Super Split BLM issue
Super Split BLM issue I have an issue with my LT fuel trims. ST are pretty good. One bank shows full rich and the other shows full lean. The problem is at idle. I tried all kinds of things tune related but none worked. I played with the individual cylinder fuel trims and still no luck. anyone ever see this issue before? if so how did you fix it?
My 94 formula has split LTs at idle. Most likely due to the exhaust leak causing one bank to read lean at idle.
Are the LT's good at cruise? If so then its something related to low airflow, low exhaust flow, or high manifold vacuum.
I would look at the O2 sensors first, bad heater, bad connection, exhaust leak, etc.
Then I would look at the injectors or spark plugs + opti to detect a misfire at idle. A misfire will read lean, driving the trims up.
What is the engine combo (cam, heads, etc...)?
Bob
Are the LT's good at cruise? If so then its something related to low airflow, low exhaust flow, or high manifold vacuum.
I would look at the O2 sensors first, bad heater, bad connection, exhaust leak, etc.
Then I would look at the injectors or spark plugs + opti to detect a misfire at idle. A misfire will read lean, driving the trims up.
What is the engine combo (cam, heads, etc...)?
Bob
the LTs are good at cruise the problem is at idle. STs are fine. Reading almost full lean on one side and rull rich on the other according to datamaster. engine freshly done didn't have this problem before prior. 385 lt1, 224/236 cam, ported and polished stock heads. O2s are both brand new.
okay so i adjusted the tb screw to ensure that my tb blades were completely closed at idle. I hoped this would fix the split somewhat but it did not. my IAC counts went up which I was hoping for but not as much as i thought they would. then my TPS voltage dropped to about 43 at idle. now if i raise the tps voltage through adjusting my slotted TPS sensor will the blades open up again thus defeating the whole purpose?
the computer will just relearn the TPS minimum. Its controlling the IAC to maintain its desired idle speed.
The intake plenum is open behind the throttle blades, so there is no bank-to-bank separation there. I don't think your BLM split is a throttle issue.
If you slightly squeeze the throttle, how much RPM do you need to pull the BLM's back into shape?
Some additional idle speed may help, as the O2's may be getting confused by the cam overlap.
Bob
The intake plenum is open behind the throttle blades, so there is no bank-to-bank separation there. I don't think your BLM split is a throttle issue.
If you slightly squeeze the throttle, how much RPM do you need to pull the BLM's back into shape?
Some additional idle speed may help, as the O2's may be getting confused by the cam overlap.
Bob
as soon as i hit the idle the LTs jump back into a reasonable range. STs are okay throughout. i'll try bumping the idle back to 900 where it was before i stroked the engine. i have it set at 800 rpm now.
So i did what i set out to do i adjusted my tps voltage to .67 with the tb blades fully closed, reset my IAC position, bumped my idle up to 900, and zeroed out all my idle underspeed and overspeed tables.
when i start the car i have to give it some gas to stabilize the rpms at 900 or it will idle real low as it tries to get up to 900. driving it around I noticed that now the idle to gas transition is super smooth i guess in part the steady timing at idle i now see after zeroing the over and underspeed tables. when coming to a stop i noticed my IAC counts drop and then climb the longer i idle as it tries to let in the necessary air to idle fine.
The good new is my LTs started to get better after doing all this. They went from being 153 / 108 while idle (cell 16) to about 143-145 / 108. The left bank even dropped into the 130s at some points. The right however is pretty much stuck at 108 while at idle.
I think what i need to do next is drill a larger hole through where the IAC passage is so that I can get some more air into the car so that the IAC doesn't have to work so much at idle. The longer I idle the higher the IAC counts go. I plan on removing the throttle body and enlarging the hole I had already drilled and then logging to see what happens.
I hope i didn't use gasket sealer on the manifold side of the gasket or its going to be a pain to get it off without getting gasket shavings into the manifold.
when i start the car i have to give it some gas to stabilize the rpms at 900 or it will idle real low as it tries to get up to 900. driving it around I noticed that now the idle to gas transition is super smooth i guess in part the steady timing at idle i now see after zeroing the over and underspeed tables. when coming to a stop i noticed my IAC counts drop and then climb the longer i idle as it tries to let in the necessary air to idle fine.
The good new is my LTs started to get better after doing all this. They went from being 153 / 108 while idle (cell 16) to about 143-145 / 108. The left bank even dropped into the 130s at some points. The right however is pretty much stuck at 108 while at idle.
I think what i need to do next is drill a larger hole through where the IAC passage is so that I can get some more air into the car so that the IAC doesn't have to work so much at idle. The longer I idle the higher the IAC counts go. I plan on removing the throttle body and enlarging the hole I had already drilled and then logging to see what happens.
I hope i didn't use gasket sealer on the manifold side of the gasket or its going to be a pain to get it off without getting gasket shavings into the manifold.
