I have experienced annoying minor hesitations for a long time, along with excessively negative long-term fuel trims indicating a response to running rich.
The hesitations typically occur soon after start up, only at relatively low rpm 2000-3000, and light engine load. Worse after being parked in the hot sun for some hours, and also immediately after filling with fuel.
After ruling out various issues, I thought it could be related to the EVAP system. EVAP system provides no performance benefit or significant gas mileage benefit.
To test this out, I disconnected the electrical connecter to the purge solenoid, which means that it stays closed, and those annoying brief hesitations disappeared.
This does however trigger the CEL due to purge solenoid open circuit.
Interestingly, while STFT adjustments continue, the LTFT is frozen. This was confirmed when I reset DTC and LTFT remained zero.
Subsequently, I have removed the purge solenoid and checked it for correct function. Using a hand vacuum pump, it maintained vacuum and application of 9V DC demonstrated loss of vacuum, confirming proper operation of the valve. Resistance 21 Ohm. Re-installed and plugged in and the hesitations were back.
My suspicion is that the charcoal canister is bad and has lost it's capacity to store fuel vapours and that fuel vapour is bypassing the canister, so being pushed/sucked through the purge solenoid faster than it should if coming from a functioning charcoal canister. Does anyone think this reasoning is sound?
So, I'm thinking of two options.
1. More costly option: Replace the charcoal filter - does anyone know how easy this would be to DIY?
2. Cheap option $10. Disconnect the distal end of the purge solenoid and cap it off, and also cap off the hose that leads to the intake plenum (because it would result in a vacuum leak if not closed). I would not be keen on leaving the purge solenoid uncapped to vent fuel vapour into the engine bay. I would leave the purge solenoid plugged, which should prevent CEL and should allow ECU to make LTFT adjustments. This would be a cheaper and simpler option. My only concern is about the accumulation of fuel vapour within the fuel tank but not sure what adverse effect that would have. I didn't notice any problems for the two weeks driving with the purge solenoid electrical connection unplugged. Opened the gas cap on a number of occasions with no discernable pressure build up. I'm not sure if our fuel caps have a built in pressure check valve to release excessive pressure or vacuum within the fuel tank, although these valves usually don't activate until pressures of 11 to 14 kPa. I wonder if excess vapour just ends up going out the vent tube (terminates in left rear wheel housing) that's attached to the charcoal canister that usually draws in air when the purge solenoid opens - our cars don't appear to have a vent solenoid.
I think I might try option 2 for a couple of weeks and monitor what happens to my long term fuel trims. However, will probably order in a charcoal filter and fit it myself if it's not too difficult. Will report results.
The hesitations typically occur soon after start up, only at relatively low rpm 2000-3000, and light engine load. Worse after being parked in the hot sun for some hours, and also immediately after filling with fuel.
After ruling out various issues, I thought it could be related to the EVAP system. EVAP system provides no performance benefit or significant gas mileage benefit.
To test this out, I disconnected the electrical connecter to the purge solenoid, which means that it stays closed, and those annoying brief hesitations disappeared.
This does however trigger the CEL due to purge solenoid open circuit.
Interestingly, while STFT adjustments continue, the LTFT is frozen. This was confirmed when I reset DTC and LTFT remained zero.
Subsequently, I have removed the purge solenoid and checked it for correct function. Using a hand vacuum pump, it maintained vacuum and application of 9V DC demonstrated loss of vacuum, confirming proper operation of the valve. Resistance 21 Ohm. Re-installed and plugged in and the hesitations were back.
My suspicion is that the charcoal canister is bad and has lost it's capacity to store fuel vapours and that fuel vapour is bypassing the canister, so being pushed/sucked through the purge solenoid faster than it should if coming from a functioning charcoal canister. Does anyone think this reasoning is sound?
So, I'm thinking of two options.
1. More costly option: Replace the charcoal filter - does anyone know how easy this would be to DIY?
2. Cheap option $10. Disconnect the distal end of the purge solenoid and cap it off, and also cap off the hose that leads to the intake plenum (because it would result in a vacuum leak if not closed). I would not be keen on leaving the purge solenoid uncapped to vent fuel vapour into the engine bay. I would leave the purge solenoid plugged, which should prevent CEL and should allow ECU to make LTFT adjustments. This would be a cheaper and simpler option. My only concern is about the accumulation of fuel vapour within the fuel tank but not sure what adverse effect that would have. I didn't notice any problems for the two weeks driving with the purge solenoid electrical connection unplugged. Opened the gas cap on a number of occasions with no discernable pressure build up. I'm not sure if our fuel caps have a built in pressure check valve to release excessive pressure or vacuum within the fuel tank, although these valves usually don't activate until pressures of 11 to 14 kPa. I wonder if excess vapour just ends up going out the vent tube (terminates in left rear wheel housing) that's attached to the charcoal canister that usually draws in air when the purge solenoid opens - our cars don't appear to have a vent solenoid.
I think I might try option 2 for a couple of weeks and monitor what happens to my long term fuel trims. However, will probably order in a charcoal filter and fit it myself if it's not too difficult. Will report results.
