Ok, so I have been fighting split BLM's on my fuel trims since I started tuning this thing...here is the situation.

At Idle my BLM's are split...by A LOT. It is Rich on the Left and Lean on the Right, so I started looking for reasons. I started with the right...if it is lean, that means that there is either a vacuum leak, Exhaust Leak, Bad O2, or Injector Missfire. I did an inspection and I have found NO vacuum leaks, NO exhaust leaks, and I switched the O2's and nothing changed. I pulled each injector plug and watched my AFR and listened to my idle and on all 4 plugs, the AFR went SUPER lean and the idle bogged which means all 4 injectors on that side are working. So I figured maybe it was the other side...

On the Left side it is Rich and that would be caused be a vacuum leak, bad O2 or an ignition missfire. I again found no vacuum leaks, and I had already switched the O2's so it wasnt that. I pulled each SP Boot (burning the crap out of myself while I was at it) and it bogged and y AFR went SUPER Rich for each plug...so all of my spark plugs are firing.

Now I went to GENERAL problems...IAC, TB, Timing, ETC. The IAC was wide open at 160 and having trouble idling, so I worked with it...now it sits at about 45-50 and the idle is good....NO change to BLM's. I have a Professional Products 58mm TB. I altered the idle a little bit to make the IAC around 60-70 to make sure the problem wasn't reversion...no change. I reduced my timing to 23-25* at idle and no change. At this point, I am SEVERELY lost and need to know if anyone else has had this issue and fixed it! The Split BLM's continue with throttle, but they are MUCH closer together.

Also, if it was a bad O2, they shouldn't be reading like they are should they? One would show WAY OFF on the MHz wouldn't it? Both of them seem to be responding the same as the other, and if I make an adjustment to my AFR or my Injectors, or my MAF Calibration, the BLM's change accordingly for BOTH sides and the O2's reflect that in their signal. WTF is going on???

Here is the video I recorded of my datalog with TunerPro RT. I give it a little throttle in the video too but does anyone see anything out of the ordinary here??? My cam is a 238/244, .592/.592 with a 111 LSA on a 106 ICL.

Thoughts?

 

OH, and for the record, this was done in Speed Density Mode. I switched it over in TunerCats when I uploaded the latest tune. This is also on an OBD1 PCM from a 94 Z28.

 

lt1 < ls1

 

Really??!! Come on jackass, be productive or move the **** on

 

Vacuum leak would cause that side to read lean, not rich.
Also, an ignition misfire leads to a lean reading on the O2 due to the unburnt oxygen passing over it. It's an oxygen sensor, not a fuel sensor.

 

Either way, those weren't my problems AT ALL.

I set the timing at idle to 18* and the BLM's got MUCH closer...I need to lean her out and rework the tune tomorrow (didnt have time tonight) but the BLM's went from being almost 30 apart, down to 6 at most, and 4 at best, but it was pulling A LOT of fuel now...they were sitting at L 109, R 112. The max it will pull is 108. So I will go in and lean it out tomorrow and see what happens.

 

What intake you running? LT1? Does it have the IAC passages that bleed right into the ports? The split fuel trim problem is pretty common in the LT1s with big cams.

 

Stock LT1 Intake Manifold, fully ported by Lloyd Elliott. The Throttle Body is a Professional Products 58mm and it has the 2 notched that bleed from the TB inlets into the chamber for the IAC. I am getting ready to drill my first hole in the TB, an 1/8" hole should work from what I have read so far. Then I will get a piece of copper tubing or something to block off those inlets and I will close the TB blades 100% and then let the IAC open a little bit more.

 

Ok, so...I spent the whole night and morning modifying my Throttle Body. Yippee.

I took it off and drilled n 1/8" hole through the IAC passage, completely closed my TB Blades, and I am about to go buy a piece of copper tubing from ACE to block off the IAC passages that bleed into the main air passages during idle...Once I get this done and re-install the throttle body, I SHOULD be able to eliminate my split BLM's completely, or as much as is possible at least, and get this tune dialed in properly!

I will keep y'all posted on the results! We have a huge car show and races at the drag strip today and I am rolling out with a few friends so the SS gets to sit in the garage until the show ends, then I will try and finish this up and hopefully have an update by this evening! Thanks for the help everyone!

 

On LT1's there is no need to drill a hole because you can just slot and clock the TPS sensor. They both accomplish the same thing either drilling or clocking the TPS.

From what I have seen LT1's like much more timing at idle opposed to LS1 engines, push timing around and find the lowest MAP at idle at ur given RPM and that is where Idle Spark should be.

 

No, they DON'T both accomplish the same thing. When you slot and clock the tps you are leaving the throttle blades partly open which is letting idle air directly into the intake manifold which is uncontrolled by the IAC. This is causing a Split BLM problem for me. I already tried that and it made the situation worse. The tecnique I am using allows me to completely close the throttle blades and it let's the IAC do all the work, and the extra air that is entering through the hole is now entering the IAC passages INSTEAD of flowing directly into the intake manifold...allowing my BLMs to even out.

So yes.. you DO need to drill a hole for LT1s.

 

http://www.noid.org/~lj/PCM%20Tutorial/SplitBLMinfo.htm

 

That is exactly what I am in the middle of doing right now LilJay, thanks

 

LOL, Air is Air no difference in cracking the Throttle blade to let air in (at least you can adjust that) when you drill a hole your stuck with it. If it worked great that fine, just a hack way of doing it IMO.

Both have the same effect on the IAC valve

Main thing is make sure your IAC counts are around 40-70 at hot idle

 

Dude, the IAC valve is not the issue here! The car is not SUPPOSED to idle off of the air coming in through the throttle body AT ALL...it is SUPPOSED to idle through the air coming through the IAC Valve. The air coming through the IAC enters into the intake manifold in a different area than the air entering through the throttle body blades. When air is coming in through the blades, and you have a large cam, you start having a problem like split BLM's. Now, just to prove to you I am not an idiot and I know what YOU mean and I know what I am talking about, BEFORE I figured out THIS fix, I adjusted the idle screw, and clocked the TPS Sensor because my IAC Valve was sitting at 160 at 0% TPS. I got it down to 40-50 IAC at 0% TPS and that helped my idle......HOWEVER, the BLM's split even FARTHER than before. SO, I closed the throttle blades a little bit more and moved the IAC Valve up to 60-70 at 0% TPS...the BLM's got a HAIR closer but not enough. Guess what that means....My aftermarket throttle body is letting in too much air into the intake manifold and it is throwing off my tune! To fix that, I need to adjust the IAC Ports, so YES, I will still get the same EFFECT ON IDLE as cracking the throttle blades, EXCEPT all of the air is now moving through the IAC passages rather than flowing into the main part of the manifold! This is going to allow me to control my idle and my BLM's MUCH better and get the split down so that I can get a proper tune on this thing!

So, you are wrong...both do NOT have the same effect on the IAC valve, they both have the same effect on the idle airflow. The hold that I drilled allows more air to flow in and helps keep the idle up, BUT it still leaves control of the idle up to the IAC Valve. Now no excess air is entering the main part of the manifold.

I can still adjust how much air is coming into my throttle body...it is called TARGET RPM and IAC POSITION. If you think that this little bitty hole is going to keep my engine running with closed TB Blades and a fully closed IAC Valve then you need to go back to school and start learning how much air is needed to operate an engine. An 1/8" hole will let VERY LITTLE amounts of air in. The IAC passage when fully open is a 1/2" hole.

For proper tuning, cracking the throttle body blades is a "hack way of doing it" IMO......

 

Ok, I guess we will have to agree to disagree. Good luck with everything.

 

No offense bro, but you don't know WTF you're talking about.

 

My thoughts exactly

 

Ed some factory LS1's have the stock TB drilled and thats fine, but on an LT1 its not neccessary. ED your a tuner you should know you don't have to drill a DAMN HOLE IN THE TB to allow more air into the engine.

Good luck with you build

 

T/A kid, so now you are going to argue against one of the best tuners out there right now, and go against the exact thing he says to do???


Ummm.....you sir are an idiot. Here is the EXACT quote from Ed on what to do...