Introduction / Prerequisites

Before you read this, make sure you have read Theory: Alpha-N, Theory: Mass Air Flow and the FordOverview.  Although not essential, it wouldn’t hurt to have at least read about Speed-Density operation as well.  This page will assume you have read and understood these pages.  This is a somewhat complicated topic and will require you to put several pieces together so don’t feel bad if you have to read this a couple times.

About the table and why it is critical

The “Load with failed MAF” (“LWFM” from here forward) table(s) are found in almost all MAF Ford Strategies.  Most strategies that make use of IMRCs (Intake Manifold Runner Control – valves that restrict air entering the engine in order to increase tumble and velocity) have two LWFM tables instead of one and  switch from one LWFM table to the other as the IMRCs open and close.  The main purpose of the LWFM table is to estimate the amount of air going into the engine without using the MAF sensor or a MAP sensor (if present) to provide the ECM with an “emergency” fallback method of running the engine in the event the MAF sensor fails.

The LWFM table is also important for normal operation of the motor because Load from the MAF (this is “Load” – the Ford-specific calculated cylinder filling value calculated from the MAF sensor, RPM and engine displacement) is “sanity checked” against the LWFM table to determine if the MAF is providing reliable information.  If there is too large of a difference between calculated Load and the LWFM table, the ECM may ignore the MAF even if it is providing valid information! This happens most commonly in forced induction situations (where load is greatly increased compared to a naturally aspirated car) but can also occur in cars with aggressive camshafts.  If you are making changes to a MAF transfer function and you are not seeing any changes in engine operation, double check your LWFM table!  Further, most strategies use “Anticipation logic” to predict airflow.  This prediction logic is based off…  Surprise… The LWFM table!  Having a sane LWFM table is neccesary for the aircharge anticipation logic to work.  You can disable this but it’s generally not necessary if you tune the LWFM table properly.

LWFM table is a classic example of an Alpha-N control strategy – it’s purpose is to provide a very crude estimation of airflow entering the engine when the MAF signal is absent or the ECM thinks it is unreliable.   The LWFM table uses only two inputs – throttle position (aka “TP”) and RPM to determine Load.  Here is a picture of a typical LWFM table: (screenshot from Binary Editor / GUFB strategy)

Here you can see the X axis is RPM and the Y axis is RELATIVE Throttle Position volts.  Each cell represents the Load that will be used to calculate fueling and timing when the ECM thinks the MAF is bad.  For example, idling with the throttle closed (0 volts relative)  around 700 RPM the ECM will assume a Load of .1602 and make appropriate fueling and timing changes.

The importance of the LWFM table varies considerably from strategy to strategy.  A rule of thumb is that the newer of an ECM you are using the more picky it will be able the LWFM table.  Fox Body and most early EECV (pre-99) are fairly tolerant of inappropriate LWFM tables where 99+ ECMs are generally much, much, much more picky.

Tuning the LWFM Table

Tuning the LWFM table is pretty simple:

First, set the Aircharge WOT multiplier, Anticipation logic scalars, etc. to make the ECM as tolerant of a bad failed MAF table as possible

Second, GUESS!   Yes, guess.  Enter values that you think are sane for the setup, starting with the stock LWFM table as a guideline.  A few examples:

• If you put in aggressive cams, decrease the LWFM table at low RPMs and throttle angles while increasing it at higher RPMs and throttle angles.
• If adding a positive displacement supercharger (roots, twin screw) multiply the whole LWFM table by approximately the highest pressure ratio you will see.
• If you add a centrifugal blower, multiply a column of the LWFM table by the pressure ratio you achieve at a given RPM

Third, drive around and log throttle position (TP Relative), RPM, Load.  Compare the Load values you log with the LWFM table.  Start changing entries in the table so they get closer to the load you really see at given TP and RPM conditions.

Note: Turbo cars present a very big challenge to this strategy due to the amount load can vary with throttle position due to spool time.  This is a very tricky case and often the only solution is to try and maximize allowed error before the LWFM table becomes active and also disable Aircharge Anticipation and other functions dependent on the LWFM table.

Introduction

As stated in our overview of MAF systems, one of their main weaknesses are air leaks.  Whenever air can enter the engine without going through the MAF, weird things happen.

There are two principal kinds of leaks that wreak havoc on MAF systems – constant leaks (like a unplugged vacuum port) and mechanically induced leaks (such as a Blow Off Valve or Bypass Valve that vents to atmosphere.)  Each leak has a tendency to affect the system differently.  In this article, we will try to take a look at what “should” be happening, what changes with a leak and what kind of odd things you can look for while tuning to identify a leak.

Reversion presents an additional problem for MAF systems.  Reversion is the technical name for when air changes direction and reverses flow.  MAFs are not one way systems – they will measure air flowing into the engine and then meter the same air flowing out of the engine when there is severe reversion, causing unreliable MAF readings.

Constant leaks

This kind of constant leak in a MAF system is the classic “vacuum leak” where a gasket, coupler or piece of tubing in between the MAF and the engine does not seal properly.  In this case, air can enter the engine without passing through the MAF.  Because air has entered the engine without passing through the MAF sensor, the MAF sensor reads artificially low.  An engine operating in open-loop mode will tend to run very lean.  A motor operating in closed loop will see very large positive trims as the computer uses the O2 sensors to add fuel to compensate for the lean condition.

The air leak provides more air for the engine at idle which will make the idle rise or sometimes “hunt” or bounce around unstably.  Generally, the idle system will also try to compensate.  On Fords you will see the ISC Integrator (“Integrator”) swing negative, indicating the ECM is allowing less flow through the idle valve than is commanded in the tune.  It is very common for the Integrator to get stuck at the minimum allowed value and have the car still idle higher than commanded.

Most MAF systems use the MAF for calculating appropriate timing values as well as fueling.  With a vacuum leak throwing off the system, the ECU thinks there is less air entering the engine than their really is.  This will mean that “load” values will be artificially low, which generally leads to timing being artificially high.  In severe situations, this combination of issues (less fuel, more timing) is a recipe for melting engine components if it goes unchecked.

Mechanically induced leaks

Bypass valves are the most common source of mechanically induced leaks although idle, purge and other vacuum operated solenoids can all be a problem.  MAF systems require these valves to be re-circulated so that air leaving the valve re-enters the intake AFTER the MAF so it does not get measured twice.

Blow off valves on turbocharged vehicles are often vented to atmosphere.  This unfortunately will severely confuse a MAF system.  When the valve opens, air that has already passed through the MAF and been “counted” is released into the atmosphere instead of entering the engine.  The ECU will supply enough fuel for all the air that has passed through the MAF while only a small portion of this air actually entered the engine.   This causes the engine to run very rich and can cause stalling or other problems when letting off the gas and the BOV opens.  Once the valve closes again and the car burns off the excess fuel delivered, things slowly return to normal operation.

Supercharger bypass valves can present the same kind of issues when they are allowed to vent to atmosphere. (or when there is a leak in the piping allowing air to recirculate.)  Failing to catch an air leak with a supercharger bypass will result in the MAF curve having a sudden change when the valve closes.  This will require complete re-tuning of the MAF transfer function once fixed so it is best to catch it early.

Reversion

Reversion is most common in engines with very large camshafts operating at low speeds such as close to idle.  Situations where MAFs read unreliably due to reversion can generally be greatly improved by moving the MAF further from the throttle body.  Increasing the volume of the intake between the MAF and the throttle body is also effective at smoothing out the pulses of air coming from an engine with a radical camshaft.  It is normally possible to get a reliable enough MAF signal in most circumstances.  Even extremely wild cams that draw 3-4″ of vacuum at idle can be tamed with an appropriately designed intake system.

Another form of reversion that is troublesome to MAF systems happens with poorly designed supercharger bypass valve systems.  In most of these systems, the pipe connecting the outlet of the bypass valve connects with the inlet of the supercharger at an angle where recirculated air flows backwards through the intake.  This causes any reverse-flowing air to be metered multiple times by the MAF, leading to unreliable operation.  This can almost always be remedied by adjusting the angle of the pipe from the bypass so it points at the inlet of the supercharger directing the flow of recirculated air away from the MAF.

Reversion is very obvious if you are logging the MAF signal. Looking at a graph of a “normal” MAF signal versus time, it will look like a line that could have been drawn without reversing the direction of travel.  The same graph of a MAF impacted by reversion will look very “shaky” and jagged, changing direction many times in a short period of time.