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Posted

Stock 1993 XJ Cherokee with automatic transmission misfires at about 4000 rpm. Engine power and operation is normal from idle to 3800 then cuts out and runs rough. Although it is a first gen obd engine management, system engine, i connected an old school inductor timing light just to see if the ignition flashes were consistent. They were until 3800 rpm and missed ignition spark corresponded with the engine cutouts. The crankshaft sensor was replaced and the spark plugs as well. This problem seemed to develop after the flywheel was replaced. There are slots in the flywheel that that the crank sensor gets its timing from. I don't recall if there was a certain way that the flywheel is bolted to the crankshaft but I never saw any marks that would indicate a certain alignment for timing. I would think if there was a problem there that it would affect the whole rpm range. I would appreciate what your thoughts are on this.

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Posted

nevadabil...The crankshaft position sensor works from hall-effect in conjunction with the flywheel/flexplate.  The flywheel or flexplate has notches that generate the hall-effect signal for the PCM.  A 4.0L six has three sets of four notches on the flywheel or flexplate.  As the flywheel or flexplate rotates, these notches pass the crankshaft position sensor.  If your flexplate was installed incorrectly (not possible), there would be an incorrect TDC reference.  The crankshaft position sensor signals TDC at #1 cylinder.  This reference at the PCM controls timing, fuel injector pulsing and other functions.

I would check the ignition coil and the distributor's indexing position if the distributor has been removed.  Ohms resistance is usually an adequate test for a coil.  A higher mileage coil may not have the capacity to fire at higher rpm.  Bad spark plug wires or a bad cap and rotor can raise resistance and overload the coil.  Dirty or faulting fuel injectors can also create this symptom, but that's unlikely if the engine runs smoothly to 3800 rpm.  A dirty fuel filter can cause this problem by restricting flow volume and pressure. 

You have a two-rail EFI/MPI system.  A defective fuel pressure regulator can sometimes limit rpm if fuel pressure is inadequate.  I'd not rule out fuel starvation from a dirty fuel filter, defective pump or defective pressure regulator.  Ignition is half the equation.  I'd start by checking fuel pressure at the rail (Schrader valve) with a test gauge and also the ignition coil, cap, rotor and ignition spark leads, which can be tested for ohms resistance.

Moses

  • 2 weeks later...
Posted

The Fuel Filter was changed, as well as spark plugs. The fuel pressure was tested at the rail and measured about 37 psi. it stayed steady with engine acceleration. when the vacuum hose was removed the pressure was about 43 psi. The distributor was removed and replaced it had a bracket to hold it in a fixed position but it cracked and broke off so a regular distributor clamp holds it. It was rough timed by turning the engine to TDC on cylinder 1 and pointing the distributer to #1 plug wire. The timing marks on the crank are really faint but since this is an electronically controlled engine, moving the engine back and forth made little difference.

Posted
14 hours ago, nevadabil said:

but since this is an electronically controlled engine, moving the engine back and forth made little difference.

The last line should have read "moving the distributor back and forth made little different made little difference"..

 

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Posted

nevadabil...The distributor housing and driveshaft/rotor indexing are important.  There is a factory method described in the FSM.  (I'm guessing you have an FSM or a CD/PDF download version.)  The distributor drive and rotor must index with the crankshaft TDC position.  Then the rotor must point to the #1 spark lead contact in the cap.  This is determined by the distributor housing position.  The PCM controls the spark advance timing, there is no centrifugal or vacuum mechanism...For this reason, the rotor tip and cap's contact alignment must be spot on.

The broken index tab on the distributor housing is telling.  Some will drop the distributor into the oil pump without indexing the rotor.  They try to correct this by breaking or ignoring the housing base tab.  This will not work.  The PCM is looking for a distributor rotor that moves in sync with the crankshaft position sensor.  The rotor may be too far from the distributor cap's spark lead contact.  A misaligned rotor can fire "okay" until the rpm loads the coil and demands more spark output from the coil.  Here, the gap resistance between the rotor tip and cap contact increases.  This can cause misfire.

Work through this to be sure the distributor drive/rotor is pointed correctly with the crankshaft at TDC for #1 piston on its firing/compression cycle.  You should wind up with the distributor housing either at or close to the original tab location for the housing.  (I want the distributor housing to fit at its original tab position with the rotor indexed correctly.)  See if this helps.

On another note, have you tested the coil's capacity yet?  Is the coil meeting specification?  Resistance in the coil wiring circuit can be an issue, including corroded pins at the PCM or PCM plug.  On your model, the 60-way PCM is vulnerable to weather and atmospheric corrosion. 

Moses  

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