Help diagnosing 2002 Mercury Villager

We have been trying to fix our van for several weeks. We took it into a shop last week and they couldn’t figure out the source of the problem either. All the info is below.

Vehicle: 2002 Mercury Villager
3.3 L V-6

Problem Description:

  1. PO300 Random/Multiple Cylinder Misfire detected
  2. Low horsepower
  3. Vehicle shuts off when idling at stop signs or red lights.

Parts replaced in the last 6 weeks:

  1. Distributor
    A. Crankshaft Position Sensor
    B. Cap
    C. Rotor
    D. Distributor

  2. Spark plug wires

  3. Spark plugs

  4. Fuel injectors (Replacements are Chinese-built)

  5. Fuel filter

  6. IAC Valve (Replacement is Japanese-built)

  7. Throttle position sensor (Replacement is Chinese-built)

  8. Some vacuum hoses
    A. To and from MAP sensor

  9. PCV Valve

  10. MAP Sensor

  11. MAP Electrovalve (on top of intake plenum near the throttle body)


  1. Fuel pressure
    A. Operating pressure: 35 PSI
    B. Static pressure: 55 PSI

The only other possible causes of this issue that I know of are a faulty PCA or a faulty wiring harness.

Thank you for your help.

Stop throwing parts at it.Get someone who can diagnose the problem and fix it.Ask around for shops that have an excellent reputation.


A compression test would be in order about now

The shop we took it to has a great reputation and spent 3 hours on diagnostics, and that was on top of giving them all this information.

The spec for the v6 3.3L sohc is 30-38 psi with engine at idle (running), and 37-43 psi with key on but engine not running. So the second measurement is higher than allowed per the spec. Was it done correctly, with the key in “on” but engine not running?

For this sort of problem, other than a good visual inspection for broken connectors, hoses & boots that have split, etc, o ther than routine tune-up stuff (new spark plugs, ignition rotor, dizzy cap, and spark plug wires) before replacing any parts I’d have suggested to start with these tests

  • are you seeing a healthy visual spark at the tip of a spark plug during cranking?
  • ignition timing at idle, rpm at idle
  • intake manifold vacuum at idle and its behavior when you briefly increase the rpm
  • fuel trims

Once all that data is known — if it is, post it here — there may be more tests to do (compression, ignition system waveforms, valve clearances and valve timing), but all that would wait until the first dataset is obtained, as above. Keep all the old parts you’ve replaced btw, you may need to re-install them to correct problems you may have inadvertently introduced by installing aftermarket parts.

PS: Has the egr function been tested? EGR problems on Ford products can produce weird and difficult to diagnose symptoms, from reports we get here. Esp the parts that decide how much EGR to apply.

According to TSB 03-22-6 that should have been accompanied by also replacing the injector wiring harness with an updated version. Did that happen?