My experience with a bad coil

[camaros67s]

It all started one summer, I had a EFI light flashing and a bad coil code pop up but when I started messing with wires the code went away and it seemed to run fine.

Then all of a sudden the next summer my gas mileage dropped from the low 40's to the low 30's. I didn't have a bad coil code so I thought it might be the air filter or plugs. Air filter was fine and changing the plugs had no effect on improving the mileage.

By now I was thinking something was wrong with one of the coils. I removed one of the spark plug wires from the rear cylinder to see if it was sparking. The first one sparked but it seemed weaker then the second one. The second one zapped me good and the first one not so much.

I then reconnected the rear and disconnected the front, at idle I could not tell any difference in how the motor idled when I removed the wires. It always sounded the same.

At this point I figured everything seems to work but my gas mileage is still sh#t. Well, way back when the code first popped up I bought a new coil (Aprilia) and threw it in my tank bag incase the EFI ever started flashing again. Even though all the coils seemed to be sparking I figured throw the new coil on one plug at a time and see what happens.

As you may have guessed by now I picked the coil that didn't zap me that bad as the first test dummy. Well low and behold the gas mileage was back up to the low 40's, and it's stayed there for the last 5 tanks of gas.

I can't say this will work for everyone but I put up with low MPG for a year and I am happy I could figuare this out on my own. If this didn't work I was going to have to take it to the shop because I new the reserve light shouldn't be coming on when the trip meter hit 140's.

Good luck.

The bike has 36,000 miles on it and it's a 2004.

[deefred]

I then reconnected the rear and disconnected the front, at idle I could not tell any difference in how the motor idled when I removed the wires. It always sounded the same.

What do you mean about the above?
For the rear cylinder you disconnected the front coil or?

[slyV]

I think he means center / side - rear plugs.
I just love your coil zapping-testing method @camaros67s

[camaros67s]

What do you mean about the above?
For the rear cylinder you disconnected the front coil or?

Sorry about not being clear, what I ment was I checked the front as well as the back cylinders and everything seemed the same. All the coils seemed to be working while the engine was running at idle.

[deefred]

Sorry about not being clear, what I ment was I checked the front as well as the back cylinders and everything seemed the same. All the coils seemed to be working while the engine was running at idle.

So for the rear cylinder you had the engine at idle and with the coil feeding the side plug disconnected the engine was still idling the same.
Then you reinstalled the side plug and disconnected the other coil instead and the engine still had the same idle.
Is the above what you did for the rear cylinder?

Sorry for being picky but for me it is important to know.

Just because you saw a weak spark outside of the cylinder doesn´t mean that it actually fired inside the cylinder as compression makes it more difficult for that spark to appear.

[camaros67s]

I removed and then replaced each plug wire one at a time listening to the motor trying to detect any change. I heard nothing to indicate that the motor was running on one cylinder. All the coils seemed to fire at idle. The only reason I thought I had a bad coil was from the drop in fuel mileage. When I picked one coil to replace it was a guess and I got really lucky because now I am getting 42 mpg regularly while for the previous year I was getting around 32 mpg. The only thing I changed before the mileage jump was 1 coil.

Hope that helps.

[deefred]

Thanks for the clearing that up.

Was the replaced coil for the center or side plug?

[camaros67s]

Thanks for the clearing that up.

Was the replaced coil for the center or side plug?

Side plug if I remember correctly.

[deefred]

I think an ignition spark tester is on my tool shopping list now

[slyV]

2 bad coils recently on my bike! Both on the rear cylinder.
Before switching to the new Renault coils [mods were required on HT & LT connectors] I've played around swapping & testing with 2 bad Aprilia coils, 2 good Aprilia coils & 2 new Renault coils for faster diagnose if it happens again [coil breakdown].
All my initial OEM coils were clean, without corrosion. The 2 bad ones were shot: the spark burnt it's way through the insulation at the lower part of the coil were the metal cylinder meets the HT part. Long story shorted: the spark was happening outside! [have video]
My conclusions: as long as you have one functional coil per cylinder it will fire & run almost OK. It does not matter if it's the side or center plug. I tested this on both front & rear cylinders.
On my setup [ K&N air filter, Iridium plugs - all newly fitted] the warning that the bike is running on both cylinders [perfect idle], but with at least 1 bad coil: small backfires in the exhaust on short, sudden accelerations.