Causes of a very rich mix, especially at idle?
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Causes of a very rich mix, especially at idle?
Hi everyone,
The problem: I recently rebuilt my motor and have been having trouble having it idle steadily.
The cause of that misbehavior seems to be that it's running very rich: the fuel trim is pegged to -25% at idle, and the AFR never really makes it to the desired 1.0 lambda. The ECU keeps advancing the timing (looking for that desired 1.0 stoich) until it gets to 2-3 degrees BTDC and then the engine falters and the ECU either catches it (and then the process repeats) or it stalls.
Under load it's better but still a bit rich, cruising at 3000 rpm meets the target AFR but the ECU still reports around -10% short-term fuel trim.
What I've tried so far: I've replaced the ignition coils, wires, spark plugs, and O2 sensors. I've had the fuel injectors professionally cleaned (it ran smoother after this, but the fuel mix remained the same). I've rigorously tested the MAF sensor, throttle position sensor, and accelerator pedal sensors (at the ECU *and* at the sensor) with an oscilloscope, they're all perfectly within spec. I've smoke tested the intake for leaks, it's airtight. Compression is fine (it's a fresh rebuild) at 100-110 psi on each face. I've re-set the ECU (both the 20 brake stomp, and disconnecting the battery) a few times throughout my diagnostic process.
At this point I'm rather stumped. What usually causes the RX-8 to run so rich that the stock ECU's short-term fuel trim is at full lean?
The engine is mostly stock: I've deleted the secondary air injection and I'm running an AEM cold air intake. Neither of these mods were a problem for the stock ECU before the rebuild, so I doubt they're the cause of my problem now. What could it be?
The problem: I recently rebuilt my motor and have been having trouble having it idle steadily.
The cause of that misbehavior seems to be that it's running very rich: the fuel trim is pegged to -25% at idle, and the AFR never really makes it to the desired 1.0 lambda. The ECU keeps advancing the timing (looking for that desired 1.0 stoich) until it gets to 2-3 degrees BTDC and then the engine falters and the ECU either catches it (and then the process repeats) or it stalls.
Under load it's better but still a bit rich, cruising at 3000 rpm meets the target AFR but the ECU still reports around -10% short-term fuel trim.
What I've tried so far: I've replaced the ignition coils, wires, spark plugs, and O2 sensors. I've had the fuel injectors professionally cleaned (it ran smoother after this, but the fuel mix remained the same). I've rigorously tested the MAF sensor, throttle position sensor, and accelerator pedal sensors (at the ECU *and* at the sensor) with an oscilloscope, they're all perfectly within spec. I've smoke tested the intake for leaks, it's airtight. Compression is fine (it's a fresh rebuild) at 100-110 psi on each face. I've re-set the ECU (both the 20 brake stomp, and disconnecting the battery) a few times throughout my diagnostic process.
At this point I'm rather stumped. What usually causes the RX-8 to run so rich that the stock ECU's short-term fuel trim is at full lean?
The engine is mostly stock: I've deleted the secondary air injection and I'm running an AEM cold air intake. Neither of these mods were a problem for the stock ECU before the rebuild, so I doubt they're the cause of my problem now. What could it be?
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