944 AFM Problems
08-14-2001, 01:23 AM
#1
6th Gear
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944 AFM Problems
Hello,
I am new to 944s and porsche in general. I recently bought a used '87 944NA that had been stored for 3 years. It has the factory alarm option, but the alarm "brain" is removed and the green wire to the factory radio is capped off. I first test drove the car and it was great. When I bought it I filled it with premium gas. Then took it home, a few days later it would no longer start. I have replaced the DME, checked all fuses and wire connections that I can think of, replaced the fuel pump, and have also replaced all filters, and full tune up. But the problem persists. When I try to start the car it kicks over as though its going to run, but dies a few seconds after I release the key from the start position. The only thing that I have done that works so far is to unplug the Mass Airflow Meter. The car will then idle around 1300 rpm but will not accelerate beyond 1600 rpm. Should I replace the Mass Air Meter or is this an indication of an additional problem. The first time it died and would not start I was going 35 mph. I have also checked the DME to see if it was wet, and it is not, there are no holes in the battery tray. Any help would be greatly appreciated. I have been reading over the lists and trying to figure this out for about 3 weeks.
Thanks,
Chris
I am new to 944s and porsche in general. I recently bought a used '87 944NA that had been stored for 3 years. It has the factory alarm option, but the alarm "brain" is removed and the green wire to the factory radio is capped off. I first test drove the car and it was great. When I bought it I filled it with premium gas. Then took it home, a few days later it would no longer start. I have replaced the DME, checked all fuses and wire connections that I can think of, replaced the fuel pump, and have also replaced all filters, and full tune up. But the problem persists. When I try to start the car it kicks over as though its going to run, but dies a few seconds after I release the key from the start position. The only thing that I have done that works so far is to unplug the Mass Airflow Meter. The car will then idle around 1300 rpm but will not accelerate beyond 1600 rpm. Should I replace the Mass Air Meter or is this an indication of an additional problem. The first time it died and would not start I was going 35 mph. I have also checked the DME to see if it was wet, and it is not, there are no holes in the battery tray. Any help would be greatly appreciated. I have been reading over the lists and trying to figure this out for about 3 weeks.
Thanks,
Chris
08-18-2001, 02:52 AM
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After checking for fuel pressure, verifying fuel pump operation and checking the DME relay (via jumping pins 87a & 87b to pin 30, under the relay) I was still having problems with the car running. ---
When I would start the car it would goto around 1000 RPM then drop off completely within a matter of seconds. However, if I removed the connection from the AFM (Air Flow Meter) my car would start and idle at the correct speed. If any throttle is given to the engine, while running with the AFM disconnected it stalls.
After a good amount of research, through the web and talking to a few professional mechanics in town, this is what I have found out:
If the AFM is bad the car computer (DME) will get a bad reading from the sensor/meter and disable the fuel pump (so that you do not flood the engine, requiring an oil change) by cutting the power from the fuel pump. One way (whether or not it is recommended) to check the AFM's operation is to disconnect it. This will cause the DME to use a factory preset idle condition (the same used when starting your car) for measurement of the fuel / air mixture. If you give the car throttle at this point it will stall out because of a change in the fuel to air mixture ratio (basically the same as a GIANT vaccuum leak) and that is why it will stall without the AFM. One of the jobs the AFM performs (it is basically a semi-advanced form of variable potentiometer) is to tell the DME how much air is comming into the intake manifold (when you depress the accelerator it opens a "butterfly valve" in the throttle body, using the vaccuum to move the "wind" vane of the AFM.) and compesate with fuel by adjusting the fuel pressure regulator and the fuel injectors. This is why the AFM is a very important and integral part of the fuel injection system. When the AFM is bad it obviously will not preform this function.
Also when you start your car the DME will perform a form of diagnostics and when it finds the AFM bad it shuts off the fuel pump etc....
A dirty AFM can also cause "flat" spots in your accelleration curve that will always happen at the same point (ie. between 2300-2500 RPM) It was suggested on one of the lists that I posted to that I take off the black plastic cover on the top portion of the DME and clean all of the contacts, slides, etc.. that are inside the AFM. this may or may not work. This method will also often times fix the "flat" spot in your acceleration.
I have found a few places out there to get these parts at a decent price:
www.partsheaven.com : $100 US - Used AFM
also check out Ian's europarts
AFM $100 US - Used
AFM $2?? US - Rebuilt
AFM $350 US - New
These price are from memory, please forgive if incorrect.
I hope this information was of help to you, I apologize if it is a little long winded. I feel that spreading as much knowledge as I can for free to be a service of good for all those out there that take the time to read it.
If there is anything else I can do please let me know.
Chris
When I would start the car it would goto around 1000 RPM then drop off completely within a matter of seconds. However, if I removed the connection from the AFM (Air Flow Meter) my car would start and idle at the correct speed. If any throttle is given to the engine, while running with the AFM disconnected it stalls.
After a good amount of research, through the web and talking to a few professional mechanics in town, this is what I have found out:
If the AFM is bad the car computer (DME) will get a bad reading from the sensor/meter and disable the fuel pump (so that you do not flood the engine, requiring an oil change) by cutting the power from the fuel pump. One way (whether or not it is recommended) to check the AFM's operation is to disconnect it. This will cause the DME to use a factory preset idle condition (the same used when starting your car) for measurement of the fuel / air mixture. If you give the car throttle at this point it will stall out because of a change in the fuel to air mixture ratio (basically the same as a GIANT vaccuum leak) and that is why it will stall without the AFM. One of the jobs the AFM performs (it is basically a semi-advanced form of variable potentiometer) is to tell the DME how much air is comming into the intake manifold (when you depress the accelerator it opens a "butterfly valve" in the throttle body, using the vaccuum to move the "wind" vane of the AFM.) and compesate with fuel by adjusting the fuel pressure regulator and the fuel injectors. This is why the AFM is a very important and integral part of the fuel injection system. When the AFM is bad it obviously will not preform this function.
Also when you start your car the DME will perform a form of diagnostics and when it finds the AFM bad it shuts off the fuel pump etc....
A dirty AFM can also cause "flat" spots in your accelleration curve that will always happen at the same point (ie. between 2300-2500 RPM) It was suggested on one of the lists that I posted to that I take off the black plastic cover on the top portion of the DME and clean all of the contacts, slides, etc.. that are inside the AFM. this may or may not work. This method will also often times fix the "flat" spot in your acceleration.
I have found a few places out there to get these parts at a decent price:
www.partsheaven.com : $100 US - Used AFM
also check out Ian's europarts
AFM $100 US - Used
AFM $2?? US - Rebuilt
AFM $350 US - New
These price are from memory, please forgive if incorrect.
I hope this information was of help to you, I apologize if it is a little long winded. I feel that spreading as much knowledge as I can for free to be a service of good for all those out there that take the time to read it.
If there is anything else I can do please let me know.
Chris
12-23-2001, 05:19 AM
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Replacing the Air Flow Meter did not fix the problem, however the diagnosis was for all intents correct. It seems, in the end, that the problem was from the DME itself. The DME was not deleivering the proper voltage (5 VDC) to the AFM. After replacing the DME, the car started and ran fine, using either AFM.
My 944 ran, at least for a week and a day, then my DME Relay died, oh well, some days are better than others. I really love driving it though.
My 944 ran, at least for a week and a day, then my DME Relay died, oh well, some days are better than others. I really love driving it though.
12-23-2001, 11:17 AM
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Please don't ever apolagise(sp) for too much information. If someone is not interested they can simply skip the discussion. Those that are interested will take the time to read it.
I personally had no idea how this circuit worked and will now be better able to troubleshoot mine if, more likely when, it gives me problems.
I personally had no idea how this circuit worked and will now be better able to troubleshoot mine if, more likely when, it gives me problems.
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12-24-2001, 03:42 PM
#8
Nordschleife Master
Just reminder that you might want to change your timing belt soon if it has been sitting for 3 years. This is just to be on the safe side or you will be right back where you stared wondering why your car won't start only to find most of your valves are bent because the timing belt broke. Enjoy that 944 they're beautiful automobiles that provide great fun!
