my 91 L98 has an issue where under part throttle acceleration, the car will stumble and buck a little bit, then like a light switch it accelerates normally again. It seems to do this even when the car is cold and hasn't gone into closed loop yet, but I notice it the most after the car has warmed up and gone into closed loop. I've watched my scan tool while it does this and when it stumbles, my oxygen sensor voltage nose dives, down to 40-60mv. Likewise, in looking at my BLMs, it seems to be contained to a certain set of cells, but I don't know if it's related to those cells, or it's just in those cells because it happens under similar conditions all the time. In either case, after my new injector swap, my BLMs are mostly in the high 140s, with a few even in the 150s and a couple at 160 (22lb injectors my *** ), but in those cells where I'm getting the stumble, the BLMs range from 118 to 130.

I've changed virtually everything in the ignition, including plugs, wires, cap, rotor, coil, & ignition module.

I've had a fuel pressure gauge on the car while driving and it seems to stay rock steady. I noticed that I had one white plug and 7 black ones, and 4 of my 8 injectors ohm checked at under 16 ohms, so I put 8 new bosch "22lb" injectors in. I use quotes because my BLMs make me suspect that they aren't 22lb injectors, but that's another matter entirely.

The car is a 91 TPI 6 speed with a TPIS base, TPIS runners, stainless roller rockers (I assume 1.6 but I really don't know) and I believe it's also got a cam. It also has an Arizona Speed & Marine PROM, level 2 or level 3... I don't really remember.

 

No its not. verify what kind of injectors you are using. if your engine is stock and ju changed to 22lb injectors AND your BLMS ar in the 150 area, something is wrong.

 

and you've got a chip programed for the other injectors..hopefully you can determine if the injectors are the same as what was in there. Think I'd also replace the 02 and check the TPS.

 

The other injectors were stock Multecs. I went with injectors that are also supposed to be 22lb/hr. Honestly, I'm not super concerned about that because I can add fuel pressure to get the BLMs where they belong. I'm more concerned about getting this lean/stumble issue straightened out. It did it with the old injectors and going all over the road map with my fuel pressure didn't change that.

TPS is functioning properly. I thought about the 02, but crosscounts go up rapidly during normal driving, so I'm not inclined to think that the sensor is malfunctioning. Plus this does this in open loop when the oxygen sensor is ignored.

 

Do you have a noid light? reason I ask is because at one time I was experiencing that stupid stumble, but I didn't have my laptop running. So I started trouble shooting the hell out of it......mine turned out to be a bad pigtail to an injector. Just throwing that out there.Sometimes these things can boggle the mind.

 

Do you have a noid light? reason I ask is because at one time I was experiencing that stupid stumble, but I didn't have my laptop running. So I started trouble shooting the hell out of it......mine turned out to be a bad pigtail to an injector. And you know what happens when you have an injector(s) going intermittent, yep, lean/rich. Just throwing that out there.Sometimes these things can boggle the mind.

 

I don't have a noid light, though maybe I should get one. What makes me question whether it's something like that is just how severe this is. I've dropped a cylinder for various reasons before, and at most the car goes slower and maybe shudders a bit. This is a rather extreme stumble that makes me think that all (or most) cylinders are effected. Also, it doesn't do it past about 1/2 throttle. Any time I really get on it, it performs flawlessly.

 

02 reading would not be bad as long as it is not staying at 60 and is fluctuating with cross counts that are moving continously,ie. 60 being the lowest and bouncing through out the number range.

I am sure wot (pe) will be affected, as the ECM makes calculations based on the last closed loop readings when it locks learn and blm. to achieve the AFR and timing set in the tune.

160 would be max. and beyond computer control, when you say "cells" are you refering to trim cell (1,2,3,4,etc.), or are you looking at the ve table itself? If trim cells you could try and remove the battery cable, to let it re-learn trim cell calculations. However with readings from 160-118 chances are that tune will need to be changed (24#'s would have brought down the 160 blm lean side, but more than likely would have thrown it rich to 108 max.in other areas of the rune).
My experience in working w/a map tune is that any mods/changes I make seem to change the blm in different kpa vs. rpm ranges in the tune.

btw, where is the FP set? it would not be a bad idea to cross check the inj. number to see if they are 22#'s, for peace of mind. MO

 

I was referring to specific BLM cells. It's a bit hard for me to be sure which BLM reading corresponds to which BLM cell because my auto x-ray can only display one thing at a time.

When I go lean, the voltage on the o2 sensor drops and stays low until it pops out of this "studder" condition.

My fuel pressure is set exactly at 43 psi, though I intend to bring it up to about 47 after work today. The injectors have the correct part numbers on them, though they aren't a GM unit, so the part number of the particular vendor I purchased them from (marren injection) is rather useless. The original Bosch numbers have been removed from the injectors. Honestly, they look like 19lb SVO injectors to me, though a 19lb SVO injector should flow pretty close to 22lbs at 43psi.

But regardless, this problem existed with the stock injectors, so I'm pretty confident that the injectors are not the issue here.

 

MO, and exactly what I was trying to point out.
The cells shown 4, 6,10, and so on, shown on an AUOTX are trim cells. Try disconnecting battery and see if the ecm will make different calculations for the trim cells (re-learning) doesn't help some. From your descriptions of lean /rich going on, I believe you need to get the closed loop tune better. Even if they are 19#'s that wouldn't account for the rich condition also being seen (118 blm).