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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Diesel Engines: Theory, Diagnosis, and Overhaul › Lesson 8
TRN-DIES20-02 · Lesson 8 · 18 clock hours · lab

Engine Diagnosis: Compression and Cylinder Leak-Down Testing

Objective. Students will perform compression tests and cylinder leak-down tests on a running diesel engine, record results, compare to OEM limits, and identify the location and probable cause of any cylinder sealing defect.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Engine Diagnosis: Compression and Cylinder Leak-Down Testing

Why Cylinder Sealing Matters

A diesel engine depends on very high compression ratios to heat intake air enough to ignite injected fuel. If combustion gases leak past piston rings, valves, or a head gasket, cylinder pressure drops, power falls, fuel economy suffers, and raw fuel or combustion products contaminate engine oil or coolant. Compression testing and cylinder leak-down testing are the two standard tools for finding and locating those leaks before an engine is disassembled.

Safety First: LOTO and PPE

Before any cranking or pressurizing procedure, apply Lockout/Tagout (LOTO) per OSHA energy-control requirements whenever you need to work near a rotating engine or high-pressure fuel system. For a controlled crank test you will briefly re-energize the starter, so the LOTO sequence must be reviewed with the whole crew: everyone must be clear of fan belts, rotating shafts, and the exhaust path before cranking. Wear safety glasses and nitrile gloves throughout. Diesel fuel is a skin irritant and an aspiration hazard; high-pressure injection systems store energy that can inject fluid through skin at low-looking pressures. Disable the fuel injection system by the OEM-approved method (electronic disable switch or fuse pull) before installing any adapter into an injector port. Never point the compression adapter hose at anyone during pressurization.

Compression Testing: Procedure Overview

A diesel compression test measures the peak pressure the engine can build in each cylinder during cranking. The test adapter replaces the injector or glow plug (depending on engine design); always use a diesel-specific adapter rated for pressures well above typical diesel compression values, which run significantly higher than gasoline engines.

Steps:

1. Warm the engine to operating temperature — oil viscosity and ring seal are more consistent when warm.

2. Apply LOTO, then disable fuel injection per the OEM procedure so no fuel is delivered during cranking.

3. Remove the injector or glow plug from cylinder 1 and thread in the compression adapter hand-tight.

4. Reconnect the starter circuit only. Crank the engine through a minimum of four to six compression strokes (check OEM for exact stroke count) with the throttle fully open where applicable.

5. Record the peak reading, then release the adapter valve and repeat for each remaining cylinder.

6. Compare all readings to the OEM minimum specification and to each other. Most OEMs flag a cylinder when it reads more than a stated percentage below the highest cylinder or below an absolute minimum. A common general guideline is concern when any cylinder reads more than roughly 15–20% below the highest cylinder, but always defer to the specific OEM specification for the engine you are testing.

Low compression in one cylinder with all others normal points to a local fault in that cylinder. Uniformly low compression across all cylinders suggests worn rings or a tired engine overall.

Cylinder Leak-Down Testing: Procedure Overview

A compression test tells you that pressure is low; a leak-down test tells you where it is going. A regulated air supply is connected through a calibrated leak-down tester to a cylinder held at Top Dead Center (TDC) of the compression stroke. The tester displays the percentage of supplied air that leaks out. Many technicians consider up to roughly 10% acceptable for a healthy diesel cylinder, 10–20% marginal, and above 20% a likely cause for concern — but again, use OEM limits when available.

Locating the Leak:

Use a stethoscope to pinpoint the escape point. Move the probe methodically around the intake, exhaust, and crankcase breather before concluding.

Reaching TDC Correctly: Rotate the engine by hand (bar the flywheel) until the cylinder you are testing is at TDC compression. Confirm this by watching the rocker arms — both valves must be closed (no rocker movement). If you apply air pressure and the crankshaft tries to rotate, the piston is not truly at TDC; recheck. Use a flywheel timing mark or OEM procedure. Keep hands clear of the barring tool.

Recording and Interpreting Data

Record every cylinder's compression reading and leak-down percentage on the data sheet before drawing any conclusion. A single low compression reading with high leak-down and air escaping from the crankcase most likely indicates worn or damaged rings on that cylinder. Low compression with high leak-down and air escaping at the exhaust indicates a burned or bent exhaust valve. Low compression on two adjacent cylinders with leak-down air appearing as coolant bubbles is the classic head gasket pattern. Your written diagnostic summary must state: the data, the OEM limits, which cylinders are out of spec, the probable defect, and the recommended repair action.

Connection to Credentials

ASE T-series testing covers engine diagnosis, including interpretation of pressure test results and root-cause identification. OEM certifications from Cummins and Caterpillar expect technicians to follow factory-specified test procedures and compare results to factory limits. Practicing these tests on a real engine with real data sheets builds exactly the systematic documentation habit that both credential programs reward.

Key terms8

Compression testA procedure in which a calibrated gauge measures the peak pressure a cylinder builds during cranking, used to assess overall cylinder sealing.
Cylinder leak-down testA procedure in which regulated shop air is introduced into a cylinder at TDC and the percentage of air that escapes is measured to locate the source of a sealing defect.
Top Dead Center (TDC) compressionThe crankshaft position at which the piston is at the top of its stroke with both intake and exhaust valves fully closed, the correct position for leak-down testing.
Lockout/Tagout (LOTO)An OSHA-required energy-isolation procedure using physical locks and warning tags to prevent unexpected energization of equipment while technicians are working on it.
Leak-down percentageThe ratio of air pressure lost through cylinder leakage to the regulated supply pressure, expressed as a percentage; higher values indicate greater sealing loss.
OEM specificationThe minimum and maximum values published by the engine's original equipment manufacturer for a given test measurement; the authoritative pass/fail standard.
Ring blow-byCombustion or compressed air that passes the piston rings and enters the crankcase, indicated during leak-down testing by air escaping from the crankcase breather or oil fill.
Head gasket failureA breach in the sealing layer between the cylinder head and block, often indicated by low compression on adjacent cylinders and air bubbles in the coolant during leak-down testing.

Worked examples4

1. An 8-cylinder diesel engine is compression tested. Cylinders 1–7 read between 420 and 435 psi. Cylinder 8 reads 310 psi. The OEM minimum is 380 psi and the maximum allowable spread between the highest and lowest cylinder is 15%. Does cylinder 8 pass or fail, and by which criterion?
  1. Step 1 — Check against the OEM absolute minimum: 310 psi < 380 psi minimum. Cylinder 8 fails the absolute minimum test.
  2. Step 2 — Check the spread criterion: highest reading = 435 psi. 15% of 435 = 0.15 × 435 = 65.25 psi. Allowable low limit by spread = 435 − 65.25 = 369.75 psi. Cylinder 8 reads 310 psi, which is below 369.75 psi. Cylinder 8 also fails the spread criterion.
  3. Step 3 — Conclusion: Cylinder 8 fails both criteria and requires further diagnosis with a leak-down test.
Answer: Cylinder 8 fails both the absolute minimum (310 psi vs. 380 psi required) and the maximum spread criterion (reads 125 psi below the highest cylinder; allowable spread is only 65 psi).
2. During a leak-down test, the tester is supplied with air regulated to 100 psi. The downstream gauge, reading pressure in the cylinder side, stabilizes at 72 psi. What is the leak-down percentage?
  1. Step 1 — Calculate pressure lost: 100 psi − 72 psi = 28 psi lost.
  2. Step 2 — Calculate leak-down percentage: (28 ÷ 100) × 100 = 28%.
  3. Step 3 — Interpret: 28% leakage is above the commonly cited concern threshold of roughly 20% and would be flagged as a significant defect requiring root-cause identification.
Answer: Leak-down percentage = 28%. This is a significant leak requiring further diagnosis to locate the escape path.
3. A technician performs leak-down tests on a 6-cylinder diesel. Cylinders 1–5 show 8–12% leak-down. Cylinders 3 and 4 both show 34% leak-down, and air bubbles are observed in the coolant overflow tank when those cylinders are pressurized. What is the most probable diagnosis?
  1. Step 1 — Note the pattern: two adjacent cylinders (3 and 4) have very high leak-down while all others are near normal. An isolated ring or valve problem typically affects one cylinder.
  2. Step 2 — Identify the air escape path: air appears as bubbles in the coolant overflow tank, meaning pressurized air is entering the cooling system.
  3. Step 3 — Combine findings: adjacent cylinders + air entering coolant = breach in the sealing surface between cylinders 3 and 4 and the cooling jacket.
  4. Step 4 — Conclusion: the most probable cause is head gasket failure between cylinders 3 and 4, or a crack in the cylinder head or block in that region.
Answer: Most probable diagnosis is head gasket failure (or a crack) between cylinders 3 and 4, allowing compressed air to enter the coolant passages.
4. A compression test is performed with the engine cranked for 5 strokes per cylinder. The technician records cylinder 2 at 390 psi on the data sheet. The instructor's verified reading for that cylinder is 405 psi. Is this within the ±5 psi accuracy requirement for the lab pass criterion?
  1. Step 1 — Calculate the difference: 405 − 390 = 15 psi.
  2. Step 2 — Compare to the allowed tolerance: the lab requires readings within ±5 psi of the instructor's verified value. 15 psi > 5 psi.
  3. Step 3 — Conclusion: the reading does not meet the accuracy requirement for that cylinder and should be flagged for retest or correction.
Answer: No. The 15 psi difference exceeds the ±5 psi lab tolerance. The technician should retest cylinder 2 and check for proper adapter seal and consistent cranking strokes.

The math in this lesson2

Learners compute leak-down percentage from regulated and cylinder pressure and compare cylinders.
7.RP.3N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Pre-test safety briefing and LOTO demonstration; students install compression-test adapter, disable injectors per OEM procedure, crank engine, and record peak compression for all cylinders on a data sheet. · Cylinder leak-down test: students pressurize each cylinder at TDC compression and measure leak-down percentage; using a stethoscope and air-escape location (intake, exhaust, crankcase, coolant), they identify probable defect (rings, valves, head gasket). · Students complete a written diagnostic summary for the engine tested, listing cylinder-by-cylinder data, comparison to OEM limits, root-cause conclusion, and recommended repair action.
Materials: Diesel engine (mounted, running-capable), compression test adapter set (diesel-specific), cylinder leak-down tester, injector disable switch or OEM procedure, stethoscope, LOTO locks and tags, OEM compression specification, data recording forms, safety glasses, nitrile gloves
Check: Instructor reviews data sheet and written summary; all compression readings recorded within ±5 psi of instructor's verified readings; defect correctly located and root cause correctly stated; pass = data accuracy 90% and correct diagnosis.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Engine Diagnosis and Repair
1. Before installing a compression test adapter in a diesel injector port, the FIRST action a technician must take is:
Engine Diagnosis and Repair
2. During a cylinder leak-down test, air is heard escaping from the exhaust tailpipe on cylinder 5. The MOST probable cause is:
Engine Diagnosis and Repair
3. A technician discovers that cylinders 6 and 7 on a diesel engine each have 38% leak-down, while all other cylinders are below 12%. Air bubbles are visible in the coolant recovery bottle when those cylinders are pressurized. Which repair is MOST likely needed?
Engine Diagnosis and Repair
4. When performing a compression test on a diesel engine, the engine should be cranked with the throttle fully open (where applicable) and the fuel injection DISABLED primarily to:
Engine Diagnosis and Repair
5. An engine's OEM compression specification is 400 psi minimum, with no more than 15% spread between the highest and lowest cylinder. The highest reading is 440 psi. What is the lowest reading that would still PASS the spread criterion?
Engine Diagnosis and Repair
6. For a leak-down test to be valid, the piston must be positioned at TDC of the COMPRESSION stroke. How does a technician confirm this rather than TDC of the exhaust stroke?
Engine Diagnosis and Repair
7. A diesel compression test shows all cylinders reading uniformly low — approximately 20% below the OEM minimum — with no single outlier cylinder. The MOST likely cause is:
Safety and Environmental Practices
8. Under OSHA's energy control (Lockout/Tagout) requirements, after a compression test is complete and the technician is reinstalling injectors, the starter circuit must be:
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