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Thread: MAF Fault?

  1. 30th March 2015, 08:20 PM #1
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    Rick1970
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    MAF Fault?

    My 3.5 '93 V8 Disco is starting to like a drink of fuel a bit more than it has in the past. Economy has slowly been dropping from 15l/100 down to around 20 in the last 2 months. It can and will blow a bit of black smoke from exhaust with a big rev. Starts and runs fine as is, no surges, no flat spots, just economy is falling away.


    Smells to be running a tad rich, plugs look as tho its running rich.


    Ignition system is good, with Bosch amp conversion. Air and fuel filters are new.


    I havn't checked fuel pressure, but reg isn't leaking into manifold, and I do intent to make an adaptor to properly check the fuel pressure.


    TPS is set to .34v. Stepper has been cleaned.


    MAF has been cleaned. Trim pot is currently set at .3, if I set trim to 1.3v, idle becomes unstable. MAF output voltage @ idle is about 1.5v with trim pot set to .3v. A quick rev to 2500rpm and it goes up to around 3.5v.


    With ignition turned on (not started), MAF output voltage jumps to around .7v, then settles to .35v after about 3-5 seconds.


    Should I be looking for another MAF?


    Cheers
    Rick
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  2. 31st March 2015, 10:29 PM #2
    angus1
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    A common cause for running rich is the water temp sensor. Not the one for the gauge but the one in front of the thermostat housing. This tells the ECU how hot the engine is and should read around 300ohms at 85C and around 2000ohms when cold. It seems to gradually get higher over time until it will flood the engine. This can be checked in a minute with a meter by just pulling the plug off and reading the resistance across the two pins when hot and when cold.
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  3. 1st April 2015, 05:13 PM #3
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    Quote Originally Posted by angus1 View Post
    A common cause for running rich is the water temp sensor. Not the one for the gauge but the one in front of the thermostat housing. This tells the ECU how hot the engine is and should read around 300ohms at 85C and around 2000ohms when cold. It seems to gradually get higher over time until it will flood the engine. This can be checked in a minute with a meter by just pulling the plug off and reading the resistance across the two pins when hot and when cold.

    Yeah, all good there. 280 ohms at running temp (82-83 deg).
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  4. 3rd April 2015, 09:53 AM #4
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    The coolant temp thing go me thinking. Rechecked at the ECU plug and got the same reading, but the fuel temp sensor shows open circuit. Double checked at the sensor, sensor fault, not working.


    Question is, will the sensor not working throw a fault code and make ECU go into a (rich) limp home mode?
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  5. 3rd April 2015, 11:10 AM #5
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    From my understanding in other vehicles, these sensors are just thermistors. 5v goes in and a lower village goes back to the ecu. If the circuit is open then there will be a fault, otherwise it's just a simple disconnect between actual coolant temp and the temp the ecu sees.

    As mentioned it is one of the key feedback inputs to the ecu. Between coolant temp, airflow, crank angle, o2 sensors and throttle position it all comes together to optimize the engine performance.
    -Mitch
    'El Burro' 2012 Defender 90.
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