Open Loop Idle

[Bartman]

I'm running an LT1 in my car, but I think this could be a potential issue for other engines as well. So let me know if this scenario makes sense in general, and if so maybe someone can help tell me how I can reprogram my PCM with Tunercat to make this change.

 

I have a pretty healthy cam in my engine and I've run into some problems with my idle. The engine idles fine in open loop (with or without the AC on), but it doesn't idle well once the system goes into closed loop. It idles a little low and rough and sometimes stumbles at low RPM.

 

I've done numerous things to try and fix this including replacing the O2 sensors, the IAC valve, and cracking the throttle blades to raise the idle speed.

 

After quite a bit of searching I found someone taling about the same type of problem along with an explanation and possible fix. The problem is stated as being a condition where the overlap in the cam allows scavenged air to get into the exhaust which makes the O2 sensors give a false lean condition. The PCM will then attempt to incorrectly add more fuel.

 

If this is indeed the case, then a possible solution would be to ignore the O2 sensor readings at idle and/or low RPM conditions. One way to do this would be by running in open loop (only up to a predetermined RPM).

 

So does this make sense, and if so has anyone done this?

[jbc3]

I run an open loop idle (have been for years). It is especially effective when you have a larger cam. since the overlap causes reverb in the intake and the BLM's go out of whack. It keeps the BLMs even at 128 at idle. Throw Datamaster on and take a look at your

 

In LT1-edit, Go to the FUEL section the WOT and drop the WOT setting below 1200 RPMs to 0. It also helps to reset your BLM ranges up to 1250, 1600 and 2100. Save your file and reload. I am not sure how to do it in Tunercat. I have an open LT1-edit, so if you need your file edited, let me know.

 

Jody

[Bartman]

I run an open loop idle (have been for years). It is especially effective when you have a larger cam. since the overlap causes reverb in the intake and the BLM's go out of whack. It keeps the BLMs even at 128 at idle. Throw Datamaster on and take a look at your

 

In LT1-edit, Go to the FUEL section the WOT and drop the WOT setting below 1200 RPMs to 0. It also helps to reset your BLM ranges up to 1250, 1600 and 2100. Save your file and reload. I am not sure how to do it in Tunercat. I have an open LT1-edit, so if you need your file edited, let me know.

 

Jody

Looking at a DataMaster file I took a while back I see my Lterm Counts up in the 140 range at low RPMs in closed loop and even breaking into the 150's.

 

I'll send you a PM about getting my file edited.

[280Z28]

I run open loop at idle as well.

[Bartman]

It looks like Jody will make the changes to my PCM for me, but I think this is the change in both LT1-edit and Tunercat. After he makes the change for me I will verify the settings in this thread. Hopefully this will be valuable to someone else because I haven't found anywhere else that describes how to enable open loop idle in Tunercat.

 

LT1-edit

WOT Table - This is the percent TPS that will be required for the WOT bit to be set in the computer when the engine temperature is less than 151 C. This can be altered so that the computer is always in PE mode at idle rpm's (thus O2's are ignored) if so desired.

 

Tunercat

%TPS Threshold For WOT Vs. RPM (Low Cool) Table - Table of differential percent throttle position thresholds to enable wide open throttle (power enrich) mode Vs. RPM. (My Cool Thresh for TPS for WOT (HighTemp) is set at 151 Deg C)

 

I'm not sure about the BLM ranges that Jody mentioned, but there is a table in TunerCat called 'BLM Cell RPM Boundaries' and my current settings are:

Low 900 RPM

Mid 1200 RPM

High 2000 RPM

[strotter]

This situation - I believe it's called "reversion" - has been discussed extensively at thirdgen.org in the /diy-prom/ section. Grumpyvette, who periodically checks in with Hybridz to share his extensive knowledge of applied mobile thermodynamics as well as his extensive collection of cool links, is very active over there, and has provided detailed explanations of the physics as well as a variety of solutions. There are some knowledgeable gearheads over there, as well as some accomplished programmers - some of those guys compile their ECM code from scratch!

[jbc3]

Bart,

It's been a long time since I had the BLM ranges explained to me, but it has to do with the range that the BLM learning is done. By raising the BLM learning range, you are putting this range in a better area, where there is less disturbance from the lumpy cam idle and low end RPMs. By moving the idle to open loop top 1200RPMs, there is no "learning" done below 1200 anyway, but as soon as the motor goes into closed loop >1200 it is pushed into the meat of the learning range and could cause the BLM's to push dramatically right as the car moves back into closed loop.. causing it to stumble as it transitions into closed loop. So, by pushing up the range, it will still allow the computer to make adjustments, but will keep it from trying to make those drastic turbulent air adjustments. You should see a smooth transition.

[Bartman]

I got the new file from Jody a couple of days ago, and I finally got it loaded up tonight. Man does it run good! From what I can tell the changes are what I thought they were going to be (and listed in my last post).

 

I'm going to have to make some DataMaster recordings, but it sounds and feels great. Many thanks to HybridZ and jbc3.