks for all your help and advice!!