Engine Mechanical Testing and Repair

When the scan tool and the ignition checks do not explain a problem, the fault may be inside the engine. Mechanical testing tells you whether the engine can seal a cylinder, hold oil pressure and keep coolant where it belongs. These tests decide whether a car gets a spark plug or a cylinder head, so they have to be done right. This guide covers the tests, what the results mean, and the repair habits that stop an expensive job coming back.

What You Will Be Able to Do

• Run dry and wet compression tests and interpret them
• Run a leak-down test and locate the leak by where the air comes out
• Identify head gasket failure and confirm it with a test
• Work a single-cylinder misfire from coil to compression
• Prepare surfaces, torque fasteners and check clearances the right way during a repair

Warning: Disable fuel and ignition before you crank an engine for testing. Keep hands, tools and test hoses clear of belts and fans. Let the engine cool before you take off parts that are under pressure. Follow your shop's lift rules, and use the exhaust extraction system.

Working a Single-Cylinder Misfire

A misfire is often the job that leads you here. Move from the cheap and easy checks to the costly ones.

1. Note the cylinder from the code. P0303 is cylinder 3. Confirm which cylinder that is on this engine.
2. Swap the coil to a known-good cylinder, clear the code and retest. If the misfire follows the coil, the coil is the failed part, so replace it.
3. If the misfire stays put, inspect and swap the spark plug.
4. Test the injector. Listen to it, check its resistance, check its pulse, or swap it if that is practical.
5. Look for a vacuum leak at that cylinder's runner.
6. If it is still misfiring, test compression.

Warning: Remove spark plugs from an aluminum cylinder head only when the engine is cool. Aluminum threads strip easily when hot, and repairing them is a big job.

Compression Testing

A compression test measures how much pressure a cylinder builds while cranking, which shows how well it seals. Low compression in one cylinder points to worn rings, a valve problem or head gasket failure.

1. Run the engine to warm it, shut it off, and let the cylinder head cool until it is no longer hot. A warm engine gives a truer reading, but hot aluminum threads strip. Follow the service procedure if it says otherwise.
2. Make sure the battery is fully charged, so all cylinders crank at the same speed.
3. Disable fuel and ignition. Pull the fuel pump fuse or relay and unplug the coils or injectors as the service procedure says.
4. Remove all the spark plugs. If one feels tight, stop and let the engine cool fully.
5. Hold the throttle wide open.
6. Thread in the gauge and crank for about 4 to 6 compression strokes.
7. Record the reading and repeat on each cylinder.

Healthy readings vary by engine, and many gasoline engines fall around 125 to 180 psi. Check the spec. The balance between cylinders matters more: readings within 10 percent of each other are generally considered acceptable.

Worked example: Readings are 165, 160, 118 and 162 psi. Take the highest and work out 10 percent: 165 x 0.10 = 16.5 psi. The lowest acceptable reading is about 148 psi. Cylinder 3 at 118 psi is clearly low.

The Wet Test

Add about a tablespoon of engine oil to the low cylinder and retest. The oil temporarily seals the rings.

• Compression rises: The rings are worn.
• Compression stays low: The valves or head gasket are at fault.

Leak-Down Testing

A leak-down test goes further. It fills the cylinder with shop air and measures the percentage that escapes, and where it goes shows you the cause. Compression tells you a cylinder is weak. Leak-down tells you why.

1. Warm the engine, shut it off, and let the head cool before you remove the spark plug.
2. Bring the cylinder to TDC on the compression stroke, so both valves are closed.
3. Connect the tester to the spark plug hole.
4. Apply regulated shop air, following the tester's directions, and read the percentage.

Warning: Air pressure can spin the crankshaft if the piston is not exactly at TDC. Keep hands away from belts and fans, and hold or lock the crankshaft so it cannot turn.

Now listen and look:

Head Gasket Failure

The head gasket seals combustion pressure, coolant and oil from each other. When it fails, any of them can cross over.

Common symptoms are:

• White exhaust smoke, often with a sweet smell
• Coolant loss with no visible leak
• Overheating
• Milky residue in the oil or coolant
• Bubbles in the coolant reservoir

Short trips can leave a little milky condensation under the oil cap, so confirm with a test before you condemn a head gasket.

Confirm it with one or more of these:

1. Combustion leak test. A block tester draws air from the coolant through a test fluid, which changes color if it finds exhaust gases.
2. Cooling system pressure test. Pressurize the cold system and look for a pressure drop with no visible leak.
3. Leak-down test. Watch for bubbles in the coolant.

Find out why it failed. Overheating is a common cause, so check the radiator, fan, water pump and thermostat too.

Reading Exhaust Smoke

A little white vapor on a cold morning is normal condensation. Persistent white smoke is not.

Checking Oil Pressure

If the oil warning lamp comes on or you hear bearing noise, confirm the problem with a mechanical gauge in place of the oil pressure sender. A sender can fail, and a mechanical gauge cannot lie about the reading. Compare the result with the spec at idle and at a set rpm. Check oil level and grade first.

Timing Belt and Chain Service

A timing belt has a scheduled interval, commonly somewhere from 60,000 to 105,000 miles. Check the schedule for that engine. It is maintenance, not a repair.

When you replace a timing belt, also replace the water pump, tensioner and idler pulleys. They wear at the same rate, and they sit behind the same covers. If one seizes later, it can take the new belt with it. On an interference engine, that can wreck the engine.

A timing chain has no set interval. Replace it at the first signs of stretch, such as a rattle on cold start or cam and crank correlation codes such as P0016 or P0017. Replace the guides and tensioners with it.

Always follow the timing marks in the service procedure, and turn the engine by hand through two full revolutions before you start it. If you feel a hard stop, do not force it. A valve may be hitting a piston.

Repair Practices That Last

Clean Gasket Surfaces

Before you install any new gasket, remove all old gasket material, clean the surface, and verify it is flat. Old material holds the new gasket off the surface and creates leak paths. Use a plastic or sharp scraper and a suitable cleaner. Be careful with abrasive discs on aluminum, because they can gouge the surface and throw debris into oil passages.

Check for Warpage

Even slight warpage stops a head gasket sealing, which causes coolant and oil leaks and compression loss. Overheated aluminum heads are especially likely to warp.

1. Clean the surface.
2. Lay a precision straightedge across it, lengthwise, crosswise and diagonally.
3. Try a feeler gauge under the straightedge.
4. Compare the largest gap with the spec. Limits are often only a few thousandths of an inch.

If it is out of spec, the head needs machining or replacement.

Torque in Sequence

Head bolts clamp the gasket with exact, even force. Follow the exact torque values and sequence, because that even clamping prevents gasket failure and head warpage. Many modern engines use torque-to-yield bolts that stretch on installation. These are one-use parts, tightened by torque and then by angle. Clean the bolt holes, because oil or coolant left in a blind hole can stop the bolt seating, or even crack the block.

Check Bearing Clearances

Rod and main bearings ride on a film of oil, so the gap between the bearing and the crankshaft has to be right. Too tight, and the oil film fails and the bearing can seize. Too loose, and oil pressure drops and the bearing knocks.

Measure clearance with a micrometer and bore gauge, or with plastic gauge strip. Clearances are often only about 0.001 to 0.003 inch. Use the spec for the engine.

Check Valve Lash

Where the engine has adjustable valves, set the lash to spec with the engine at the temperature the procedure calls for. A gap that is too tight can burn valves, and one that is too wide makes noise and wears parts.

GDI Carbon Cleaning

On direct-injected engines, carbon can build up on the backs of the intake valves, causing rough idle, misfires and power loss. Confirm with a borescope. Follow the approved cleaning method, usually walnut shell blasting with the valves closed, and vacuum out all the media afterward.

Common Mistakes

Check Your Understanding

1. Cylinder 2 reads 95 psi. After a wet test it reads 150 psi. What is the likely cause?
2. During a leak-down test on cylinder 4, you hear air at the throttle body. What is leaking?
3. Cylinders 2 and 3 are both low, side by side. What do you suspect?
4. An apprentice wants to reuse a set of head bolts because they look fine. What do you check?

Answers

1. Worn rings, because the oil sealed them and compression rose.
2. The intake valve on cylinder 4.
3. A head gasket leaking between those cylinders. Confirm with a leak-down test.
4. Check whether they are torque-to-yield bolts. Those are one-use, so follow the service information.

Key Takeaways

• A compression test shows that a cylinder is weak, and a leak-down test shows why.
• Readings within about 10 percent of each other are acceptable, and a wet test separates rings from valves.
• Air at the oil fill cap means rings. Air at the tailpipe means an exhaust valve. Air at the intake means an intake valve.
• Confirm a head gasket failure with a test, then find what caused it.
• Replace the water pump, tensioner and idlers with a timing belt.
• Clean surfaces, check flatness and clearances, and torque in sequence every time.