Diagnosing Engine Timing Faults

Timing faults are costly to get wrong. A cam correlation code can mean an oil change is overdue, a solenoid is stuck, a chain has stretched or a belt has jumped. The repairs for those range from an oil service to a job that takes days. On an interference engine, a timing fault left alone can bend valves.

You already know how to replace a belt, and you know that a chain rattle needs attention. This is about proving what is wrong before a timing cover comes off, and working out what the failure has already damaged.

What You Will Be Able to Do

• Explain how crank and cam timing relate, and work out what a one-tooth jump means.
• Diagnose cam correlation codes in a sensible order.
• Separate oil, solenoid, phaser, sensor and chain faults with live data.
• Verify mechanical timing and judge chain stretch.
• Assess the damage after a timing failure on an interference engine.

How Timing Goes Wrong

The crankshaft turns twice for each camshaft turn, so the cam sprocket has twice the teeth of the crank sprocket. A cam degree is worth two crank degrees. Timing specs may be given in either, so note which one you are reading.

Worked example: A crank sprocket has 20 teeth, and the cam sprocket has 40. One tooth on the crank sprocket is 360 / 20 = 18 crank degrees. One tooth on the cam sprocket is 360 / 40 = 9 cam degrees, which is the same 18 crank degrees. So if the belt jumps one tooth, the cam is 18 crank degrees out. That is a large error. The engine will run badly, if it runs at all.

Modern engines use variable valve timing (VVT). A phaser, which is a hydraulic actuator on the end of the cam, turns the cam ahead of or behind its normal position. An oil control valve, a solenoid valve commanded by the engine computer, sends oil pressure to the phaser. So VVT depends on clean oil at the right pressure and grade.

Reading the Codes

Sensor A is usually the intake cam, and sensor B the exhaust cam. Confirm this for the engine. These are generic meanings, and what sets each code depends on the maker.

The pattern matters more than a single code. Correlation codes on both banks, or on every cam, point to something shared, such as the crank sensor, the crank sprocket, oil supply or the main chain. One cam only points to something at that cam, such as its phaser or solenoid.

A Diagnostic Order That Saves Money

Go from cheap to expensive.

1. Check the oil. Check level, condition and the grade the maker asks for. Thick, dirty or low oil is a common cause of VVT codes. Note service history. Long oil change intervals are also a common reason chains and tensioners wear early.
2. Read the freeze frame. When did it set? Oil temperature, rpm and load tell you whether it set hot at idle, which suggests low oil pressure, or during a cam change.
3. Graph desired and actual cam position. Compare them at idle, then raise the rpm. Actual should follow desired within a few degrees and settle quickly.
4. Command the solenoid. With a bidirectional tool at idle, command the cam to advance. A working system changes the cam position, and the idle gets rough or the engine may stall. If nothing happens, the fault is the solenoid, its oil supply or the phaser.
5. Test the solenoid. Check its circuit, inspect its small oil screen for debris, and listen to it click. Many have a screen that plugs with sludge.
6. Measure oil pressure. Use a mechanical gauge if the phaser responds slowly when hot.
7. Check the sensors. Scope the crank and cam signals together, and compare with a known-good pattern.
8. Verify mechanical timing. Only now does a cover come off.

Reading Cam Data

Worked example: A V6 with 110,000 miles has P0016 and P0018, and it rattles on cold starts. Oil is at the right level, but the records show long gaps between changes. Warm at idle, both intake cams read about 7 crank degrees retarded while the computer asks for 0. Both respond normally when commanded. So the phasers work, but each starts from an offset position, and both banks show the same offset. That points to the shared chain, not to two phasers failing at once. A timing cover inspection follows.

Scope Checks

A lab scope shows the crank and cam signals on one screen. Trigger on the crank signal, usually at its gap or missing tooth, and note where the cam edges fall. Compare with a known-good capture from the same engine.

• Edges in the right place, code still returns: Look at wiring, a sensor signal with noise, or the reluctor (the toothed wheel the sensor reads).
• Every cam edge shifted the same amount: Mechanical timing is off.
• Shift varies with rpm: The chain or tensioner is loose, or the phaser is moving.
• Missing or stretched teeth on the crank signal: A damaged reluctor, or a harmonic balancer that has slipped on engines where the wheel is on the balancer.

Replacing a good cam sensor because of a correlation code is a common waste of money. These codes come from a comparison between two signals, so the sensor is often just reporting the truth.

Verifying Mechanical Timing

1. Disconnect the battery if the procedure calls for it, and remove what is needed to reach the timing marks or tool locations.
2. Turn the crank by hand in the normal direction of rotation until the crank mark lines up. Do not use the starter. Many engines must not be turned backward, because it can let the chain or belt go slack.
3. Check the cam marks, or fit the special locking tools the maker specifies. Many modern engines have no usable marks and rely on tools. Do not guess.
4. Inspect the belt or chain, guides, tensioner, and how far the tensioner has extended.
5. Measure stretch by the maker's method, if there is one. Some makers publish a scan tool value, and others publish a length over a stated number of links.

Warning: If you feel a hard stop while turning the engine by hand, stop. A piston may be touching a valve. Do not force it, and do not use the starter to get past it.

Replace chains, guides, tensioners and sprockets as a set. A new chain on worn sprockets wears quickly. Oil-fed tensioners also need the oil passages to be clean, so find out why the old parts wore.

After a Timing Failure

A broken or jumped belt on a non-interference engine usually leaves no damage. On an interference engine, pistons can hit open valves. The question is what the failure damaged. Do not quote a new belt until you know.

An interference engine is one where the valve and piston share the same space at different times. If timing fails, they can touch.

1. Confirm the failure. With the belt cover off, turn the crank by hand and watch the cam. If the cam does not move, the drive has failed.
2. Set the timing with a temporary belt or chain, if practical, then run a compression or leak-down test on every cylinder. Readings near zero mean bent valves.
3. Look inside. A borescope through the plug hole can show marks on the piston crowns and a valve that does not sit right.
4. Decide. Bent valves mean the head comes off. Check for damaged guides, a cracked piston and broken cam parts. Then the choice becomes whether to repair or replace the engine.

Worked example: A four-cylinder interference engine snapped its belt at highway speed. You fit a new belt and set the timing. Leak-down reads 90 percent on cylinders 1 and 2, 85 percent on cylinder 4, and 12 percent on cylinder 3. Air comes out through the intake on those three. Three cylinders have bent intake valves. The borescope shows fresh valve marks on the pistons in those cylinders, with no holes. The head comes off for valve work, and the block stays, if the pistons pass inspection.

Common Mistakes

Check Your Understanding

1. A belt-driven engine's cam sprocket has 44 teeth. How many crank degrees is one tooth? 360 / 44 = about 8.2 cam degrees, which is about 16.4 crank degrees.
2. P0016 and P0018 both set, and all cams show the same offset. Which do you suspect first, shared parts or individual phasers? Shared parts, such as the main chain, crank sprocket or crank signal, once the oil is confirmed good.
3. You command an intake cam to advance at idle, and nothing changes. The solenoid clicks. What next? Check its screen and the oil supply, then the phaser.
4. The engine runs fine, but P0017 keeps returning. On the scope, the cam and crank edges are where they should be. What next? Timing looks right, so check sensor wiring and connections, and look for a noisy or intermittent signal.
5. A belt snapped on an interference engine. A coworker wants to fit a new belt and hand it back if it starts. What do you tell them? Test first. Set the timing and test every cylinder. If it has bent valves, the customer has paid for a belt on an engine that still needs a head job.

Key Takeaways

• One cam degree equals two crank degrees, and a single tooth is a big error.
• Check oil before anything else, because VVT runs on it.
• Compare desired and actual cam position, then use the solenoid command to split the problem.
• One cam points to that cam's parts. All cams point to shared parts.
• Correlation codes compare two signals, so do not replace the sensor by default.
• After any timing failure on an interference engine, test every cylinder before quoting.