1997 Ford Taurus Misfire, much worse above 2,000 RPM

  • 1997 FORD TAURUS
  • 6 CYL
  • FWD
  • 153,000 MILES
At 148000 the car developed a minor misfire on starting. At highway speeds, a mis fire sometimes occured and when the torque converter locked up, the engine stumbled. Pulled all plugs that have about 25,000 miles on them, they are like new. Replaced sparkplug wires and there was small improvement. Scanned OBD-II codes. Nothing set or pending. The mis fire and rough engine performance is very gradually getting worse but still no codes are present. Fuel economy is not effected, and if anything may have improved slightly. Driving at a steady speed of 40-50 mph, it runs fine, give it the gas at these speeds and it stumbles, runs rough, etc. Recently I hooked the computer up and measured sensor outputs. The only thing I noticed to me that was odd was reving the engine in park to 4000 rpm, the short term fuel trim increased from near zero/+/-3% to +40% and the engine would not go over 4,000 rpm. Still has a misfire on cold start that goes away in a minute or two and still has no codes set or pending.
Do you
have the same problem?
Monday, May 11th, 2009 AT 9:52 AM

1 Reply

I would start to suspect the Mass Air Flow sensor at this point.

A few notes to point out.
Revving the engine in Park will usually result in not going over the programmed rpm limit in the PCM. This is to protect the engine from excessive "free revving" - that is revving the engine without a load.

Also to check is fuel pressure while driving at the affected speeds. A loss of fuel pressure or restricted injectors can also cause a miss. Injector O-rings are another possible cause of trouble. They do get old and crack - allowing unmetered air to enter the intake stream and lean out one or more cylinders.

But. My gut feeling is that the MAF is out of calibration or that there is an air leak in the intake path.
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Tuesday, May 12th, 2009 AT 12:24 PM

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