First a description, then some data.
Usually noticed first in cold weather on the highway, in transition from coasting to light throttle. Much more severe than a miss - feels more like the engine cut out. Then noticed on light acceleration when engine and air are cold. Then becomes an intermittant problem on heavier acceleration, with a more pronounced and sustained effect. Randomly causes rough idle. Car runs fine otherwise. Rarely happens above 2000 RPM, never over 3000. PCM throws no trouble codes. Changing plugs, boots, coils, filters, pcv valve doesn't fix it.
I bought a bluetooth OBD reader and have been trying to catch something misbehaving when the problem occurs. Suspected throttle position sensor and mass air flow sensor; no evidence. Suspected vacuum leak; old fashioned vacuum gage (in garage) ok, OBD vacuum ok, gumout squirted around intake manifold seals ok. No outstanding problem with long or short term fuel trim other than a consistent few % difference between banks. But I did find an indicator. The timing advance sits around +12 when behaving normally, and moves smoothly upward on reasonable acceleration. But now and again on a little heavier acceleration, it drops to 0 and the bucking proceeds. It also occasionally idles at 3, accompanying a slight roughness.
Either something is telling the pcm the engine is under very heavy load or the pcm is having a brain fart. I don't think its the pcm but I'm having trouble isolating what sensor is misbehaving. Web searches tell me the pcm calculates advance from crank and cam position, O2 sensors, coolant temperature, MAF, throttle position, intake air temp, and heaven knows what else. Has anybody who has experienced this rather common problem figured it out? Or if we have any pros in the group, could one of you demystify the Timing Advance?
Usually noticed first in cold weather on the highway, in transition from coasting to light throttle. Much more severe than a miss - feels more like the engine cut out. Then noticed on light acceleration when engine and air are cold. Then becomes an intermittant problem on heavier acceleration, with a more pronounced and sustained effect. Randomly causes rough idle. Car runs fine otherwise. Rarely happens above 2000 RPM, never over 3000. PCM throws no trouble codes. Changing plugs, boots, coils, filters, pcv valve doesn't fix it.
I bought a bluetooth OBD reader and have been trying to catch something misbehaving when the problem occurs. Suspected throttle position sensor and mass air flow sensor; no evidence. Suspected vacuum leak; old fashioned vacuum gage (in garage) ok, OBD vacuum ok, gumout squirted around intake manifold seals ok. No outstanding problem with long or short term fuel trim other than a consistent few % difference between banks. But I did find an indicator. The timing advance sits around +12 when behaving normally, and moves smoothly upward on reasonable acceleration. But now and again on a little heavier acceleration, it drops to 0 and the bucking proceeds. It also occasionally idles at 3, accompanying a slight roughness.
Either something is telling the pcm the engine is under very heavy load or the pcm is having a brain fart. I don't think its the pcm but I'm having trouble isolating what sensor is misbehaving. Web searches tell me the pcm calculates advance from crank and cam position, O2 sensors, coolant temperature, MAF, throttle position, intake air temp, and heaven knows what else. Has anybody who has experienced this rather common problem figured it out? Or if we have any pros in the group, could one of you demystify the Timing Advance?
