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Q: Why does it turn over, have spark, and fuel pump works, but will not start? on 2000 Mercury Mountaineer

We replaced the crank shaft sensor and it still will not start. The check engine light was on for about 2 weeks before it broke down. We also rented a computer and it is showing no codes found. Could it be as simple as a fuel filter? We have no clue what is wrong with it.
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If you are certain that you have spark and PROPER fuel pressure (the pump running is not enough to go on),and since you must be fairly mechanical to replace the crank sensor, check for injector pulse. Having spark & fuel, but no injector pulse, would indicate a failure of the camshaft position sensor circuit. I don't know what computer you rented, but what is needed is likely a scanner with graphing capabilities. There needs to be certain wave forms generated by both the crank and cam sensors, and they must be in synch, for the engine to be capable of starting on the ignition side of things. Unfortunately, the engine needs to be turning RPM's of at least 675, steady for about 10-20 secs, with no "start" signal present, before the ECM will even consider starting to run "after start" self-checks.

However, start by verifying fuel pressure at the port on the fuel rail. If ok, & I think the spec key-on/ engine off, w/o looking it up to be sure, is about 40-44 psi, at a minimum. If pressure ok, since you know you have spark, then take some brake parts cleaner, a flammable spray, and sprayy a couple times in the throttle body or air intake tube every 5 secs while cranking. If it starts, you know the injectors are not doing their job, and the most likely cause, being not 100% sure, but maybe 90%, is the cam position sensor.
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I understand the idea behind this as a first step, as it should be to eliminate outside possibilities, but many vehicles, especially of this age, will not retain a code for a no-start condition if the fault is in the ignition circuit. There are no sensor feedbacks in that circuit except for theft deterrent/responder failure.
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We actually get it at least once monthly - CEL/SES light on, no faults in hitory, since last clear, or current, from the powertrain or body control. We figure the ECM knows something is "up", but doesn't know what, so it turns on the light. I'd guess you'd need to ask the engineers who built the thing - or at least someone smarter than me - why.
Voltage issues will very often cause the MIL to illuminate without a stored DTC.Also frequent source of stored DTC's in other control modules that don't necessarily put a light on. Also it wasn't stated any where if along the way , the battery went dead and needed to jump itthat could clear codes on a 2000 model ,and the 'readiness' codes have not 'set' yet to trigger codes again.
Yes the battery did die and we had to jump it quite a few times. Thank you for helping me out.
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