[KOG]

I believe 2006 was the first year Jeep offered the 5.7 Hemi V8 with MDS (multiple cylinder displacement). Cylinders #1, #4, #6, #7 are deactivated to reduce fuel consumption. I wonder if the MDS solenoids are sticking mechanically and causing misfire issues on certain cylinders? I found this message in a Chrysler 300C forum.

P0300 random misfire, 300C 5.7L

300C 5.7L - Had been getting a P0300 random misfire for six or more months. Finally took vehicle to dealer to have engine scanned. Long list of possible problems, bad plugs, ignition harness, possible bent valve or push rod, etc. Expensive and no real solution. Had previously had heads off to...

www.300cforums.com

If the engine will not idle then the solenoids could be stuck such that all or some of cylinders #1, #4, #6, #7 have valves deactivated (not opening and closing). You would have to remove valve covers to confirm this theory and watch individual cylinder pushrod action.

MDS was my first thought, but the cylinders affected aren't just the MDS ones, They're the rear four on the engine, half are MDS, half aren't.

 

[KOG]

Below is a description of how the MDS works.

View attachment 73408

The firing order for the hemi V8 is 1 - 8 - 4 - 3 - 6 - 5 - 7 - 2. When MDS is engaged #1 cylinder lifters are causing a problem and the PCM (powertrain control module) detects and mistakenly assigns a misfire to #8. #4 lifters are functioning properly and no misfire issued on #3. #6 lifters have an issue and are mistakenly causing the PCM to detect a misfire on #5. #6 and #7 have more severe lifter problems and a misfire is assigned to those cylinders.

Misfire in this context means that when a power pulse in the combustion chamber is expected, the expanding air - fuel mixture drives the piston downward and causes the crankshaft speed to increase slightly and then decrease slightly before the next cylinder in order fires. If a cylinder misfires and does not increase speed but continues to decrease crankshaft speed, the PCM detects this and assigns a misfire code to that cylinder that did not make its expected contribution.

But one would then ask that if a cylinder is deactivated through MDS, it is not expected to fire or combust and make an equal contribution towards increasing crankshaft speed. When the deactivated valve lifters are functioning properly, air is trapped in the cylinder. It takes work to compress the trapped air on the upward stroke of the piston but that is regained when the gas expands and helps drive the piston downward on the down stroke. So the net effect on the crankshaft is unchanging. But if the lifters are stuck or hanging up and causing the valves to open and close slightly, the trapped gas can escape so the pressurized contribution on the downward stroke is lost and the crankshaft slows down excessively and the PCM detects this and assigns a misfire code. This is just my hypothesis of the MDS behavior issue.

I do not believe you have an electrical problem with the wiring to the MDS solenoids. If there was a wiring problem with short to ground, short to power, or open circuit, an appropriate diagnostic code would be set. No other diagnostic codes have been presented so that leads to the conclusion that the issue is a mechanical one with the valve lifters.

So I am thinking that you will need to inspect the MDS solenoids and lifters for cylinders #1, #6. #7. It could be a problem with the solenoids, internal oil passageways, or the lifters themselves.

That sounds plausible. I'm already planning to pull the valv

I second the motion.

New lifters are out that supercede all previous versions. There is a fair possibility that they're available, so do not use previous/superceded part numbers.

For this application, they are showing a cam of 53022065AB, and lifters are 5038785AD (front) and 5038786AD (rear). Plus gaskets, coolant, and the beer.

For the cost of a full repair (pulling the oil pan, drop the crank, flushing the block, checking the bearings, replacing the oil pump, timing chain/sprockets/tensioners, VVT solenoid, etc) you're further ahead getting a Mopar reman long bock, as the newer Eagle and Apache engines do not use EGR (except for the 6.4 truck engine).

 

[KOG]

FIXED! Finally had time to get back on it. Started by unplugging and spraying Deoxit on all injector and coil connectors. That was it.