Tuning: MAF systems, air leaks and reversion


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 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.