Here we investigate my own experiences with a pre-VVT 2005 Jeep 5.7 MDS Hemi engine. At 70,000 miles, one of the MDS solenoids stopped working (there are 4 on this engine).
The resulting symptoms where the engine would not go back to 8-cylinder mode, it stayed in 4-cylinder mode resulting in terribly rough running. Only cutting the engine and restarting cured this.
This is a major engine repair so you can manage the situation as I did for 6 years and 50,000 miles by just locking the Mopar HEMI Multi Displacement System or MDS off.
Sometimes I forgot to do this and got caught out with a repeat performance of non 8 cylinders running, resulting in me having to stop the engine and restart it. (Not a good idea on the UK’s new “Not so Smart Motorways”) or long tunnels.
If you have an eco-light fitted, it will tell you that you are running on 4 cylinders, but as the one MDS solenoid not working plays havoc with the whole system and delays the 8-cylinder reactivation, no frantic pressing of the Tow/haul button will fix this on a run. You cannot use the Tow/haul button or shift out of top gear 5th to 4th to switch back into 8 cylinder running mode when this failure occurs. I know I’ve tried! Just pull over.
Diagnose the fault properly before you go any further. I used the excellent OBD2 JScan app and caught the error code one day while the terribly rough running was happening. OBD Error Code: P1414 Definition: Cylinder 4 reactivation control performance failure. That confirmed which MDS solenoid had failed.
You will eventually need to get your Just Empty Every Pocket Jeep wallet out and buy some new parts. Or with Rockauto your PayPal will do the same. Its not that expensive.
Buy all 4 Solenoids Dorman or Mopar are fine. You will need a new intake gasket set. If you drive a 2005 model, get new set of ignition leads, also do all of your injector seals or o rings, replace any plastic parts that have gone brittle. The time to do this is now as you are stripping down quite a bit of the engine. Also do an oil service. Future proof the car or truck.