Do the same four cylinders go inactive each time the Hemi goes into fuel saver mode or does different cylinders go inactive each time. I have been wondering about this ever since I read about the spark plugs having to be changed at 30K miles.

 

Thought I read somewhere where it is the same 4 cylinders that deactivate. Don't think the mds has anything to do with the 30K plug change. My 2006 tacoma with the v6 called for new plugs every 30K.

 

Yes it's the same 4 cylenders. Do a search and you'll find more info. Plenty other threads on this.

 

Thanks for the replies. I was just curious as to the method used. Now I know.

 

The Multiple Displacement System (MDS) provides cylinder deactivation during steady-speed, low-acceleration and shallow grade climbing conditions to increase fuel economy. Both four- and eight-cylinder configurations have even firing intervals, providing smooth operation. Two cylinders on each bank are active when the engine is in four-cylinder mode – every other cylinder in the firing order. All of the cylinders that are deactivated have unique hydraulic valve lifters that collapse when deactivated to prevent the valves from opening. Engine oil pressure is used to activate and deactivate the valves. It is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes a latching pin on each valve lifter, which then becomes a “lost motion” link. Its base follows the camshaft, but its top remains stationary, held in place against the pushrod by light spring pressure but unable to move because of the much higher force of the valve spring.

NOTE: It is critical to use the recommended oil viscosity in 5.7L engines that use MDS.

Deactivation occurs during the compression stroke of each cylinder, after air and fuel enter the cylinder. Ignition then occurs, but the combustion products remain trapped in the cylinder under high pressure, because the valves no longer open. No air enters or leaves. During subsequent piston strokes, this high-pressure gas is repeatedly compressed and expanded like an air spring, but fuel is not injected.

The Multiple Displacement System (MDS) provides cylinder deactivation during steady-speed, low-acceleration and shallow grade climbing conditions to increase fuel economy.

MDS can provide a 5-20 percent fuel economy benefit when operating in four-cylinder mode, depending on driving habits and vehicle usage. For EPA rating purposes, fuel economy is 8-15 percent higher than if the engine was operating on eight cylinders at all times.

MDS is integrated into the basic engine architecture, requiring a minimum of additional parts - four additional solenoids, an oil temperature sensor and a wire harness. Eight unique valve lifters and a modified camshaft are also required.
Source(s):

Dodge Tech

 

So the spark plug change interval has nothing to do with MDS. It's b/c we have copper plugs, right?

 

Wonder where those solenoids are located.....seems to me that is a wearable item with high time. I noticed since I put a flowmaster on, I can hear the MDS begin to kick in. It seems to think about it for a while if you are on the very edge of it not kicking in. Seems almsost to surge for a bit...then MDS light comes on....

 

The new GEN Hemi is Platinum. This was discussed in the spark plug thread but some kept glazing over it.

As I stated in that post I will be interested to hear what the gaps are on 30K plugs in the new GEN Hemi when someone gets to that point.

 

I thought it was because the heads are aluminum and if the plugs stay in too long the can get stuck or damage threads???? What do I know just what I read and hear and what my wife tells me is right.

 

I do not think the aluminum heads have anything to do with it. The TA is an all aluminum engine... Recommended change interval, 100K.

It could just be the way the Hemi is designed, in terms of plug location it is very unique.