stumble at idle / too much air?

[Craig 94 TA GT]

Is there a such thing as "too much air" for the LT1 at low RPMs? I've always had stumble/hesitation when you hit the gas harder than normal from idle. Here's a sample reading of when it goes from just hitting the gas pedal to ~3 mph with the stumble:
1) TPS Volts: 1.18
RPM: 575
MPH: 0
Cell/BLMs: 18 / 129/124
Knock retard: 0
MAP: 0.56
MAT volts: 1.84
Air flow: 18.89
Spark timing: 28

2) TPS Volts: 1.31
RPM: 700
MPH: 0
Cell/BLMs: 18 / 128/124
Knock retard: 0
MAP: 0.88 <--- normally not this high?
MAT volts: 1.84
Air flow: 37.91
Spark timing: 12 <----- this looks abnormal?

3) TPS Volts: 1.35
RPM: 875
MPH: 2
Cell/BLMs: 18 / 128/124
Knock ret: 0
MAP: 0.90 <---- getting higher
MAT volts: 1.84
Air flow: 31.77
Spark timing: 12 <----- still this low??

4) TPS Volts: 1.35
RPM: 1275
MPH: 3
Cell/BLMs: 18 / 128/124
Knock ret: 0
MAP: 0.84
MAT volts: 1.84
Air flow: 37.84
Spark timing: 19 <---- getting back to normal?

5) TPS Volts: 1.33
MPH: 4
Cell/BLMs: 10 / 138/132
Knock ret: 0
MAP: 0.75
MAT volts: 1.84
Air flow: 39.63
Spark timing: 27 <----- back to this number and beyond after this point.

The only thing I'm seeing here a dip spark timing, yet no knock...and the MAP (and the air flow a bit) goes way beyond any other time I normally accelerate from idle. Does anyone know what this is a sign of?

Thanks so much in advance

[Joe 1320]

A couple of thoughts:

The MAF wires can become contaminated by oil type filters, not to mention normal accumulated crap from years of service. A cleaning of the element wires with electrical contact cleaner could only help.

Have you tapped the MAF with a screwdriver while the engine is running? Any changes in the engine running parameters and you've found a MAF on it's way out the door.

The IAC is directly responsible for metering additional air when the throttle is closed. It is responsible for idle quality but can affect throlle tip in to a minor degree. Thoroughly cleaning the IAC and throttle body can help if this is the culprit.

The TPS sensor can have a bad spot or two and not trigger a code. I've seen that many times.

A EGR valve that is partially fouled may affect throttle tip in.

[Craig 94 TA GT]

Thanks for the tips Joe...I will clean the MAF and do as you suggested...hopefully that will be the problem. I did notice that when I switched back to the stock MAF ends, the BLMs were a lot closer to normal (previously at 143/128 at cell 16) (now at 133/120).
The IAC has already been cleaned and looked fairly decent when I took it out. However, I guess it might be worth changing just to see.
I'll also check out the EGR valve. When I had the intake off, it "looked" OK, but I am a bit leary of it, especially because I think I have a slight vacuum leak somewhere.

BTW, how are the repairs going for your hourse? How is the GN? I hope everything's going good for you. Take care man...

[Injuneer]

I don't really see anything odd in your numbers. Your MAP climbs because the throttle is opening, but there is little air flow (low RPM). As the airflow starts to increase with RPM, the MAP starts to come back down, as there is added pressure drop in the partially open throttle blades. See difference between 875RPM (0.90 MAP) and 1275RPM (0.84 MAP).

Your spark timing is right out of the stock timing table. Go to a stock timing map and look up your 5 combinations of RPM + MAP. You get numbers within 1-degree of what you reported (the PCM interpolates between entries, so its easy to account for a degree or 2 of difference, when looking at the table).

You can see a stock timing table in my ScanMaster writeup.

http://members.aol.com/InjuneerZZ/ScanMast.htm

You might be getting a little roughness due to the transition from Cell 18 to Cell 10. There's about a 6 to 8% shift in the amount of extra fuel being added between those Cells. I'd also wonder why its operating in Cell 18 at all. It should be in Cell 16 at idle, and transition to Cells 08, 09 and 10 as the RPM goes up.

[Craig 94 TA GT]

Hmm...that's what I thought just now...I looked at the chart and the SP advance looked dead on to the RPM vs. MAP. But everytime I accelerate normally, I don't see nearly has much MAP and air flow as when it stumbles. But on every time I accelerate from a dead stop, the cells go from 16 to 18 then to x. At idle, it stays at 16 normally, then at ~575 RPM, goes to 18 and stays there until past ~900 to 1200 RPM.

I just noticed, however, the stumble "seems" to be getting lighter. I'm not sure if it's in my head though. But I did just switch out my MBA MAF ends back to stock a few days ago. I've driven it 3-4 times since then. So maybe the PCM is recalibrating because the ported ends screwed up the calibration? Is that possible?

[Injuneer]

The stumble (misfire) will increase MAP (decrease vacuum). You may be seeing a result of the stumble, more likely than the cause. Is it possible you have a bad lifter or sticky valve?

[Craig 94 TA GT]

I don't think that's possible now...but I thought it was before, so I just replaced all the lifters with the Comp Cams OEM ones (looks like I did have a collapsed/lazy lifter too), also the 1.6 RRs, LT4 springs, pushrods, etc. I'm almost certain I adjusted the rockers correctly at 1/2 past 0 lash. I originally thought the collapsed lifter was causing this, but after everything was done, the stumble is there there! A bit frustrating.
Thanks for your reply