Engine mechanical misfires and noise
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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:
- 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.
- 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.
- 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.
- 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).
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!