Slight Miss At Idle -- Only in Drive/Reverse

[boxyjeep]

Got a weird issue with my '88 MJ w/ 4.6L ATK stroker and AW4 transmission.  The truck starts and idles fine, but once I put it into gear (reverse, drive, 3, 1-2, etc) it will have a periodic miss.  Putting it back into park immediately eliminates the problem.  The issue seems to be exacerbated after the engine is warmed up and I've driven for awhile.  Initially I thought there was a short somewhere in the wiring harness as the engine/transmission slightly torqued or rotated when in gear, but I've jiggled/moved/stretched everything on the harness and haven't discovered any wiring issues.  I also have brand new motor mounts.  My assumption is a bad TPS.  The ECU side is adjusted correctly, but I haven't checked the voltage on the TCU output.  Here's a vid (you can see the slight idle miss):

 

 

 

[pizzaman09]

My guess is that it isn't from a short but due to the extra load that is on the engine fighting the torque converter.

That said, there could be many things that cause a miss, I'd start by checking that the engine has a good ground .  Then I'd probably check if the header bolts are tight to reduce the possibility that it's an intake leak.  In my experience, a miss at idle or low load is usually the engine running super lean.

[boxyjeep]

14 minutes ago, pizzaman09 said:

My guess is that it isn't from a short but due to the extra load that is on the engine fighting the torque converter.

That said, there could be many things that cause a miss, I'd start by checking that the engine has a good ground .  Then I'd probably check if the header bolts are tight to reduce the possibility that it's an intake leak.  In my experience, a miss at idle or low load is usually the engine running super lean.

I’m running a new O2 sensor with new injectors and REM is showing proper close loop operation, plus that would likely also be exhibited when in park.

 

It’s tough to see in the video, but the RPM decreasing in gear is expected—but the random misfire isn’t…. I’ll dig into it a bit more tomorrow.

[cruiser54]

Double check the CPS harness routing. And, the harness routing at the front of the engine. 

 

 

[boxyjeep]

2 hours ago, cruiser54 said:

Double check the CPS harness routing. And, the harness routing at the front of the engine. 

 

Thanks for sharing this.  I had an issue a long time back where my CPS wiring was rubbing up against the exhaust manifold, so I replaced it and wrapped it in a couple layers of heat-resistant foil and tape and secured it out of the way.  That was the first thing I checked.

 

My harness is heavily shortened/modified and I'm concerned that a solder joint broke somewhere.  I'm swapping in an OE harness to see if that changes anything...