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Amedee, Former ASE Master Tech
Category: Car
Satisfied Customers: 26627
Experience:  ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector
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99 jetta: replacing..coil..idle weak and stalls after a few minutes

Customer Question

early '99 jetta still not running right. After replacing the coil it was very hard to get started due to extreme flooding. Now that it starts it seems to idle weak and stalls after a few minutes. Could the o2 sensor have become fouled from the excess fuel and backfires. If so how can I test it or clean it?
Submitted: 9 years ago.
Category: Car
Expert:  Amedee replied 9 years ago.

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 or left loose from working under the hood. 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. Whether it be the sensor or the snorkel, you can even do it yourself!
Amedee and other Car Specialists are ready to help you
Customer: replied 9 years ago.
Reply to AMEDEE's Post: how can i test the mass air flow sensor to know for sure it is bad?
Expert:  Amedee replied 9 years ago.
DO you have a voltmeter?
Customer: replied 9 years ago.
Reply to AMEDEE's Post: YES
Expert:  Amedee replied 9 years ago.
Lets start by Checking Voltage Supply
  • Disconnect 4-pin connector from Mass Air Flow sensor.
  • Connect multimeter with test leads from VW 1594 to measure voltage at terminals 1 and 3. Specification:9.0-14.5 volts

If no voltage is present:

  • Switch ignition off.
  • Check wiring according to wiring diagram.
Customer: replied 9 years ago.
what does that tell me about the operation of the sensor itself, if all I am checking is if voltage is getting to the sensor?
Expert:  Amedee replied 9 years ago.
Because it is like a sensor, it acts and responds electricity like a computer. Garbage in, garbage out. Before we test to make sure its outputs are good, we have to check inputs first.
Customer: replied 9 years ago.
doesn't seem to be getting voltage there.
Expert:  Amedee replied 9 years ago.
Is the key on?
Customer: replied 9 years ago.
Expert:  Amedee replied 9 years ago.
Is there battery voltage at any one of the terminals going to the mass air flow sensor?
Customer: replied 9 years ago.
Expert:  Amedee replied 9 years ago.
Well than this is a problem.

We need to trace this down.

Would you like a wiring diagram?
Customer: replied 9 years ago.
I think I'm just going to get it towed to a repair shop. It's my understanding that there are special vw tools and testing equiptment needed for further troubleshooting. Thanks for your help.
Expert:  Amedee replied 9 years ago.
Well I would be more than happy to help you if needed. Let me know if there is any more questions.
Customer: replied 9 years ago.
what further steps would I take from here? Do you think that I will be able to diagnose this problem in a timely matter?
Expert:  Amedee replied 9 years ago.

It all depends what it is. SOme problem are easy, and some are hard time find.