Missfire problems

[S197Anthony]

I have a 2008 full bolt on and cammed gt and have some problems with misfires. Cylinder 5 is misfiring and i have a random misfire on all the cylinders. The sparkplugs have been changed and so have the coil packs so now im thinking its either the timing or the Bama tune i have on the car. Any input would be appreciated.

 

[stkjock]

It’s not the tune if you’ve had the tune installed a while.

Have you checked the harness for damage? Check the output of the alternator and battery as well.

 

[RocketcarX]

Alternator is putting out AC voltage. Time for an 09-10 alternator upgrade.

 

[07 Boss]

I would check the grounds for the coils. Each side of the motor splices into a common ground. If it is random on all or one bank of cylinders I would start there. #5 might just be the one being detected the most or there is another issue.

 

[JeremyH]

Did you gap the plugs? Swap the cylinder 5 stuff with another cylinder one part at a time, (cop, plug, injector) and see if the misifire clears from cyl 5 and moves to the new cylinder. Which ever part moves the misifre is your culprit.

 

[46addict]

Alternator is putting out AC voltage. Time for an 09-10 alternator upgrade.

Do you know the acceptable range of AC voltage? Or does this need to be close to 0 as possible?

OP, aftermarket coils can present issues. Did you use Ford OE ones?

 

[JeremyH]

No more than .5

 

[Pentalab]

No more than .5

If you use a DVM ( on AC volts) to try and read the AC content at the battery terminals, make sure you use a DVM, that when in AC Voltage mode, blocks the DC component. Typ they will use a dc blocking cap in each test lead output. Some DVM's don't, in which case they will read 14.4 volts, even when on the AC voltage position. I have a B+K DVM that doesn't have the dc block caps. So it's useless when trying to read AC content at the battery terminals. However, my Fluke 87 does include dc block caps, and easily reads AC content. You will know in 2 secs if the DVM will do the job or not.

 

[JeremyH]

You want to read from alternator post to alternator case, reading at the battery can hide a ripple ac problem.

 

[Pentalab]

You want to read from alternator post to alternator case, reading at the battery can hide a ripple ac problem.

Why is that? On paper, it shouldn't make any difference which end of the alternator cable the dvm is placed at, but I will take your word on it.
When/ if it stops raining this week, maybe, I will measure the AC ripple Vac at both ends of the cable. I can do that simultaneously with 2 x dvms. If there is any amount of AC ripple, its should get worse with an increased load on the alternator. If the AC ripple is really bad, that can easily damage a battery. We once had a battery charger on an emergency diesel generator setup at work, where the start battery was fried, and replaced twice in 2.5 years. 3 batteries in all. Turns out one of the diodes in the FWCT had failed shorted, which in turn pumps raw AC into the start battery on half cycles. DVM's way back then, would not read the ac component at all, since they didn't have dc block caps. It was an easy fix, but batteries don't like AC going into em....and neither do most electronics.

 

[JeremyH]

It depends on the quality of your meter, your fluke will see it for sure, I have a similiar model. But like you said battery doesnt like it, tends to smooth out and acts like a capacitor, it can hide it when going right off the battery terminals. Also the noise naturally dissipates as it travels away from the alternator. So your most accurate reading is at the alt.

Yes you want to load it up, turn headlights, radio on, ac etc and hold some rpm when you take your reading as well.

It can read fine at the battery at idle, when its really not.

 

[Juice]

My "quick" check for A/C ripple and general health of the charging/battery system: put on an AM station, start and rev engine. I you hear humm in speakers that go with rpms, further investigation is needed.

 

[ixtlan]

Check the coil plugs.
I had a miss I chased for some time until the plug finally fell apart internally giving me a hard fail.
I bet I looked at it 5 times then it came apart in my hand.