My friends car:
1995 Legacy AWD 5speed with hill holder. EJ22 Engine.
TYPE OF STUTTER/STUMBLE:
Stuttering/hesitation randomly for a few seconds while foot is on the throttle, then it's fine. Sometimes it stalls while coasting in neutral or clutch is pressed.
MODS: None
MAINTENANCE:
New spark plugs.
Inspected the Mass Air Flow Sensor visually but not electrically.
New front oxygen sensor.
New fuel pump and sump filter when the pump failed recently, new engine compartment fuel filter as a precaution, the stuttering/hesitation was there before and after the replacements.
No check engine lights.
We hooked a multimeter to the fuel pump to watch how much current it was pulling to let us know if it was shutting off, current stayed steady while hesitation occurred while driving.
Hooked a wire to one of the spark plug wires and ran it inside the car, when the hesitation occurred while driving we brought the wire 1mm away from a grounded part of the car and saw the arcs, meaning the coil was firing just fine during the hesitation, ignition is not the cause of the hesitation.
Then we hooked an oscilloscope to one of the fuel injector wires and watched the pulses while driving and sure enough we saw the pulses cut down to almost non-existent when the hesitation occurred.
Something is telling the engine computer to kill fuel delivery to a very small amount while foot is still heavy on the throttle.
Throttle position sensor?
Mass air flow sensor?
Coolant sensor?
Loose wire somewhere?
I don't see a knock sensor or MAP sensor anywhere on the engine or in the shop manual.
Swapped the front oxygen sensor with the rear one and it didn't fix the hesitation. Then he bought a new oxygen sensor for the front and it still didn't fix the problem.
