I've got split BLMs in cell 16 (idle), 17 and 18. All other cells are real close to each other, although I've got to tweak for the injectors still. I've updated the injector constant and offsets but they still need adjustment ("all other cells" are 120 range). I found it difficult to adjust for the injectors with the split there.

Car is a 95 Z28 M6
383: .030" over 350 and 3.75" crank
10.9:1 compression
XE 224/230 .534 .544 112
Ford SVO 30#
stock throttle body
hooker LTs
stock ported heads
26deg closed TPS timing

I perused LJ's site and tweaked my throttle stop to no avail. My baseline was .55v TPS and IACs in the 45 area. I backed the throttle stop all the way out, incrementally, until it's all the way out and the TPS reads .47v and the IAC is ~63 and still split BLMs. I moved the stop, incrementally, to .75v which took the IAC to ~8 at idle and the split is there as mentioned above. The BLMs at this setting, at idle, are ~ 119/138. I plan to swap the
O2 sensors and extenders from left to right tomorrow to rule them out but......I figure a bad O2 would cause problems across the board. Would an exhuast leak be more apparent at closed TPS (higher vacuum)? I don't think I've got a leak but I'll get the car up on ramps tomorrow and really check things out. I haven't tuned the 26 deg closed TPS timing for this setup....that was a "carry over" from my last configuration (basically the same mods except 350/LPE 211/219 .533 .560 112 cam/11:1 comp). Oh yeah, and I've bumped the idle to 900rpm.

I don't get any stumbles, misses or SES lights so it seems like the ignition system is working well.

Thanks,
Rob
95Z28

 

I have a very similar setup (less stroke) and had fought the split blm. Modding my 52mm TB helped but I kept geting 112/134 +-. Finally ran it down to a header leak at #8 tube. I found a good way to find exhaust leaks, use a mechanics stethoscope and replace the solid end piece with 1/8 in copper tube, that well carry the sound of even the smallest leak and is bendable so you can get 360 degrees around each tube.

 

Hey...now there's a good idea for hunting down exhaust leaks! I've got a mechanic's stethoscope and that ting works damn good.

I got some 1/4" copper tubing that I'll run instead of the solid probe and "resonator" chamber. I tested this out by blowing across the end and it's REAL sensitive. I should get some time later this evening to check things out.

In the meantime, I swapped the left and right O2s and wire extensions.....problem's still on the passenger side so I've ruled out O2s and wiring.

If I can't find vacuum or exhaust leaks, I think I'll start tweaking the closed TPS timing and see what that does.

Thanks,
Rob
95Z28

 

I used the copper tube and stethoscope and couldn't find any leaks. To back up that finding, I ran some Seafoam through the intake and watched for smoke....that method found leaks on my previous setup that didn't have any indication they were there, other than the smoke! Anyway, the Seafoam didn't indicate any leaks either.

Any other thoughts? What all could cause greater than 128 BLMs on only one side of the engine, other than these:
exhaust leak
vacuum leak
weak spark
leaky injector on the driver side
clogged injector on the passenger side

maybe contaminated cylinder (i.e. coolant/oil in the chamber)?
Wasn't sure how that would affect the burn (rich vs lean)

Thanks,
Rob

 

Reversion in the intake manifold due to the cam?
Have you tried the "drilling a hole in the tb" trick in order to let more air in at idle?

 

My first thought was that it was the larger cam doing this. By tweaking the throttle stop I am basically doing the same thing as the "drilling a hole in the TB" and that didn't seem to cure my symptoms. I think my direction now is to rule out anything/everything on the passenger side that could be causing the PCM to add fuel. Thanks for the reply!

Rob
95Z28

 

Try swapping the injectors from side to side and see if the split follows.
Other than that I think you've hit on most of the possible causes.

 

Yeah, I've thought about doing that. I think I'll do a more thorough check for vacuum leaks first and then look more closely at the injectors. The injectors I have are "new" (less than 1200 miles) Ford SVO 30# but I'm not *real* familiar with them yet. I still have my stock ones that I could swap out with them too.....they should still be big enough for idle and some part throttle tests.

Thanks,
Rob
95Z28

 

With a cam like that intake reversion should be a null issue, or one would think anyway. As for drilling the TB, if your car idles fine, at an IAC count of 63 you say, thats not too bad, I'd leave it alone. Its the aftermarket TBs that require a "fix."

Is your BLM splig across the board, or only within a certain RPM range?

IF you had an exaust leak, WOT should show up even the most minor leak, via a lean warning on whichever O2 was reading as such.

 

Personally, I'm against drilling the TB as a "fix" so I won't be doing that as any solution. If I had to, I'd enlarge the IAC opening.

The BLM split is odd. At low load and rpm, the passenger side is above 128 and the driver side is below. Mid load and rpm (cell 6 or so) it starts to transition and in higher load/rpm cells it flip-flops (above 128 on driver, below on passenger).

The more I think about it the more I like the idea of swapping my stock injectors back in. The offsets are well defined and I wasn't having any issues with them when they were removed (other than being too small in the upper rpms). Putting these back in would remove any injector performance/offset issues from the equation.

Thanks,
Rob
95Z28

 

Some people drill the TB blades, I dont advocate that. However there is an IAC bypass hole that the aftermarket TBs often do not have, my BBK did not. I forget the drill bit I used but it was fairly small, not much larger than stock.

 

Just get a Holley TB & your probs are over, or do the misterious mod just do a search on google for "AS&M split BLM" & you can do this on most TB's except the Holley cuz it doesn't need that stuff

 

Bunker, are you suggesting that a Holley TB will fix the problem I'm having? I have a stock TB right now and I really don't know if that's the problem. Heck, I really don't see how it could be.

Rob

 

I don't think it's a TB issue, normally the TB mod is only needed for aftermarket. Did you change your injector constants for the 30lb svo's?
You might try the injector swap, side for side, and o2 swap (pita)
I made two extra long pigtails so I can swap connecters with out physically removing the o2's.

 

I agree, the TB shouldn't be the issue since it's stock.

Yep, the injector constant was updated for the Ford SVO 30# running at 43.5 psi. I think the constant I used was 31.68 or something close (from memory).

I swapped the O2 sensors and extensions and the higher BLMs were still on the passenger side so I've ruled them out as a cause.

Rob
95Z28