01 XJ Misfire

[Trent]

I have a 2001 XJ 4.0L automatic that has a misfire on cylinder 1 when coasting down hill that I can't seem to figure out. The cel will flash when im coasting down hill for longer than 5-10 seconds. When I scan it all that comes up is misfire on cylinder 1 code. It also idles kinda rough but it is all in cylinder 1.

Parts that have been rulled out:
Coil pack/ replaced but no change
spark plugs/ New and gaped to .035
injectors/ newish out of a tj
injector wiring/ tested with noid light and multimeter
Recalibrated the camshaft sensor
Has 43psi on fuel rail
compression test was about 130 or so all the way around I think

Any help would be greatly appreciated
Thanks

 

[Falko]

Have you tried cleaning and re-gapping the CPS?

When you say the idle is a bit rough, but it's all in cylinder 1, does that mean the idle doesnt change when you kill cylinder 1?

 

[Trent]

I didn't know you could regap the crankshaft position sensor. How whould you do that?

As for cylinder 1 I can pull the injector wire off and there is no change in the idle but it only shows a misfire when coasting down hill

 

[Falko]

It is technically not an adjustable parameter, but since the CPS could cause a misfire and also reads the misfire condition, and it's free to check, it's worth a shot. I know @Jody Treadway has a favorite procedure; I haven't messed with one in a while.

Since the drop test says something is wrong with cylinder 1, start by swapping the coil, spark plug, and injector to other cylinders one at a time and see if the symptoms follow any of the parts.

 

[Jody Treadway]

Do this first to rule out a valve issue:
While the engine is idling, hold a dollar bill at the tailpipe. See if it gets sucked into the tailpipe at all. If it does, you have a poor sealing exhaust valve. On 2000 and newer 4.0 engines this is pretty common.
Try this first and let me know. I'll guide you through diagnosing it.

 

[Trent]

I have swapped coils spark plugs and injectors around before and no change. I actually replaced all of the injectors because the ones that were in it were cracking on the outside. After I replaced them and it didn't help I swapped them around too.

As for the dollar with the tailpipe I have done this several times and it hasn't once sucked into the tailpipe at all. Also I don't know if its just me but it does seem like there is a lot of excess pressure pushing out of the tailpipe but maybe it's because it is cut off 2 inches from the muffler.

 

[Jody Treadway]

I have also seen the stands of wire break at the injector connectors. Inspect them really closely.

 

[Trent]

I used a noid light on them and #1 looked the same as the rest even when wiggling the wires around there was no change in the pulse. I also checked voltage on them with a multimeter and all were the same even with wiggling the wires.

 

[mysterync]

Could be the driver in the pcm for that injector.

 

[Jody Treadway]

One other thing to visually check, the engine harness tends to rub on the driver's rear head stud. It can rub through and cause numerous electrical issues. Check that out.

The fact that it misfires only while coasting downhill lends itself to a vacuum/ valve sealing issue just by nature. I'd have to really and I mean really verify that a valve is not sealing properly.

 

[Trent]

I didn't find any vacuum leaks (I have found some in the past so not completely ruling it out yet). I have a bare patch of wire that runs to one of my o2 sensors located at the back of the valve cover and now that I have messed with the injector connector on the #1 cylinder so much I have a break in the insulation just above the connector but the wire is still intact.

Jody I have actually been meaning to call you to see if you could do some work on this jeep. Along with the engine problem I need the front pinion bearings replaced in my Dana 30. I would normally do the work myself but I don't have the tools or the knowledge to do it.

 

[01XJ]

Usually this tech tip is for 3# cyl but its possible that the fuel rail is heat soaked (Hot Soak) boiling the fuel before hand. Do you run hot under the hood and do you have the EC Heat Shield over the exhaust header? Just a thought. I went with new injectors from Precision Auto Injectors and it took care of #1 and #3 misfires and smoothed the idle out. I also wrapped my fuel rail with heat tape.