I'm starting to run out of things to tinker with on my venerable PNP2. One of the last things to try to get figured out is a recurring issue with the idle AFR's leaning out during extended driving periods. I have a '99 and I chalked it up to the returnless fuel system on the NB's heat soaking. So I converted the fuel system to a full-return over the winter and while that solved a hot-start heat soak problem that I was having, it's done nothing for the running hot lean out situation.

Here's what happens; In the morning - regardless of the ambient temps - the car will start up just fine and idle with the AFR's in the 12's (because of ASE and WUE) and then settle into the mid 14's after all of that goes away. After driving normally for about 20 minutes, the idle AFR's will be in the high and upper 15's and eventually will peg at 16 (which is as far lean as my AFR500 will go). Blipping the throttle will bring the readings back to expected momentarily, but they will eventually drift lean again. If I change the VE table to compensate for the hot idle lean condition, the cold and warm idle values will be stupid-rich and that just wastes gas.

Now, here's where it gets weird(er)...I thought that I could use the MAT Air Density table to add a touch of fuel at higher temps (yes, I know, that's not a good way to do this - but I'm grasping at straws). But when I look at that table while scrolling through a log (which, up to now, I've never done) the green bar that tracks across the graph is stuck at zero! And the "Est. Intake Air Temp" gauge just sits there. I'm getting good readings from my GM IAT sensor and my MAT values in the logs are non-zero. So have I overlooked a setting in the tune or am I SOL with regards to the Air Density table?

Inquiring minds want to know.

 

Use EGO

 

The coldest temp(45) to the hottest temp(120) is a 2% swing in fuel. If you're at 16 instead of 14.5, you'd want 10% more fuel. The correct way to do this, if the fuel temp is indeed the cause, would be to run a flex fuel sensor to monitor fuel temperature, make sure ethanol fuel multiplier is zero, and add (through tuning) an appropriate amount of fuel. You could do this by making a table of fuel temp vs. engine speed or something similar, adding or even subtracting when necessary. But wait, the MS can't do this.

So just use EGO.

 

EGO correction at idle! Isn't that blasphemy? I think I read somewhere that EGO control in the idle cells is a no-no. I have EGO Correction on, but not below 1300 RPM.

 

this is because people don't know how to tune. Of course EGO at idle is the way to go.

 

Then EGO at idle it is. My MSPNP2 lives for another day!

 

Only if you don't know what you're doing. I would avoid those sources for info. Some don't trust EGO in the higher boost/rpm range for some voodo reason. I run ego (closed loop) from top to bottom. These engines seem to excessively heat the fuel and fuel components which is the cause. Below is a pic of your ego table that I changed to somewhat represent what I run. You might want to change the scale on the timing table so that you have a more dedicated range for the idle area. The typical idle range is 25-30kpa at 900rpm. Yours is set up so that as the rpm varies a small amount it will vary your timing. I recommend making the bottom 25kpa and make a 900rpm cell. The more constant the timing is the better your idle will be.

 

The old adage is to not use auto tune for tuning VE on idle cells. The fact that idle requires so much EGO (both add and remove from time to time) is EXACTLY why not to auto-tune.

I allow +/-17% authority EGO in idle cells to keep AFR at target over summer / winter / hot restarts / etc.

DNM

 

I guess I can see how someone could confuse EGO correction with autotune and I do agree with autotune not being ideal for tuning the idle VE cells.

 

I have seen the light! Now I've applied EGO to the idle cells and have adjusted the authority to +/-17% (up from 10%, and then 15%). Right now I'm tuning VE for spring/summer but I'll revisit when temps cool down for the winter. Right now, the challenge is to re-tune the PID loop for EGO in idle. Right now it's very much over-damped, but I'll get that straightened out quickly.

One day I'll spring for a MS3-Pro, but not until I've wrung out every last iota of tuning wizardry out of the old girl (or when/if the PNP2 dies - whichever comes first).