A recent thread set a light bulb off for me in my idle BLM split. I have about 10 point split between the banks at hot idle. The thread talked about the idle circuit for the IAC in the manifold and how aftermarket TBs don’t utilize the circuit and have idle issues and split BLMs.

I have a stock 48mm TB. But I decided to check if the stop screw on the cable cam was keeping the blades cracked open and therefore allowing airflow into the plenum during idle. To my surprise, not only was it not holding it the butterflies open but it was not even in contact with the throttle cam at all. I could easily slip a cereal box thick piece of cardboard between the screw and the metal tab on the cam…. So not that. And I’ll probably adjust that so there is a positive stop on the screw (surprisingly the TB doesn’t “stick” or get caught up on the butterflies).

Next I looked at the butterflies and I could see some slivers of light around them. Not much and hard to capture in a pic. But I presume that is normal as well? They can’t be a perfect fit or it would stick.

So should I be chasing a vacuum leak somewhere instead? It does idle fine and IAC counts are around 30 if I recall correctly. I’ll have to go back to my last datalog to validate that number.

 

What year?

 

1994

I’ll mention I found a very slow/minor leak in the purge solenoid vac line and will be replacing it. I’ll also admit that last time I had the TB off I didn’t have a gasket handy and reused it. I have a new gasket in hand for when it goes back on. Maybe these things added up to make an impact. Maybe not.

if the consensus is that the leakage around the butterflies (the light gap seen in the pic above) is normal, then I’ll throw it back together with the new vac line and gasket and see what happens.

 

Yes, put the new pieces on, and a 94 wont have any issues. With the sequential fuel injection, it was a far better system than the batch fire the years before.
That small of a leak in the throttle bodies will be handled by the IAC system,

 

if you have play in the bushing near the throttle cable cam it too can be allowing unmetered air into the engine. any competent carburetor shop can rebush the throttle body, be sure to use oem gaskets. out of the literally 10 tpi/lt1 throttle bodies i have had my hands on, they all have wear after 75,000 miles. fixing it will help cure the condition you are describing. clear the block learn after repair, make sure the tps is adjusted properly (if it applies to your year) it will idle smoother.

sometimes carbon build up can cause some split blm at idle/off idle too on cars that sit more than they are driven. look at the o2 voltages on a scanner capable of giving a live data stream, if one or the other seems sluggish change them both. voltage should fluctuate around rapidly. you can google what the exact voltages should be centered at. Havent had to mess with a narrow band o2 in a while, and the exact numbers escape me. also, worn ignition parts, leaky plug wires, worn cap, rotor can all contribute to split blm. look at the short term and long term tables. if they are pegged out you know you should do a thorough and comprehensive tune up, wires, cap, rotor, pcv, fuel filter, and if miles on the odometer dictate also the o2 sensors, making sure you have no broken or cracked vacuum lines, etc.

any leakage around the butterflys is not good, regardless of consensus here, if you want like new car performance, mechanical wear on parts must be addressed. there are also kits with bushings and the correct reamer available for the tpi/lt1 throttle body, it is the same size as the qjet secondary shaft for the 85-92 cars. cannot say for sure on the 93-96 cars, once they switched to the bolt on iac valve the throttle body design was altered significantly.

 

I rebuilt the engine last year and it has less than 500 miles on it. At that time, the TB came apart, cleaned in an ultrasonic and back together. Can’t say I noticed any wear/slop in it but not sure precisely how to check for that. The shaft does wiggle in any direction and there is no binding. The TB is the original unit with ~125k miles.

It went back together this afternoon with a new gasket and new hose to the purge solenoid. I did also make sure the purge solenoid itself was holding vacuum with a MightyVac. It does. Haven’t started it yet. I’m going to put the MightyVac on the remaining vacuum lines to be certain no issues there. I did replace the PCV 180* hose and manifold grommet with one from a spare manifold that was in much better shape. If all of this checks out and I still split BLMs I’ll look into a TB rebuild but that will come next winter… too soon to driving for me to be waiting on anything.

Both o2s are brand new. Maybe 30 mins of run time on them to datalog my new injectors at idle. Plugs/wires brand new. IAC a few years old.

 

Split BLM’s are normal and Ive had them ever since i started checking for them

i started checking for them when i did the hotcam and i was using stock 48mm TB at the time

 

I know Split Blms are very common on batch fire LT1s, but they were supposedly reduced significantly on 94 and up.

 

I have it running in Speed Density currently (MAF off in tune) I want to nail down the VE tables and then go after MAF tune in the future. I have a wideband as well pinned into the PCM. That said, I’m not sure if running in SD also forces it to batch fire instead of sequential. I’m seeing a ~10 pt BLM split between the banks but this was based on only about 30 mins run time and 2 iterations of running data through Trimalyzer and reflashing. I had just upgraded to 30lb Bosch 3 and wanted to get some quick baseline updates into the tune before I start datalogging on the street. I didn’t consider wideband data, yet, as I really only idled it with a few revs. The narrowbands are brand new.

 

Interesting you are switching over to SD when you have MAF car. As long as you adjusted the table to state you have 30# injectors and, in my personal opinion change the Air Fuel ratio to 14.2 due to alcohol in fuel, vice 14.7 stoich.. But I digress.
Keep running trim analyzer and dial it in.
I am sure blms will settle out.

 

It’s not a permanent switch, but oh so easy to tune. I’m tuning a heads cam 355. Eventually, I will tune with the MAF.

The injector constant and offset table has been updated. Even so, it ran quite rich per the BLMs (not maxed on how much it could pull, but close!). With just 2 quick iterations through Trimalyzer I got it within reason of 128. I believe my split was something like 122/132 but it’s been a few weeks so I don’t quite recall. I need to check the other vac lines still then will run and datalog once more.

 

Sounds great. That was quick.