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Hits and misses: Variables of variable displacement engines

Monday, April 1, 2019 - 06:00
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Hemi MSD crazy case study

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A very interesting case study came up in a John Thornton class I had the pleasure to attend a couple of years ago. It brought to light an unusual side effect of a faulty COP (Coil on Plug) on a Dodge Hemi engine. The faulty COP (even though it was providing spark) was spiking the PCM that controlled the coil’s primary winding. That spike sent the PCM into a software tail spin referred to as a PCM reset.  When the PCM resets on a Dodge, the ASD (Auto Shutdown Relay) turns off. PCM resets can occur occasionally or several times per second. In the car John was diagnosing, the reset occurred multiple times per second, causing a buzzing sound at the ASD relay. As John said in his class: “ASD relays shouldn’t buzz!” Along with a drivability fit, the car had an MIL for numerous DTCs. The DTC that seemed to set most often was an MSD solenoid circuit fault. Presumably the ASD clicking on/off rapidly as the PCM reset caused the MSD solenoid’s voltage status to be one of the first things the PCM looked at when it “regained consciousness” after each reset. The tech John was assisting had naturally focused on the MSD relay circuit DTC, which was only a symptom of the root cause – the faulty COP was causing the reset! 

Variable displacement engine complaints and diagnostics

Transition too abrupt

Some drivers on today’s variable displacement vehicles can feel the transition. Some calibration changes have been made to reduce the noticeability of the feature either by increasing torque smoothing strategies (Figure 6) or reducing the instances on when the system activates. The Acura I worked on recently had received a software update to address VCM issues (including oil consumption), which significantly reduced highway apply time for the system. On road tests with the Honda factory scan tool, I ran out of patience trying to capture the apply of VCM cylinder shut off during steady interstate driving. As soon as I left the interstate, transitioning to moderate highway/rural driving conditions, the system began its process of running in 3-cylinder mode. I had a hard time feeling any transition though. Performance minded drivers of GM AFM and Dodge MDS equipped vehicles may complain that they can feel their V-8s transition to 4-cylinder mode or dislike throttle response. For many older drivers, there is sometimes historically induced trepidations dating way back to the old complaints from the Cadillac 4-6-8 days. Quite a few Cadillac owners complained about abrupt activation/deactivation transitions and the transmission’s fourth gear input circuit (a major enabling input for the old 4-6-8 system) so that circuit “mysteriously developed an open circuit condition” at the dealer thanks to some customer satisfaction minded Cadillac technicians back in the day. Despite tremendous improvements in these systems today, there are still problems and disadvantages that some owners are unwilling to put up with. The processes involved with deleting this feature on today’s vehicles is covered in numerous owner web blogs.

Figure 6

Contamination/Oil sludging issues

As with engines equipped with variable valve timing and variable valve lift, variable displacement models are every bit as dependent on an ample supply of clean oil. Engine oil sludge and other sources of contamination can cause the pins in the lifters or rocker assemblies to lock in the deactivation state (misfires) or activation state (variable displacement feature not working) resulting in drivability and DTC complaints. Many variable displacement engines include a screen near the oil passages feeding the displacement control solenoids (Figure 7). Today’s variable displacement engines require preventative maintenance services (in this case the common LOF) at intervals based on factory recommendations, real-world experience and the customer’s driving habits. As important as the oil’s viscosity and synthetic/semi synthetic status is the engine builder’s spec for that application.

Figure 7

Oil consumption and fouled spark plugs

Regarding motor oil, a common complaint on variable displacement engines is oil consumption.  Theories vary as to why extra oil consumption seems to plague these vehicles compared to their fixed displacement counterparts. There are lots of folks in the aftermarket promoting the deletion of AFM and MDS on GM and Chrysler products. I personally prefer to stick with what the OEM says on the subject of 2007-2011 models equipped with AFM. Take a look at this abbreviated summary of the General Motors TSB 10-06-01-008F:


This TSB focuses on the common problem of engine oil consumption of vehicles with higher mileage (approximately 48,000 to 64,000 km (30,000 to 40,000 mi) experiencing an MIL and/or rough running engine. Verify that the PCV system is functioning properly. If the customer understands that some oil consumption is normal and still feels the consumption level is excessive, more than 1 quart per 2,000 to 3,000 miles of driving, perform the service indicated in this bulletin.


This condition may be caused by two conditions:

  1. Oil pulled through the PCV system.
  2. Oil spray that is discharged from the AFM pressure relief valve within the crankcase. Under most driving conditions and drive cycles, the discharged oil does not create any problems. Under certain drive cycles (extended high engine speed operation), in combination with parts at the high end of their tolerance specification, the oil spray quantity may be more than usual, resulting in excessive deposit formation in the piston ring grooves, causing increased oil consumption and cracked or fouled spark plugs (#1 and/or #7)


Redesigned rocker covers address the PCV oil usage issue. For the excessive oil discharge from the pressure relief valve GM says you may also need to install an oil reflector in the crankcase oil pan near the pressure relief valve and clean/free up the piston rings or even replace the pistons in severe cases along with the spark plugs (of course) if they are oil / carbon fouled. See the TSB in its entirety for complete details.

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