Unlocking the mysteries of variable valve timing

Here is a scantool shot of the fuel trims. Notice the difference in the bank 1 to bank 2 LTFT. Bank 1 LTFT is showing a rich condition.

On this scope capture you can clearly see that the timing is off. Channel 1 is the camshaft sensor and channel 2 is the crankshaft sensor.

Here is the camshaft gear on bank 2. The timing marks are correctly aligned.

Here is a scantool screen capture of the multiple cylinder misfire history. This is why the PCM flagged the code P0300.

When an engine is at TDC or top dead center, that means the number 1 cylinder is at the top of the compression stroke and the intake and exhaust valves are in the closed position. This is for all combustion engines. This is your base starting point in your diagnosis as well.

Oil is a key component for proper operation and longevity of the VVT system. Many of these designs are controlled by oil pressure. The use of the incorrect oil can cause these systems to malfunction. Lack of maintenance can allow sludge build up in the VVT components (solenoids and actuators) and that will prevent them from doing their job. Failures related to proper lubrication and viscosity are the most common ones you will face. 

Troubleshooting VVT — An example
When starting your diagnosis on one of these engines, it’s important to determine if this VVT issue is related to a fault internally or externally. What I mean by that is with an internal fault you must determine if the actuator sticking, lifters are failing or some other mechanical or even an electrical fault is causing the problem. On an external fault you could have a timing belt that is off a tooth due to improper belt installation or the belt is just plain worn out. It could be that the key way on either the camshaft or crankshaft is loose, worn out or broken. Most modern engines today will set a code or codes in the Engine Control Module (ECM) that will lead you in a good direction. Hook your scan tool up and look at your engine data, especially the camshaft data or timing data Parameter Identifiers (PIDs). You can gain a lot of knowledge about what the engine is doing or not doing.

Let’s look at a very simple engine that has a VVT problem — a 2007 Saturn Vue with a 3.5L engine. The only stored code in this engine was a P0300 (Random Misfire Detected). This engine has four valves per cylinder but incorporates the use of five rocker arms. It uses three intake rocker arms and two exhaust rocker arms. The extra intake rocker arm is part of the intake rocker arm control system. The intake rocker arm control system is designed to change valve timing and lift, depending on engine speed, vehicle speed, engine load, and other sensor inputs. It operates by controlling intake valve rocker arm actuation. The intake rocker arm control system achieves low-end torque and low fuel consumption when operating in the low-engine speed range, while maintaining high power output when operating in the high-engine speed range. The engine has a normal four-valve-per-cylinder arrangement. At low-engine speed, the primary and secondary intake valve rocker arms operate at normal lift. At high-engine speed, the primary and secondary intake rocker arms are mechanically connected to the mid intake rocker arm to allow high valve lift. Synchronizing pistons connect and disconnect the three intake rocker arms. Hydraulic pressure, or oil pressure, against a timing piston moves the synchronizing piston outward. A stopper piston and return spring move the synchronizing piston back or inward when hydraulic pressure is reduced. Learning how the system operates goes a long way in your diagnosis. Remember, this particular system is not like your normal VVT system. There isn’t any camshaft actuators or solenoids to contend with. So how does a misfire code become a valve timing problem?

With the scan tool hooked up and looking at some of the data PIDs we can see that there is a history of a misfiring engine. Looking at our scan tool screen shot, we can see that all cylinders are showing numerous misfires. On this engine, the cylinders run one, three and five for bank one and two; four and six for bank two. It appears that cylinder No. 2 seems to be the biggest culprit. Let’s not focus on a specific cylinder or cylinders for a misfire problem yet. Let’s look at some of the other data we’ve gathered. I want to mention here that this vehicle runs relatively well without any real engine roughness. What caught my eye was the long-term fuel trim numbers. Look at the scan tool screen shot that was taken here. Do you notice a difference between cylinder banks? Bank No. 1 is running a fuel correction of -14 percent compared to cylinder bank No. 2. Remember the only code was a P0300. There isn’t any air/fuel-related codes stored as the fuel trim isn’t excessive enough to set those codes.

Let's analyze what we’ve got so far. We have multiple cylinder misfires and a fuel trim reading from bank to bank that’s not correct. Sure, we could go after some more data like oxygen sensors or some of the other input sensors, but let’s look at what we have here first. The crankshaft sensor (CKP) tells the ECM what position the crankshaft is in. The camshaft position (CMP) sensor input is used to detect an engine misfire. The ECM also uses the CMP sensor signal as an input for modifying the fuel injection timing and for modifying the ignition timing. Based on the fact that the camshaft sensor is wh at monitors the misfires and we have multiple cylinder misfires, let’s actually look at what the camshaft sensor is seeing.

Here is a scan tool screen capture of the multiple cylinder misfire history. This is why the PCM flagged the code P0300.

Here is the camshaft gear on bank 1. The timing marks are misaligned. This is the negative fuel trim reading on bank 1.

Here is the camshaft gear on bank 1 after the timing belt was realigned. 

To do this I hooked up my scope and set channel one to my camshaft sensor signal circuit located on the left cylinder bank. I set my scope’s channel two to the crankshaft position sensor signal circuit. After starting the vehicle and getting my scope settings correct I could see right away that cam timing on this engine was a problem. What’s nice about using a lab scope is that I didn’t even have to pull anything apart to see this! Removing the belt covers revealed what the scope captures led me to expect — the timing belt was out of time.  

The camshaft actuator solenoid on a 2013 Chevrolet Cruze

This is a camshaft actuator that failed. Notice the spring that came apart. Lack of oil pressure is what caused this failure. This is not off of our Chevrolet Cruze but the fundamental characteristics are the same.  

Another exampleOn our next vehicle, let’s look at the VVT system on a much newer vehicle with more parts in play. This particular vehicle is a 2013 Chevrolet Cruze with a 1.4L engine. This vehicle has four camshafts that are controlled by the VVT system. It came in with a P0010 (Intake Camshaft Position Acuator Solenoid Valve Control Circuit). This particular engine has a CKP sensor, CMP sensor, an intake camshaft actuator solenoid and an exhaust camshaft actuator solenoid. These camshafts are driven by a timing chain as opposed to a timing belt. On VVT engines, valve timing or valve overlap occurs. Valve overlap is the period during the valve timing where both the intake and exhaust valves are open. Occurring towards the end of the exhaust stroke, the intake valves are opened just before all the exhaust gases are released, providing more time for the intake air to enter the engine. On this engine when it’s at idle there isn’t any valve overlap or change in valve timing. As the RPMs increase, valve timing changes and valve overlap will change as well. The valve overlap will change based on what the ECM sees and what the current driving conditions call for, which is based on throttle input by the driver. The movement of the intake camshafts will advance the valve timing and the movement of the exhaust camshafts will retard the valve timing. As with our last vehicle, oil quality and quantity are important factors in VVT performance. On this engine, the camshaft actuator is driven by oil pressure. The oil pressure is routed through the camshaft actuator solenoid valve. It is controlled by the ECM, which activates the camshaft actuators’ solenoids plunger and spool valve and oil flows to the camshaft actuator assembly. The actuator assembly varies the camshaft position based on oil pressure and flow. Since this vehicle came in with a P0010 code, the ECM has determined that the commanded state of the driver compared to the actual state of the control circuit does not match for five seconds or more. In other words, the intake camshaft actuator solenoid is malfunctioning. It either has a stuck spool valve, a plugged-up screen,(which is located at the base of the solenoid and acts like a filter to keep debris out) or there is an electrical failure of the solenoid actuator. The intake actuator solenoid on this vehicle was removed and inspected. There was a small amount of debris on the filter screen and there was debris inside the valve itself. That debris can also inhibit the solenoid’s valve operation by getting lodged inside the plunger and slowing or stopping the valve movement. When debris is found it usually means a lack of maintenance or the engine has had some internal work done and the sediment or sludge particles were disturbed. Cleaning out the passages with a shop towel, along with replacing the solenoid actuator and a fresh oil change fixed this vehicle’s problem. I can’t stress this enough on VVT engines: Maintenance, maintenance, maintenance!!

With our first vehicle, I explained the importance of valve timing and the effects it can have on your diagnosis. The second vehicle explained the effects that a lack of good oil quality, quantity and maintenance can have on a modern engine of today.

When working on an engine with a VVT system, it’s very critical to take the time and gather all the facts. Understand how the systems works, who the players involved are and try, if you can, to see how the engine was maintained. Spending a little time and doing some detective work upfront will go along way to help you reach a successful conclusion as to what could be a tough diagnosis.