Posted in MoparForums.com

2009 Ram 1500 5.7 MDS – weird #5 misfire problem

   April 30, 2024
(I posted this on Ram Forum as well so please ignore if you've already seen it.)
I have a weird thing happening at 135K miles, with a clean well-maintained, original-owner 2009 Ram 1500 Hemi MDS. It’s actually two things that are probably related:
  1. After a cold start, it runs fine. I drive about 1 minute and if/when it kicks into eco/MDS mode, it runs poorly (95% sure due to cyl #5 - see below) until I either pull over and turn off the ignition completely, and restart, or until it warms up over ~165F or higher. This NEVER happens if I start out in Tow/Haul mode or with shifter in a lower gear i.e. 5th, in order to keep it from going into eco/MDS mode until warmed up (or over 165F). Once it’s warmed up, I can use eco/MDS mode just fine, and it purrs along great – like new,
  2. BUT under normal operating temp, if I put relatively heavy throttle into it, typically on the highway, somewhere between 3-4K RPM, it throws a P0305 #5 cyl misfire code. When I let up, the light usually goes away pretty quickly. But this will repeat fairly consistently under heavier throttle, and when it’s occurring, I can usually feel the roughness until it clears.
This original 5.7 has new spark plugs and I tried swapping the #5 coil pack with a new NGK, even though the original still looks new, as do all the rest. Since turning off the ignition and restarting seems to clear either condition, it makes me think this is computer related. The code is always #5 cyl, and she doesn’t seem to be burning any oil or coolant (via head gasket), so other than possibly the injector, I’m still scratching my head at this. If I remember to turn on Tow/Haul or keep it out of eco mode until she warms up, as well as take it easier when passing or using the on-ramp, she runs fantastic like a sewing machine all day long. No tick noise - ever. I’d love to get thoughts on this and what I can try. Thanks in advance!
Author: cuda341