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by Bob O’Neill and Bob Lincoln
The MAP sensor’s function is to tell the computer how much air pressure is present in the intake manifold. Technically, it is a strain gauge device which uses air pressure or vacuum to vary a voltage which it reports to the ECU.
Code 13 indicates a failure of the vacuum or pneumatic failure which prevents the MAP output from changing. The Power Limited light will be lit and the ECU will enter limp mode. Because it can’t rely on the MAP sensor, it uses the TPS and RPM (from the Hall Effect sensor) to indicate manifold pressure.
This code should present just as the engine starts. If the MAP sensor output voltage doesn’t chance more than .04V the code 13 is stored, the Power Limited light is lit and the ECU enters limp mode. At this point the AIS closes and the ECU relies on the TPS and RPM as reported by the HEP to calculate manifold pressure. On turbo engines RPM is limited to 2000 when the TPS reports that the throttle is open more than 22 degrees. This prevents over boost while in limp mode and Code 13 is set.
Occasionally a MAP sensor will ‘drift’, so that the vacuum does not provide just the right output voltage. Since the ECU in its error reporting will only trigger if the voltage is below the minimum or above the maximum, no code 13 or 14 will be set. Instead, the mixture will be off a little (usually too rich), and it may even trip an oxygen sensor code. In such cases, the O2 sensor is good; it’s just reporting the mixture is off. This condition is hard to detect and diagnose, and is not all that common.
In case of emergency — the car refuses to run at all — the driver may try unplugging the MAP sensor, which sometimes brings back limp-mode functioning, when no code was set because the readings, while incorrect, were "within normal parameters."
Leaking, disconnected or broken vacuum hose – Read ‘How to troubleshoot drivability issues’. The MAP sensor is a strain gauge which provides a signal to the ECU based on the vacuum it sees delivered by the vacuum line. The MAP sensor is attached to the ECU on ’86 and earlier models. The MAP sensor is located on the passenger side strut tower. The vacuum line to the MAP sensor often gets disconnected. If this happens simply reconnect it. If you notice that the connection doesn’t feel snug replace the hose with 1/8” inside diameter vacuum hose. Make sure it’s long enough to provide slack in the event of something moving the hose. This will prevent it from becoming accidently disconnected.
Defective Barometric Read Solenoid – If the barometric read solenoid is not operating it can contribute to code 13. Bypass this solenoid by connecting the MAP sensor directly to a known good vacuum source to see if the engine runs better. Keep in mind that this procedure may not prevent code 13 from being set. If this procedure does result in the engine running better and if code 37 is not also present repair or replace the solenoid.
Moisture in MAP sensor – In cold weather water can condense in vacuum lines and inside the MAP sensor then it can freeze. In ’86 and earlier models where the MAP sensor is located on the ECU the issue of moisture in the MAP sensor is more common. If the problem doesn’t disappear once the inside of the car has warmed up, remove the sensor and bring inside to warm up. In severe cases you may have to shake any water droplets out of the sensor and put it in a warm dry place. It may take a day or more for all moisture to be removed. It is important that you not disassemble the sensor. If it gets to that simply replace it with a new one. Moisture entering the MAP usually affects models where the MAP sensor is located on the ECU. If you are experiencing this issue frequently you may want to investigate the source of moisture contamination. Vacuum lines and vacuum leaks are a prime cause. All ’87 and newer models have the MAP sensor located under the hood on the strut tower to prevent the issue of moisture contamination.
Defective Fuel Pressure Regulator – The fuel pressure regulator can develop a tiny leak in the diaphragm which will permit fuel to enter the vacuum system. If fuel enters the vacuum system it can set code 13. It is very important to insure that the vacuum system is sound with no leaks, no disconnected hoses and in the case of the fuel pressure regulator it is important to insure it is operating properly. If you discover fuel in the vacuum system, replace the fuel pressure regulator.
Defective MAP sensor – When the MAP sensor fails it will usually set a code 14 to indicate an electrical signal failure. A failed MAP sensor is not usually the cause of a code 13. If you have experienced moisture in the MAP and are still getting a code 13, replace the MAP. Moisture in the MAP can destroy it especially if it freezes and perforates the diaphragm.
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