Odd misfire at certain revs on steady throttle
Discussion
Got me a bit stumped, this.
Wife's Puma. Drives fine, pulls very nicely, well serviced, just had some head work done (but it has done this since she's had it, sometimes more, sometimes less).
If you are driving along on a steady throttle at about 2000-2500 rpm you can feel a weird constant uneven misfire.
My first thoughts are leads, coil packs, fuel filter, but I was wondering if anybody had any ideas
Wife's Puma. Drives fine, pulls very nicely, well serviced, just had some head work done (but it has done this since she's had it, sometimes more, sometimes less).
If you are driving along on a steady throttle at about 2000-2500 rpm you can feel a weird constant uneven misfire.
My first thoughts are leads, coil packs, fuel filter, but I was wondering if anybody had any ideas
leads - a dodgey lead will be there all the time.
coil packs - possibility but would happen all the time
fuel filter - if its blocked then you would notice it at hard acceleration and top end
could be a dodgey throttle potentiometer. try holding it in this rev range at different throttle positions. if you can isolate it to a certain TP then its probably that. Although this is general mechanics I am not sure if the car you talk about even has/uses one.
coil packs - possibility but would happen all the time
fuel filter - if its blocked then you would notice it at hard acceleration and top end
could be a dodgey throttle potentiometer. try holding it in this rev range at different throttle positions. if you can isolate it to a certain TP then its probably that. Although this is general mechanics I am not sure if the car you talk about even has/uses one.
If everything is ok outside this rev range and at high throttle then it can't possibly be fuel filter, fuel pump or indeed be very likely to be electrical. It sounds a bit like a weak cruise mixture stumble if it were a carbureted engine but again on modern lambda controlled efi this shouldn't be possible.
It might not even be engine but a drivetrain vibration or resonance issue. You might want to double check it does it at the same rpm in all gears and that it isn't actually road speed related. However when in doubt start with the basics. Compression test, cam timing, ignition timing.
Several decades ago when my transport was two wheels not four I had a Honda 550 that had a minor but irritating low rpm stumble that neither new spark plugs nor any amount of tweaking ignition or carb mixtures could cure. It finally vanished when as an experiment I swapped from the original fitment NGK plugs to Champion ones. I have no idea why.
It might not even be engine but a drivetrain vibration or resonance issue. You might want to double check it does it at the same rpm in all gears and that it isn't actually road speed related. However when in doubt start with the basics. Compression test, cam timing, ignition timing.
Several decades ago when my transport was two wheels not four I had a Honda 550 that had a minor but irritating low rpm stumble that neither new spark plugs nor any amount of tweaking ignition or carb mixtures could cure. It finally vanished when as an experiment I swapped from the original fitment NGK plugs to Champion ones. I have no idea why.
syncro. said:
Is it definitely a miss fire? Could be the lambda sensor on its way out causing the fuel to switch rich/lean slowly.
At rest, will it hold any given throttle input, or do the revs flare up and down?If it cannot hold 2500rpm steadily then the lambda is likely on the way out.
Otherwise, sounds like a lead starting to break down, or coil pack.
NiceCupOfTea said:
I know what you're saying - but it's had a top end rebuild at a Puma specialist, new plugs (old ones were wrong, thanks Ford main dealer) so all timing should be fine...
Fairly sure it's there in 4th as well as 3rd.
So was the problem there before or after the specialist worked at it ?Fairly sure it's there in 4th as well as 3rd.
What you describe sounds like surging to me which is usually a sign of a borderline weak mixture. An air leak is where I would start, look for split rubber hoses in the intake manifold area. A spray of carb cleaner on the leak often causes the idle to surge and is a useful way of finding it.
I see there's a Ford technical service bulletin on the Puma about engine idle hang up during gear changes, engine running rough/hesitant or engine speed undershoots during deceleration to a stop. Cause is indicated as a probable bad connection of the throttle position sensor to the wiring loom requiring the old connectors to be cut out of the loom and replaced. Affects 2000 model year only though and may not be related to your symptoms anyway.
Might be worth checking with Ford for any later TSBs which match your symptoms.
Might be worth checking with Ford for any later TSBs which match your symptoms.
NiceCupOfTea said:
Will go and have a poke about when I have a mo. I would have thought air leaks would be obvious under hard acceleration though?
I've had the HEGOS (oxygen sensor) suggested too.
no, on the contrary. The manifold vacuum is highest on part throttle, over run and idle. At wide throttle there is very little vacuum so small air leaks are almost totally maskedI've had the HEGOS (oxygen sensor) suggested too.
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