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Engine quits unpredictably (ignition)


Model: 944S2, Year:1989, Mileage:160000, Type of use:Street use only
The car is a 944S2, the basic symptom is engine quits as if turned off, at any speed or condition. On occasion there is just a hiccup. Nothing is absolutely consistent in either the occurrence of, or in reproducing the symptom. If coasting after the engine quits with the clutch still engaged and turning the engine over, cycling the ignition key switch to off and back to run will often get it to restart, but not always and then the clutch must be disengaged and the starter has to be used. The only way to reproduce symptom in the garage is by disturbing the ignition final amplifier module (IFAM) push or twist the plug, tap on the module, or at times merely touch it. On a very few occasions it would not restart with the starter, simply disturbing the IFAM was all that was needed to get the engine to start again. Only sometimes while driving can it be directly associated with any vibration or bumps in the road.The IFAM was replaced as the apparent cause of the problem. The new IFAM results in a no-start condition. After re-installing the original part, the engine will start but basic symptom remains. A second replacement IFAM, separately sourced from the first, produces same no-start result as the first replacement. (It is this result that really has me floored) Considering that it might be a team effort of weak links I have gone thru the whole ignition system, replacing some parts just due to age.---Troubleshooting Accomplished / Parts Replaced ----Cleaned engine ground points.Cleaned fwd LH frame ground point.Checked ignition coil resistances (while warm). No faults found.Continuity checked and voltage measured at connectors for ignition system in engine compartment. No faults found. ECU signal at IFAM plug connector is around 0.7 V while engine is running, essentially zero when not (ignition on).Disassembled connector plug to IFAM for visual inspection and testing. No faults found.ECU sent out for bench check by Specialty ECU Repair. No faults found.Replaced ECU/fuel pump relayReplaced the Hall effect sensor in distributor (aged, plug disintegrated and wires exposed).Replaced reference sensor (aged, cracked connector plug and brittle wiring).Replaced spark plug wires (aged, two found to be leaking).Replaced spark plugs (eliminate any cracked plug).Replaced battery (10 yr old, little reserve capacity left).-------Other observations ------Attempts at cushioning the IFAM or otherwise isolating it from road vibration have limited success, seeming to reduce (or maybe wishful thinking) but not eliminating the problem.At times it seems to be more sensitive while the cooling fans are cycling. Supply voltage variation at IFAM during fan cycle is less than 0.5 V .

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