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Auto-choke problem on 1.6 Montego
Bit of an odd one this, it gets murkier the more I investigate and do, and I now have a Montego 1600 with a seemingly kaput Auto Choke.
From the end of February, there had been a seemingly random misfire while trying to accelerate in the higher gears. Random? It happened occasionally when the engine was warm/hot with the engine under load in the higher gears at engine speeds around 2000 to 3000rpm - the engine would seem to 'jolt' fleetingly then return to normal running in the early stages, then it became more of a 'loss of power' sometimes as the problem progressed over several hundred miles. Occasionally the rev counter would 'drop' but not very often. Easing the accelerator allowed acceleration to resume as it was before.
Around 2 weeks ago, I resolved to get to the bottom of this problem. Various checks were made on the ignition system, and changing the Ignition ECU was the only thing that stopped the misfiring (there was a random misfire at idle, with the rev counter needle dropping for the duration as well, by this stage).
But! Immediately on starting the engine for the second time with the 'new' (used) Ignition ECU (it has the same part number as the original one, from memory), I noticed that the temp gauge had started reading much lower and then the engine started idling faster at around 1600rpm. The idle speed dropped from the fast idle of 1600rpm to around 1300rpm pretty quickly and stayed there.
Next morning, the car started beautifully, but the high idle speed of 1600rpm remained until the car had warmed up and then stopped in traffic, whence the revs dropped to 1300rpm and stayed there until the car was driven and stopped in traffic when the same pattern occured. The temp gauge shows the sort of readings where the engine would be coming off its fast idle after starting from cold.
Removing the coolant temp sender plug caused the idle speed to drop, which threw me a little.
Despite this, the car runs really well, but the fuel gauge is dropping a little faster than it usually would.
Suspecting the fuel ECU to be faulty, fitting another (used) fuel ECU produced the same behaviour, and nor did fitting a used coolant temp sender make any noticeable difference either.
The Fast Check showed the 'Stepper Motor HI' LED as well as the expected LEDs, so after checking for continuity in the wires between the fuel ECU connector and stepper motor plug and finding no breaks I changed the carb stepper motor for a used spare I have - and removing it from its carb unfortunately loosened the spindle so that the outer part could be turned by hand.
Having reset the position of the hole in the outer part by eye, and fitted this stepper motor to the carb, the car runs beautifully when warm/hot but there is no 'throttle jacking' and the idle speed remains around 900rpm (the fast idle rod is 'out' a few mm, and just stays there) even when the coolant temp sender plug is disconnected. Meanwhile, the temp gauge reads low still.
Wielding the Fast Check again, not only is the 'Stepper Motor HI' LED on still, but now the 'Coil' LED has gone out (it should be illuminated). And while the fuel ECU is disconnected the temp gauge still reads 1/4 with the Ignition Key at 'II', just as it did before the fuel ECU was disconnected (when the Ignition key is removed, the needle drops to Cold, as usual).
Realising I now have a car that won't start in the morning, I am now back in the MG1600...
I'd dearly like the keep the Auto Choke on the Montego, but a lack of time for the next week or so plus an MoT booked for Tuesday makes me lean towards fitting a manual choke and rewiring the temp gauge circuit just to get the car back on the road for now
.
Can anybody inspire me as to what might be going on and where to start checking? Thanks in advance
.
From the end of February, there had been a seemingly random misfire while trying to accelerate in the higher gears. Random? It happened occasionally when the engine was warm/hot with the engine under load in the higher gears at engine speeds around 2000 to 3000rpm - the engine would seem to 'jolt' fleetingly then return to normal running in the early stages, then it became more of a 'loss of power' sometimes as the problem progressed over several hundred miles. Occasionally the rev counter would 'drop' but not very often. Easing the accelerator allowed acceleration to resume as it was before.
Around 2 weeks ago, I resolved to get to the bottom of this problem. Various checks were made on the ignition system, and changing the Ignition ECU was the only thing that stopped the misfiring (there was a random misfire at idle, with the rev counter needle dropping for the duration as well, by this stage).
But! Immediately on starting the engine for the second time with the 'new' (used) Ignition ECU (it has the same part number as the original one, from memory), I noticed that the temp gauge had started reading much lower and then the engine started idling faster at around 1600rpm. The idle speed dropped from the fast idle of 1600rpm to around 1300rpm pretty quickly and stayed there.
Next morning, the car started beautifully, but the high idle speed of 1600rpm remained until the car had warmed up and then stopped in traffic, whence the revs dropped to 1300rpm and stayed there until the car was driven and stopped in traffic when the same pattern occured. The temp gauge shows the sort of readings where the engine would be coming off its fast idle after starting from cold.
Removing the coolant temp sender plug caused the idle speed to drop, which threw me a little.
Despite this, the car runs really well, but the fuel gauge is dropping a little faster than it usually would.
Suspecting the fuel ECU to be faulty, fitting another (used) fuel ECU produced the same behaviour, and nor did fitting a used coolant temp sender make any noticeable difference either.
The Fast Check showed the 'Stepper Motor HI' LED as well as the expected LEDs, so after checking for continuity in the wires between the fuel ECU connector and stepper motor plug and finding no breaks I changed the carb stepper motor for a used spare I have - and removing it from its carb unfortunately loosened the spindle so that the outer part could be turned by hand.
Having reset the position of the hole in the outer part by eye, and fitted this stepper motor to the carb, the car runs beautifully when warm/hot but there is no 'throttle jacking' and the idle speed remains around 900rpm (the fast idle rod is 'out' a few mm, and just stays there) even when the coolant temp sender plug is disconnected. Meanwhile, the temp gauge reads low still.
Wielding the Fast Check again, not only is the 'Stepper Motor HI' LED on still, but now the 'Coil' LED has gone out (it should be illuminated). And while the fuel ECU is disconnected the temp gauge still reads 1/4 with the Ignition Key at 'II', just as it did before the fuel ECU was disconnected (when the Ignition key is removed, the needle drops to Cold, as usual).
Realising I now have a car that won't start in the morning, I am now back in the MG1600...
I'd dearly like the keep the Auto Choke on the Montego, but a lack of time for the next week or so plus an MoT booked for Tuesday makes me lean towards fitting a manual choke and rewiring the temp gauge circuit just to get the car back on the road for now
.Can anybody inspire me as to what might be going on and where to start checking? Thanks in advance
.
:laugh: 
. 
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