The vehicle: 1982 Mazda 626 - 2.0l I4, SOHC, *QUASI-ELECTRONIC* ignition (basically a traditional ignition system with "solid state points") *NO* computer of any kind in this vehicle, unless you count the radio/CD/MP3 player in the dashboard - purely mechanical/thermal/vacuum controlled emission control system. Distributor uses mechanical (centrifical force from moving weights) and vacuum for advance control.
The misbehavior: Stumbling, lurching, and loss of power (occasionally severe enough to cause a stall) under load/acceleration. Particularly noticable if I hit the gas hard with the tach showing roughly 2300-2700 RPM while climbing a slope, or trying to accelerate while moving at a speed that's near the "top end" for the gear I'm in. This is particularly bad, since according to both book and experience, the "sweet spot" in the power band for this engine is in the 2400-2600 RPM range, depending on whether you want to talk about torque or "raw horsepower" output.
I needed to go in and pull the distributor today as part of a "clean up some oil leaks" campaign - As I suspected, the O-ring on the distributor shaft and the valve-cover gasket were shot. Before putting things back together, I went through thte distributor carefuly - weights are free and cause the rotor plate to move as expected, springs are strong and operating, and applying vacuum to the fitting for the vac. advance rotates the rotor plate as expected. As part of my "post-distributor removal drill", once everything was back together, I hooked up the timing light to make sure I hadn't screwed things up. This turned me on to a new, not previously directly detected symptom...
Hooking the inductive pickup of the timing light on the #1 plug wire with the engine idling gives me normal flash from the timing light, but only at idle. When I "wind it up", I start seeing "dropouts" in the flashes from the timing light. Holding at high-ish revs (estimated
2500-3000 RPM) produces periods of anywhere from a fairly obvious "one lonely miss" to multiple seconds with nothing at all from the timing light until the throttle is released and the engine "winds back down" toward idle. Once back at idle, steady flashes at what looks to be a perfectly reasonable rate from the timing light.Move the pickup from #1 to any other plug wire, and the timing light flashes (obviously it's "out of time" since it's on the wrong plug, but the flashing is constant and regular in relation to engine speed) steadily through all speed variations. Which seems to me like a pretty good "rulling out" of the timing light itself malfunctioning.
It follows logically that the "dropouts" I'm seeing in the timing-light flash are indicating that #1 isn't firing for some reason. Inspection of cap and rotor shows nothing unusual about either the tip of the rotor, the center contact, or the #1 post. Pulling the #1 plug shows nothing of any particular interest - Gap is good, and the deposits on that plug are the same thin layer of powdery, pale tan, dry looking "dust-cake" that the other three show. Plugs, wires, cap, and rotor have somewhere in the neighborhood of 5K miles on them since being installed. No visible cracks or other problems with the wires. Coil-to-centerpost wire likewise looks fine.
I'm stumped - As far as I can figure out from eyeballing wiring diagrams, there's nothing different about #1 that should let it "misbehave" without also causing 2, 3, and 4 to be having trouble - They all fire from the same coil, with the only distinction between them being the moment when the spark is supposed to happen.
So, if this car were to show up showing these symptoms, what would *YOU* be looking at as the source of the trouble?