I realise that some of you dislike these long posts and it's probably bad manners or poor netiquette to write them, they're a bit like Fidel Castro's 6 hour speeches, I know, but I like to tell the full and detailed story of any findings. I'd like to apologise to the mods in advance if this is a problem.
The paragraphs in red
by the way, are worth remembering for future reference. A couple of surprises there, I've certainly learned something new.
Manifold Absolute Pressure Sensor (MAP sensor), sounds impressive eh?, what is it?
Some EFI systems, like our Speed-density set-up, do not use airflow measurement of the sort provided by air mass sensors (MAF) like on more sophisticated car systems to determine the base pulse of the injectors. Instead, the base pulse is calculated on manifold absolute pressure (MAP) among other sensors and fixed, calculated or estimated fuelling "maps".
The MAP sensor measures changes in the intake manifold pressure that result from changes in engine load and speed. The pressure measured by the MAP sensor is the difference between barometric pressure (outside air) and manifold pressure (vacuum). At closed throttle, the engine produces a low MAP value. A wide-open throttle produces a high value. This high value is produced when the pressure inside the manifold is the same as pressure outside the manifold, and 100 percent of the outside air is being measured. At sea level this is an average of 14.7 psi. This MAP output is the opposite of what is measured on a vacuum gauge. The use of this sensor together with the barometric pressure sensor also allows the ECM to adjust automatically for different altitudes.
The bike's ECM sends a voltage reference signal (+5 volts) to the MAP sensor. As the MAP changes, the electrical resistance of the sensor also changes. The computer can determine the manifold pressure by monitoring the sensor output voltage. A high pressure, low vacuum (high voltage) requires more fuel. A low pressure, high vacuum (low voltage) requires less fuel.
The output of the MAP sensor is fed into the ECU where it is referenced against revs, intake air temperature, barometric pressure and a pre-calculated table that takes into account things like BMEP (brake mean effective pressure), a measure of engine efficiency determined during development. These inputs, plus the fixed estimated table, allow the ECM to calculate the engine's mass intake air flow, enabling it to provide the appropriate fuelling and ignition timing.
The MAP sensor provides information to the ECM which is only used at shallow throttle angles (very small throttle openings, up to 6% and 2-3k rpm) to provide accurate engine load indications to the ECM. This degree of engine load accuracy allows the ECM to make very small adjustments to fuel and ignition which would otherwise not be possible from throttle angle data alone. The throttle angle data is provided by the TPS (throttle position sensor).
Where are they fitted and how are they connected?
We have a pair of them roughly situated three inches on top of the right hand throttle body, they're attached to the chasis and connected to the two intake manifolds by equal length tubes. These tubes are of a rigid PVC type material rather than rubber, to prevent possible collapse by the vacuum acting upon them. The ends that connect to the manifold and sensor ports are flexible rubber and the one for the right cylinder also has a neoprene sleeve over the rigid PVC pipe to protect it from heat and abrasion by the rear cylinder head bracket. More details about these couplings later in this post.
What is it used for on OUR bikes, exactly?
We have a 2 way Manifold Absolute Pressure sensor (that is two MAP Sensors are fitted), these can have three functions:
1. Accurate fuelling during low speed running. More sensitive control at lower engine speeds. By lower engine speeds I mean 2-3000 rpm and up to 6% of throttle opening. After that the engine relies on other fueling maps derived from other sensors like the TPS, crankshaft position sensor, etc.
2. On some bikes (The Rocket III for example) they also act as cam position sensors. Crank sensor is able to sense 2 revs out of 4 stroke cycle. Map sensor differentiates which cylinder is on compression by using vacuum from each throttle body. This allows system to use sequential sparking, over wasted spark. On our Bonnies the ignition is of the wasted spark variety so this feature is not used.
3. Throttle body balancing. The two MAP sensors electrical signals can be read and monitored individually using the factory diagnostic tool or software like DealerTool, Tuneboy or TuneECU. This enables throttle body balancing in a quick, easy and accurate manner without the use of vacuum gauges which would be very difficult to use on our systems. Disconnecting the MAP sensors to connect the vacuum gauges would cause havoc unless a proper set-up using tight-fitting "T" pieces as shown by JohnyC's post Nº5 later on this thread.
These ilustrations pinched from Triumphs training manual show the working range of these sensors:
What problems can a faulty or poorly connected MAP sensor give?:
Anything that interferes with the MAP sensor's ability to monitor the pressure differential may upset the fuel mixture and ignition timing. This includes a problem with the MAP sensor itself, bad electrical grounds or open circuits in the sensor wiring, and/or vacuum leaks in the intake manifold or hose that connects the sensor to the engine.
Typical driveability symptoms that may be MAP sensor related include:
* Surging, snatching and poor low-speed behaviour.
* Rough, uneven idle or, in extreme cases, no idle at all.
* A rich fuel condition, which may cause spark plug fouling and stop the plug/cylinder from firing altogether.
* Detonation or pre-ignition due to too much spark advance and a lean fuel ratio.
* Loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio.
A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The computer will try to compensate by richening the fuel mixture and retarding timing -- which hurts fuel economy, performance and emissions. In some cases, specially with leaking sensor hoses, the cylinder won't be able to burn the excessively rich mixture, fouling the plug with raw fuel.
This, of course, is the opposite of what would happen if there was a vacuum leak on a carburetted engine. On those it would merely run a little leaner at idle, creating idle instability and stalling and a little off-idle acceleration hesitation or flat spot. An air leak on a MAP sensor vacuum line is a lot more problematic.
Conversely, a restriction in the air intake (such as a dirty and blocked air filter), may produce higher than normal vacuum readings. This would result in a load-low indication from the MAP sensor and a lean fuel condition.
Notice that, once more, this is the opposite of what would happen with carburetted engines, where a blocked air filter would create a rich mixture condition. Very confusing, specially for those that believe that EFI and carburation are essentially the same...
The fitting of big, lumpy hot cams with large valve overlap specs can cause pressure fluctuations in the intake manifold at light load. These fluctuations are known to confuse MAP sensors. You'll find that vacuum drops at low rpm and the engine will run too rich. Not likely to happen with mild road cams though.