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Category: BMW
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Experience:  ASE certified tech ADVANCED LEVEL SPECIALIST
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my car will not pick up speed when i press the gas pedal. It

Customer Question

my car will not pick up speed when i press the gas pedal. It continues to go at about 20km per hour. What could be the cause.

Submitted: 6 years ago.
Category: BMW
Expert:  AMEDEE replied 6 years ago.

You could have a fuel delivery problem. I would recommend to start by checking fuel pressure. Fuel injected cars are very fuel pressure sensitive. Fuel pressure is very important and must be within specifications! But if the pressure is low, it does not mean the pump is bad. A plugged fuel filter, a pinched line, and low voltage at the pump can contribute to low fuel pressure. All three must be check before replacing the pump! You can get a fuel pressure gage at a local auto parts store for cheap.

Fuel pressure testing is frequently overlooked but it is one of the common causes of poor performance. The test procedure must include testing that will insure normal
fuel pressure under all operating conditions.

Key Factors

1. Fuel demands are low at idle.

2. Wide open throttle at high engine revs uses the most fuel.

3. Port injection fuel pressure must be reduced when manifold vacuum is high. Vacuum pulls fuel out of the injector causing it to deliver too much fuel if the pressure remains high. Manifold vacuum is used to open the pressure regulator at a lower pressure when vacuum is high.

Also, You could have a bad mass air flow sensor. Or the snorkel from the mass air flow sensor to the throttle body could be ripped. Both are very common and can cause you vehicle to loose power along with many other drive ability issues. First look for cracks or rips or anywhere air can enter the engine that is not accounted for by the mass air flow sensor. Next comes the Mass Air Flow (MAF) sensor. This is a very important sensor input to the computer. It uses a hot wire sensing element to measure the amount of air entering the engine. The MAF sensor then outputs an analog voltage signal to the PCM proportional to the intake air mass. The PCM calculates the required fuel injector pulse width in order to provide the desired air/fuel ratio. If the sensor is bad than this air/fuel ratio will be off causing severe performance issues. In extreme cases, I have seen these two problems cause a no start condition. This input can also be used in determining transmission Electronic Pressure Control, shift and torque converter clutch scheduling. The check engine light may even pop on if the sensor is bad, the snorkel is ripped or a tube has fallen off. This will result in a lean condition.

Just be sure the exhaust is not plugged and that the engine vacuum is good at idle. (about 19 inches or so). THIs will ensure that the valve timing is good.

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