Hits and misses: Variables of variable displacement engines

April 1, 2019
What would you say if that new full-size Chevy truck you’re thinking about buying could run on only two out of eight cylinders?

How about only one out of eight cylinders? That’s true for 2019 and for the last 20 years there have been plenty of other models in your service bays every week that can run on half of their cylinders thanks to the continually advancing technology of variable displacement engine technology. In this article, we’ll pass along information on how the brand new and older systems work plus give some diagnostic tips on what to do when they you get common variable displacement engine complaints like misfires and oil consumption.

Variable displacement? The whys and the variations

Figure 1

To meet increasing CAFÉ requirements, OEMs have been downsizing engine sizes and adding technologies such as turbocharging and GDI for years. Another technology seen in your service bay to accomplish the same CAFE goal is variable displacement. A full-size pickup or SUV weighing over 2.5 tons travelling 65 miles per hour only needs around 25 HP to maintain speed on a level surface. In 2005, the Generation IV Vortec 5.3L engine in the GMT 360 platform (Chevrolet Trailblazer and GMC Envoy) were the first engines equipped with Displacement on Demand (DoD) if we don’t include those early Cadillac 4-6-8 engines from the early ’80s. The DoD engine control system has the ability, under certain light load driving conditions, to provide maximum fuel economy and reduce emissions by deactivating four of the eight engine cylinders. Cylinders 1 and 7 on left bank and 4 and 6 on right bank are always the cylinders disabled. GM changed the name to “Active Fuel Management” or AFM shortly after that and other OEMs joined in with their own variable displacement technologies such as Chrysler/Dodge/Jeep with their Multiple Displacement System (MDS) used on Hemi engines (almost identical to GM’s system) and Honda/Acura with their Variable Cylinder Management (VCM) system. Each manufacturer has released subsequent variations with minor changes to their systems over the years. Until the 2019 GM full-size pick-up/SUV model engines (5.3 L84 and 6.2 L87) debuted this year with Dynamic Fuel Management (DFM), there had been one aspect of commonality – the same cylinders were always shut down during reduced displacement operation. With DFM or as Delphi Technologies refers to it, “Dynamic Skip Fire”, almost countless combinations of cylinders activated and deactivated will likely become the new normal for variable displacement technology. Delphi teamed up with the software company Tula in Silicon Valley to come up with an innovative technology that constantly changes which combination of cylinders are deactivated. GM’s DFM utilizing eight separate solenoids (Figure 1) to control all 16 valves can rotate cylinder deactivation patterns as well as run fixed patterns. DFM differs from other systems in its charge trapping strategy with a low-pressure combustion charge trapped in deactivated cylinders, requiring deactivating and activating the intake valve before the exhaust valve. For rotating patterns, which cylinders are being deactivated can change with each subsequent engine cycle.

AFM and MDS mechanical operation

GM and Chrysler/Dodge/Jeep use oil pressure control solenoids to move a spring-loaded pin in both the intake and exhaust lifters essentially allowing the lifter to collapse/shorten on command. The solenoids are housed in common assembly GM refers to as a LOMA (Lifter Oil Manifold Assembly) (Figure 2). On both OEMs these locking pins connect the inner mechanism of the lifter to the outer housing. The inner mechanism interfaces with the pushrod; the outer housing contacts the camshaft lobe through a roller. Thus, the lifter doesn’t lift as far when the cam lobe moves it up therefore the push rod doesn’t push as high and the rocker arm doesn’t open the valves (Figure 3). Although one solenoid controls lifter pin release pressures for both intake and exhaust valves, solenoid activation is timed so the exhaust valve is disabled first. This traps a burnt exhaust charge in the cylinder which contributes to a reduction in oil consumption, noise and vibration levels and exhaust emissions. If all enabling conditions are maintained for variable displacement operation, the PCM calibrations will limit cylinder deactivation to a cycle time of 10 minutes in V4 mode, and then return to V8 mode for one minute. Transitions from 8-4-8 only takes 250 ms. Fuel is disabled while spark remains active to reduce the potential for plug fouling and cylinder wall wash.

Figure 2
Figure 3

Honda/Acura’s VCM system differs in that it uses solenoid-controlled oil pressure circuits to move spring-loaded pins to cause the overhead cam’s rocker arm assemblies to uncouple from the cam lobe followers (Figures 4 and 5).

Figure 4 Figure 5

Electronics and software strategies

All variable displacement systems work via oil pressure control solenoids. The engine management computer’s job is to activate these solenoids while monitoring for current draw and voltage status.  Resistance for each solenoid is around 12 ohms. If, for example, the solenoid’s low-side ground control circuit is NOT commanded to ground the circuit should be at battery voltage. If the solenoid’s low-side ground control is being grounded (to activate the solenoid) the low-side control circuit should be near zero volts. In either state the voltage should be as predicted and if not, a DTC sets for a circuit fault. Nothing new there. The enable criteria for any of these systems to work is lengthy. Basically, there can be no high load or over heat condition nor any DTC setting conditions present for these systems to activate.

Hemi MSD crazy case study

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:

Condition

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.

Cause

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)

Correction

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.

Engine mechanical misfires and noise

These problems can occur no matter what OEM or design is used in variable displacement systems.  Regardless of whether lifters don’t lift (GM, Dodge) or rockers don’t rock (Honda), it’s a small pin operated by the electronic control of oil pressure that does the job. When it doesn’t move to allow for cylinder deactivation there are torque management processes still going on to prevent the driver from feeling cylinders cut in and out. Torque management will continue for cylinders that are NOT deactivated properly until a DTC sets, meaning if the cylinders aren’t shutting off due to stuck mechanical parts, there will be surges noticed under steady cruise conditions. If the engine enters reduced displacement and remains there due to sticky mechanical parts (stuck lifter/rocker pins or solenoid oil passageways), a stumble is noticed followed by a misfire(s). Honda/Acura models will sometimes “hammer” when their V-6 Odyssey minivan models get one or more cylinders stuck in displacement reduction mode. This abnormal noise may even be described as “it sounds like a machine gun under my hood!” Honda addresses normal noise and vibration on their VCM systems via special motor mounts and noise cancellation via the factory audio system. Whether your customer has excessive engine noise or a misfire DTC, you’ll need to add variable displacement system testing to your diagnostic approach when working on these vehicles.

Variable displacement mechanical diagnostic tech tip

Kent Moore has a special tool (EN-46999) for GM’s AFM systems that checks the oil control solenoids’ electrical integrity as well as the actual flow from the solenoids via compressed air and a pressure gauge tied into the system. As with any OEM specialty tool with a high price tag (MSRP of $1,400), unless you find a used one for a low price, you’re probably not going to purchase it. Adapters to the harness and the pressure connections further run up your costs and hassles to do diagnostics the “OEM way” so one alternative to using this and other special tools on variable displacement engines is to perform a running compression test while activating the variable displacement system:

  1. Connect a scan tool that has the bi-directional control capability to activate variable displacement oil control solenoids for the vehicle you’re working on.
  2. Disable the fuel and spark for one of the cylinders that is controlled by the variable displacement system. Choose two variable displacement-controlled cylinders – one that is NOT having a misfire and one that is for this test.
  3. Remove the suspect cylinder’s spark plug and install a compression gauge with the hose’s Schrader valve removed. If you’ve never performed a running compression test you should practice this procedure on a few known good vehicles to get your bearings on what to expect.  Generally, the compression you get during a normal speed cranking compression test is at least twice as high as what you encounter on a running compression test. Running compression tests are excellent methods for catching problems such as faulty valve springs which sometimes don’t show up when performing conventional cranking compression tests.
  4. Activate a suspect cylinder’s variable displacement solenoid via your scan tool (or do so manually with fused test leads connected correctly) while observing the compression with the engine running. A properly functioning variable displacement system will cause the running compression readings to increase and decrease as you activate and deactivate that cylinder’s oil control solenoids. If an oil passage way is plugged up, a solenoid is not functioning or there’s a stuck pin in the lifter (GM or Dodge) or rocker (Honda/Acura) you will not see a change in running compression as you activate that cylinder’s solenoid. A deactivated cylinder will typically run around 20 PSI on a running compression test while the same cylinder will jump up to around 50 PSI upon reactivation if that oil control solenoid is working, the oil passageways are clear, and the lifter is working as designed.

Note: If you know how to perform an in-cylinder pressure transducer test with your lab scope, leave the compression gauge in your tool box and use the transducer to look for changes in cylinder pressure as the oil control solenoids are actuated. Your initial pattern upon cylinder deactivation will show two larger pressure pulses for both intake and exhaust (almost identical in amplitude) followed by lower and lower pulses until the cylinder is reactivated (Figures 8 and 9).

Figure 8
Figure 9

There’s an abundance of known fixes for popular variable displacement engines as they near their 15th year in the field. The brand-new GM DFM/dynamic skip fire systems that fire and skip cylinders with deliberate and intelligent patterns will undoubtably bring a whole new set of problems and fixes yet to be discovered and developed.  Rest assure of one thing that’s not variable – Motor Age’s mission to keep you informed!

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