Question About #13 On Surging Idle Checklist

Is your car / computer Cobra or GT? What size injectors do you have? What type of MAF is the 24lb?

rThis is always a really confusing point due to the way the aftermarket tweaks a MAF characteristic line to try to compensate for high flow injectors.

Background: The MAF measures air flow. There is a characteristic line (transfer function) in the ECU calibration that converts MAF voltage to air flow. There are slope and offset values in the ECU calibration used to characterize fuel flow for the injector size. The measured air flow from the MAF is used along with the desired AFR (determined by laod/speed maps in the ECU software) to calculate how much fuel flow is needed from the injectors. That fuel flow in converted into an injector pulse width using the injector slope/offset calibration values. The MAF's air flow measurement is also converted into a load signal, usually % based (volumetric efficiency), which indexes other maps in the software for setting things like spark and desired AFR.

A good example of this is that both the '94/'95 GT and Cobra use the same MAF sensor with a nearly identical MAF characteristic line calibratoins. However, they use different injectors and have different calibration settings for the injector slope/offset values (19 lb/hr GT and 24 lb/hr Cobra). This combination of ECU calibration settings matched to the MAF sensor and injectors allows the engine to provide the proper amount of fuel for each specific hardware combination.

The bad aftermarket way: A lot of aftermarket companies say that their MAF's are calibrated for a higher flow rate injector than stock. What they do to accomplish this is lower their MAF's transfer function relative to the stock meter by the same percentage as the increase in injector flow rate (ex. 19 to 24 lb/hr swap), so that the fueling from ECU is still correct. This lower magnitude air flow signal tricks the ECU into sending a lower pulse width to the injector so that the higher flow injectors don't over-fuel the engine. Potential problems come from the ECU's load calculation since the MAF's tweaked signal is going to give the ECU an incorrect load calculation, which messes up how the ECU software indexes the majority of the critical engine maps, like the spark maps, and how it controls the idle air valve. This type of hardware trick generally works to a limited degree, until you get into significantly larger injectors where the MAF characteristic has to be shifted so dramatically low that the air flow and load calculation are a fraction of what they should be.

The good aftermarket way: The best approach which resolves a TON of driveablity and idle problems is to change your ECU calibration to match the MAF sensor and injectors that you're using. In essence, you need to update your ECU calibration data so that it still measures the air flow properly through the correct MAF characteristic line, which allows the system to command the right fuel injector pulse width for the injector size on your engine. You can do this yourself through chip tuning tools like the Quarterhorse or the Tweecer, or you can have a tuner shop burn you a chip with the proper calibration data - assuming that they know how to do this properly.

To answer your specific question: I think that point 13 is saying - Idle problems can come from running an aftermarket MAF sensor (tweaked for a non-stock flow rate injector) with the wrong flow rate injector. For example, running a MAF tweaked for 24 lb/hr injectors with 42 lb/hr injectors in your engine is going to result in your engine running pig rich. Conversely, if you run the same MAF tweaked for 24's on an engine with 19 lb/hr injectors, it's going to run on the lean side and you'll have problems. (This sounds like it might be what's happening with your car.)

Regardless of the scenario, your best bet for overall performance is to have the correct MAF characteristic line for your meter and injector flow rates for your injectors set in your ECU calibration.

I hope helps and good luck!

He can easily correct a hanging idle through the tune. It's actually likely not related directly to the injectors / MAF.