One cool day last fall, my aunt fueled my mother's 112k 2001 Grand Marquis at a Philips 66 near her house, and within a few miles, the car started chugging horribly. It couldn't make enough power to keep up with traffic at 35MPH. My aunt had it towed to a reputable shop.
This was quite out of the blue. I'd driven the car off and on for 6 weeks prior to my aunt taking it for a spin, and there was no indication that anything was wrong. Quite the opposite, in my opinion. I really enjoyed driving it on the interstate.
The shop's diagnostic scan found multiple coil failures, specifically, cylinders 4, 5, 6 and 8. That's the last half of the firing order.
The service quote was around $800. Mom can't afford that, so I drove the car off their lot and battled it back to her driveway.
I pulled a plug, found it a little old, so I changed them all for good measure. Just PM, you understand. I didn't expect a miracle fix.
I bought a single Accel replacement coil from AutoZone and used it to verify that the dead cylinders were in fact dead. Interestingly enough, the known-good coil did not cause any audible or visible change in the idle when it was substituted for a suspected bad coil.
I'm tempted to R&R all the coils, but before I do, I thought I'd shop around for opinions.
What could cause the last half of the firing order to scan out with bad coils?