I finally fired up the 5.7 in the '85 pickup. Runs well, but cylinders 1,4,6 & 7 are dead at an idle. My best guess is that I installed MDS lifters without looking at what I was installing. Does anyone know if I can just drop in a set of MDS solenoids in order to get it to fire on all 8? I really don't want to pull the heads and swap lifters if I don't have to. Engine is a '05 5.7, running an SRT cam with Eagle heads. Block has plastic plugs in the MDS holes right now.

Looks like you answered a question I posted a few days back. I wanted to know if I left the mds lifters in would they work ok. It seems the mds sender will have to be energized so they will open the valve. Aluminum plugs and reg. lifters without the stock PCM. Seems you will have to pull the heads. 1-4-6-7 cylinders are the mds.

If you want it to run, you need to pull the head.

Pretty sure fail safe mode on MDS is the lifters don't lock out. I think without the mds solenoids in place (pretty sure the those plugs just keep oil from exiting the motor), there is nothing to prevent engine oil from getting to the circuit that locks the lifters. much easier to put the mds solenoids in and trying it before pulling the heads off to swap the lifter.

this is from a chrysler training manual:

5.7L HEMI® MDS Operation
Both four- and eight-cylinder configurations of MDS 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. Oil is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes latching pins 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. 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. 2009+ MY 5.7L HEMI® MDS lifters where redesigned to allow for more camshaft lift when deactivated. These newer lifters can be used in prior model year 5.7L engines.

"can just drop in a set of MDS solenoids in order to get it to fire on all 8?"

Yes you can.

Not sure if anyone has ever posted a pic of the lifters and see lots of confusion about these, so here is what they look like. The first pic is a conventional Hemi lifter. The second is a an MDS lifter. They are dimensionally identical. The hole in the side where you can see the pin is what makes them work. This lifter beat out the trunion bearing which let it bounce and destroy the internal body, which is why the pin is turned. Normally the pin sticks through the hole and locks the lifter halves together, making it a conventional lifter. When oil pressure is applied from the solenoid (MDS mode activated), the pin is pushed in, and the internal body can collapse enough to prevent valve lift, but not enough for the pushrod to loose tension to the rocker.

Nice explanation and pics. I see a lot of the MDS lifters turning and tapping lately in our dealership. Mostly on the later VVT engines.

FYI, just replaced the plastic plugs with a set of MDS solenoids. Firing on all 8 cylinders again. So yes, you can leave MDS lifters in your engine and just leave the solenoids unplugged, engine will run on all 8 all the time.

Ok what was he talking about the newer lifter allowing more lift? I think all eight of mine have turned that pin, I'm going to pull one apart and give a looksee. Of course I have eight Reg. litters out of the engine that swapped 3 intake seats, messed the heads and pistons up bad.

I take that as it has more plunger travel in MDS mode for higher lobe lift to be used.