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Discussion Starter #1 (Edited)
Hi. I own an unmodified UK spec Fiat 124. Still the factory ECU map with a whopping 140bhp.

Previously I had a CEL for a cylinder 3 misfire. Changing the coil made no difference. When I removed the spark plug is was heavily fouled. It did look wet. Changing the spark plug or cleaning the plug instantly solves the misfire. The CEL clears and the car drives back to normal. This has occurred on cylinder 3 twice, the other spark plugs never have an issue.

Just out of interest I was playing around with OBD2 readers and noticed that when the car is idling at 800rpm and warmed up (so its closed loop), the Long Term Fuel Trim is always a negative percentage. Suggesting the car is running rich and its trying to back off the fuelling to compensate.

Any ideas is this is normal or what could be causing it?

screen shot from idling. The short term hunts around 0% as expected, the long term always stays around the -7 to -12%

81907


Real time graph of various driving conditions. With a few steady gear changes at the start upto 100 sec, then a constant 3000rpm from 100 secs to 165 secs, then a lift off (coasting), idle at a set of red lights from 180 sec to 190 sec, then a short acceleration upto 5000rpm with 2 or 3 gear changes around 190 sec, then coasting again, then steady drive from 210 sec.
81908
 

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Leaky spark plug tube seals are common - maybe try replacing that?
 

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Discussion Starter #3
Leaky spark plug tube seals are common - maybe try replacing that?
There was no fluid on the coil or the coil side of the sparkplug. I didnt see anything inside the tube. The fouling was just on the very tip where the electrode is.
 

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Try changing/swapping coil positions. Try moving the coil from the offending #3 cylinder to #1 and current #1 to the now offending #3 position and see if your problem stays with cylinder #3 or if it travels to cylinder #1. If your issue travels problem solved - coil.
 

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2018 Abarth 124 Spider, Mare Blue / Nero Abarth Leather, Brembo's, Automatic
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Whereas the condition is only on one cylinder, and the ECU is controlling a rich condition properly and within normal parameters, one could assume O2 sensors are working properly and no exhaust leaks that would affect the O2's exist. It appears you have addressed the ignition side, new plug and coil having been installed. But, it doesn't end there. Examine the coil connector carefully - terminal tension or corrosion, incl. terminal integrity at the ECU also, ohm wires. Will the ECU reliably drive a noid/test light? ( If you have access to an oscilloscope that's even better.) If not, ECU may be faulty. Or, a leaking fuel injector can cause this issue, delivering too much fuel to just one cylinder. Same wiring checks apply here also. s.
 

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Discussion Starter #6
Try changing/swapping coil positions. Try moving the coil from the offending #3 cylinder to #1 and current #1 to the now offending #3 position and see if your problem stays with cylinder #3 or if it travels to cylinder #1. If your issue travels problem solved - coil.
ive already tried this with the coil and it made no difference to the misfiring cylinder. Only the spark plug did.
 

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Discussion Starter #7
Whereas the condition is only on one cylinder, and the ECU is controlling a rich condition properly and within normal parameters, one could assume O2 sensors are working properly and no exhaust leaks that would affect the O2's exist. It appears you have addressed the ignition side, new plug and coil having been installed. But, it doesn't end there. Examine the coil connector carefully - terminal tension or corrosion, incl. terminal integrity at the ECU also, ohm wires. Will the ECU reliably drive a noid/test light? ( If you have access to an oscilloscope that's even better.) If not, ECU may be faulty. Or, a leaking fuel injector can cause this issue, delivering too much fuel to just one cylinder. Same wiring checks apply here also. s.
Using an OBD2 reader, both the O2 sensors read as expected...

O2 S11 0.09mA
O2 S12 0.705V

The exhaust was recently replaced, just before the second occurrence of the misfire. But afterwards I took the car to 2 different places who put it up on their lift and couldn’t find a leak from the exhaust flanges. I’m guessing the exhaust pipe is a coincidence.
 

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2018 Abarth 124 Spider, Mare Blue / Nero Abarth Leather, Brembo's, Automatic
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Me too! Esp. seeing what it already happened once before the exhaust work. (By the way, little exhaust leaks that can change O2 readings can be tricky sometimes. I use an adjustable blow gun and a bit of compressed air, blowing it into the exhaust and using just crumpled up paper towels or cloth shop rags to seal the exhaust tips - you dont want more than maybe 1 PSI! Then spray the exhaust with soapy water. Or you can run the engine, seal the tips with towels and do the same thing - but this way you have to be quick - doesn't work well on hot exhaust pipes). Is your OBD reader capable of showing coil and injector control waveforms? Or do you have access to a noid/test light to view coil control pulses? I like using a light, as you can have voltage, but not enough current to operate the coil. Injectors dumping extra fuel is somewhat common, I replaced one on another brand of car just last week.
 

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2018 Abarth 124 Spider, Mare Blue / Nero Abarth Leather, Brembo's, Automatic
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Just thinking a faulty injector is fairly likely as it appears you don't have a hard misfire at first, only after having run for a bit. But a weak coil primary circuit could cause a weak spark so some further diagnostic work is required - hard to do from this side.
 
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