diagnosis tool shows misfire on cyl 1 & 6 . Replaced spark plugs and coil rail. No help. Starts fine, idles ok, no issues running up until about 50 mph, then begins to shake pretty bad. let it coast no shaking...accelerating through the shake works. My daughters car so I want to make sure its safe for her to drive
Did smell gas on spark plug removed from Cyl 6, assumed that was due to misfiring
Thoughts?

 

Fuel pressure test

 

Firing the parts cannon is an expensive way to troubleshoot.


No other codes? How old are the plug wires?

 

 

No plug wires here!

Apparently coil #1 isn't getting a signal from the PCM. Coil #1 fires cylinder #1 & cylinder #6 simultaneously. You should check the wiring between the coil rail and the PCM, paying close attention to the pins in the connectors and the backwiring of those pins. The way this works is all 3 coils are powered up all the time and are grounded thru drivers in the PCM. When a spark is called for, the PCM breaks the ground on whatever coil is needed to spark, and the magnetic field breaks down causing the spark. There are only 3 coils that fire 6 plugs on compression AND exhaust. This is called "waste spark" and is nothing new. The idea is to lower NOX emissions by re-burning anything left over from the power stroke.

#6 is usually the first to misfire because the spark is created in coil #1 directly over plug #1, but also thru a long connector link inside the rail to plug #6 on the other end. However, since you've replaced the coil rail, I suspect the primary wiring is at fault. What I mean by "backwiring" is the wires are attached to the pins on the back side of the plugs and sometimes break because of being flexed.

 

Nothing beats good solid troubleshooting like throwing parts at the problem! I like when the manual says "Replace with known good part" to see if that fixes it.

 

I'm too lazy to look: What is the coil rail, does it contain the coil? Just curious. Could he maybe interchange the coils and see if the problem follows to another cylinder?

 

The coil pack is a self-contained unit with 3 coils that mount directly on the plugs and bolts to the head. They have a history of internal shorts, so a lot of mechanics replace them just because! After 190K miles on my WJ, I had a problem of weak spark in #5, so I changed mine. Problem solved.