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Unread 07-07-2014, 16:49
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Re: Ford auto mechanic?

Quote:
Originally Posted by Ether View Post

Just for the record I did check the air filter and air intake box when the problem first manifested. But the nest had apparently not yet been sucked up into the air filter box where it could easily be seen. I didn't pursue it any further since it didn't seem to fit the symptoms I was observing (engine always starting and idling smoothly, quietly, at correct speed for first few seconds), and therefore I directed my efforts toward other tests/inspections.

In hindsight, I can concoct reasons why the rodent nest could cause the problem, but nobody I spoke with prior to that suggested such a mechanism.

The following is probably a question that only a Ford engineer could answer, but I'll toss it out here just in case someone knows:

On the engine in question, does the PCM completely ignore the mass airflow sensor and fuel pressure for the first ~3 seconds after starting, and use some sensorless fuel and air metering strategy? Is that what would cause the engine to always start immediately and idle smoothly, quietly, and at the correct speed for the first ~3 seconds before beginning to misfire?



That is interesting that the nest got sucked up to the air filter. That's a new one on me too. In my (limited) rodent experience they go right for the airbox in the first place.

I have a theory that would explain your car's symptoms (and to off a speculative answer to your question):

In my stand-alone ECU I program a parameter called 'cranking pulse width' which varies based only upon engine coolant temperature, not MAF, MAP, or any other normal fuel-related sensor. Your engine starts just fine with this, and then, after a few moments of running, enough of the nest got sucked up against the air filter to 'suffocate' the engine and force it to stall.

The reduced flow should not have affected the functioning of the MAF sensor (it still would have measured the correct amount of air flow, pressure/temperature measurements are not required) nor the FPR (it still would have adjusted the fuel rail pressure to the proper level). But it would stop an adequate amount of air from reaching the engine.
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