I guess when cruising in closed loop with the MAF just about anything will work. This is due to how closed loop feedback from the O2 sensors and the Mass AirFlow meter work. It really takes the guesswork out of tuning. No doubt exactly why GM use it.
Well throw a cam in there and when starting cold the car will be in open loop and at low rpm speed density airflow calculations come into play.
That sorts out those with good tunes from those that just do the easy stuff.
Anyway following on from those thoughts, Ive done a load of changes recently which threw my tune out. Just spent the morning dialing it back in. Now its nice again. (For the next day anyway)
Take a look at this first image, it shows the idle hunting up and down, spark, vacuum, rpm and if it was shown, IAC counts basically wavering to and fro. If it gets bad enough it will stall. Sometimes the oscilations will increase and get worse. Often when AC is turned on or the Radiator fans startup.
So all in all, it idles, but its horrible. Ok so how do we normally fix this? First we go back to basics and make sure that the engine is getting enough air at idle, or at least getting what it wants. To do this we log both short and long term idle corrections to airflow. These corrections are made by the PCM to try and get a stable idle. In efilive they are known as calculated pid's as they are summed and made into a single value for easy logging. So we make a map with ECT vs the RAFIG pid (Only this is needed for M6 cars, Auto's use RAFPN as well) as the car heats up it will log a map of adjustments required to the Desired Airflow table. Once warmed up, we shutdown and update this table. If you cant get your car to idle initally, you might need to start logging to figure out which way its going and just guess to get it to run long enough to map the adjustments.
So once the airflow is updated. Its worth going into the bidirectional controls utility to see if adjusting the spark and fuel mix would help the idle. In my case fuel was ok (set to 15:1 at idle) but spark was now quite a bit out. Dropping it from around 38 degrees to 28 degree's sorted out a lot of the hunting. Im assuming the new injectors and injector offset tables I have been messing with caused some of this. So easy done, all fixed. Now back to a smooth idle. I just made sure my high octane table and base spark tables were close in the idle and off idle areas, so that transitions were smooth between both tables and all is well. The resulant logs are shown below. Notice RPM is now almost dead flat and there is some slight adjustment to spark by the PCM to hold it there.
So there we go. A short run down on tuning idle for anyone who is having issues in this area.
1 comment:
The most important thing with idle is to find what it wants, without any modifiers kicking in. Live controls are wonderful for this purpose. I just scan until I find the nicest vacuum, then I set all the spark tables to the same values (remember there's low and high, then there's idle spark in D and idle spark in P/N for automatics). Usually i go with 28 for small cams, 32 for medium ones and more for really big ones, depending how big. Once all the spark tables agree, I redo RAF (although very often LTIT is just stuck at some seemingly random numbers so I usually just go off STIT), and that solves 95% of idle issues. Of course having a good VE and IFR tables helps to start off.
Have you tried the MNR-0's new method? It's interesting, but I haven't tried it yet m'self.
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