On my 95 lt1 today I was doing some fuel trim watching and I noticed on the highway when I'd let off the throttle and drop to cell 2, my short terms weren't going under 128 very often at all if ever at times, but they would go up into the 130's at times and the blms as a result were climbing and hovering around 144-145 both banks.

I know i've got an exhaust leak somewhere i'm trying to find where exactly so i'm assuming that may be an issue but since they're not really split in any cell. I have a hard time pointing the finger to only one side showing lean to cause one bank to increase fuel.

The other cells were running a bit rich as well but not that bad, mostly all were in the mid 130's. I'm either running lean somehow because of a leak in the intake tract somewhere I guess, or the coincidence both banks have an exhaust leak before the 02's, or to my actual question, could this be a sign my 02 sensors are starting to kick the bucket? they've dealt with header coating burning off through them before I put it up a couple years ago.

They still seem to be jumping above and under 450mv like they should but I dunno why my cell 2 hangs out at 128 on both banks in my short terms, and the only corrections are to add fuel, very rarely did I see it dip below 128. There were times it'd sit at 128 for several seconds with neither bank budging. .

 

Since it's apparerently affecting both banks equally, it may actually be pointing to a lean condition. Have you checked the fuel pressure under load? Has the MAF sensor been cleaned recently?

You indicate that on closed throttle decel it drops to Cell 2. Usually you will see Cell 17 in decel.

 

yeah I recently cleaned the maf last weekend but was very careful and only sprayed the sides and small mists toward the small fragile wires. Also I let it dry completely before reinstalling it. I will get a new fuel pressure check tomorrow. When I let off the throttle, it goes to cell 2 according to my auto xray. Cell 16 at idle. In 6th gear around 65-70, rpm were under 2k rpm obviously, so i was seeing cells 6 and 10 jump around with partial throttle both running lean as well in the high 130 range.

Aside from the lean blms, the car runs perfectly fine. No idle hunting, surging, hesitations etc., but the exhaust reeks of gas at idle. I will post back with fuel pressure findings soon.

Edit: Come to think of it, while I was in cell 2 I may have had my foot resting on the pedal applying a very small amount of pressure but thought I was off the gas entirely, also I cleaned the maf with cbc mass air cleaner and nothing else. Just sprayed it a few times, sent a few mists through the wires and let it sit for a little bit to dry before I put it back on. Everything looked fine, and the car runs perfectly fine as I said aside from running lean.

 

Fuel pressure results were 33 psi at idle, 41 psi with fpr vac line off covered or not. 36-37 psi cruising low rpm, jumping back to 40-41 psi at wot. So it seems its slightly lower than expected at idle at least. Not sure if it would make enough difference to cause the 140 range blms.

Edit: oh and as far as cell 2, it was in fact me resting my foot on the throttle, letting completely off the throttle, it did indeed go to cell 17 on decel.

Edit2: One last thing, I am running a 93 performance tune I had done back several years ago from madwolf. And this is the first time I have had the car out of storage for a couple years. I had drained the tank of the old fuel and put in a new full tank of 93. I also added a bottle of heet for any water that may still be setting at the bottom of the tank. If I remember right, alcohol can oxygenate the fuel mixture right? Could a single bottle of heet running through a car with a aftermarket tune cause a lean condition at all?

 

You're running a 1993 LT1 tune in a 95? Confusing.

 

93 octane. Sorry about that. Guess I should have said premium. I have a habit of referring to it as 93 cause thats what premium typically is around here.

 

Looking around at how the other cells are reacting in sync with the fuel pressure at the same rpm. I'm starting to wonder if the blms are raising from the fuel pressure. I have svo 30#s. which if ion set the constants with 43.5 psi in mind when he did my tune, the amount the blms are raising when compared to the fuel pressure at the time seems to go hand in hand.

Or I could be completely wrong as i'm just learning tuning aspects.

Maybe a slightly clogged sock or something is causing the low idle and cruising pressure. Cause the tune itself was fine before and nothings been changed. I wouldn't mind as much if they were in the upper 130's, but mid 140's is reading too lean for my liking.

 

Spoke with Ion and he thinks the same as well, so I'm just going to address the fuel pressure issue even though its somewhat close to spec. It appears the pcm is probably compensating for the lack of fuel pressure at idle and cruise speeds. Thx anyway.

 

The loss of flow from running 41psi rather than 43.5psi is only 3%. That would correspond to a BLM increase of about 4 counts.

When you measured the fuel pressure at WOT, was the engine at max RPM? If not, you're not checking the pressure under max load.

 

yes, and in cell 15 at least when I went into wot at around 5500 rpm my blms were only 133-132 in both banks. Fuel pressure was 41 psi. I am not 100% sure how the pcm adjusts for the pe mode blms when in wot like that, cause I know it ignores feedback during PE, but thats what my cell 15 looks like when im into it at max rpm.

The only time they are really high is at idle and very low rpm cruising in cell 2.

Edit: actually looking back at the captured data it was 131 in left bank and 134 in the right bank in cell 15/PE mode. Dunno if 3 counts is large enough to consider them split or not, or if it even matters in PE. That's what i'm getting anyway. The 133-132 in both banks was somewhere in the mid range load/rpm in I wanna say cell 11. right before I went wot.

 

If the BLM's were adding fuel (above 128) in Cell 15, the PCM uses Cell 15 for "power enrichment" mode, and the BLM's. It isn't adjusting them - its just using what the PCM learned in closed loop.

If the BLM's were subtracting fuel (below 128), the PCM will either stay in Cell 15 and lock the BLM's at 128, or default to Cell 18 with 128 BLM's.

You do realize that a 131 or 134 BLM is not indicating a problem??? Anything within +/- 5% (122 - 134) is fairly normal.

 

yeah I wasn't worried about those, its the 140-145 in cell 16 at idle and cell 2 that had me a little concerned. Am I beating my head for no reason there too?

 

Your problem is specific to low load/low RPM. Possibly a very tiny vacuum leak, or possibly the impact of the low fuel pressure. It may have more of an affect with the very short pulse widths required at low load. Maybe there's someting in the configuration of your air intake track that upsets the MAF calibration at low air flow.

 

Yeah It does seem like the blms follow my fuel pressure almost perfectly. It seems to me the fuel pressure is the deal, and since you and ion point at the fuel pressure as well without even seeing the car. It makes it seem even more so the fuel pressure fluctuations are to blame.

What you said about the the short pw's at low load and where my fp is during that time. It just fits too well.

 

Here's something interesting. I just decided to watch the blms while pulling the vac line to the fpr and holding my thumb over it to simulate an idle psi of 40+. Lo and behold the blms started to drop into the 130's in cell 16.

Think i've found my "problem"