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TRN-AUTO19-07 · Lesson 7 · 20 clock hours · lab

Misfire Diagnosis: Cylinder Contribution, Compression, and Injector Testing

Objective. Students will diagnose an assigned misfire complaint by interpreting misfire monitor data, performing a cylinder contribution test via scan tool, conducting a compression and relative compression test, and testing injector balance on a gasoline engine, then identifying the specific cylinder and root cause.

⚠ 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.

Misfire Diagnosis: Why Systematic Testing Matters

A misfire is an event in which a cylinder fails to produce its expected power contribution. Left unaddressed, misfires damage catalytic converters, waste fuel, and can cause drivability complaints that escalate into expensive repairs. Modern OBD-II systems count individual misfire events and store them by cylinder, giving you a road map before you ever open the hood. Your job is to follow that road map in order: retrieve data, rank suspects, confirm mechanically, then pinpoint the root cause.

Safety First

Before any under-hood work, don safety glasses and nitrile gloves — engine oil, fuel spray, and battery acid are all present in a running or recently operated engine. When performing a compression test with the engine cranking, keep hands and tools clear of the accessory belt and cooling fan. Disable the ignition system in the approved manner before removing spark plugs to prevent accidental starts. When working near the fuel system or injectors, confirm there is no ignition source nearby and handle fuel-soaked components over an approved container. Follow your shop's lockout/tagout procedure any time you are working inside an engine compartment with the vehicle secured on a lift.

Step 1 — Scan Tool Misfire Data Review

Mode 6 and enhanced data. OBD-II Mode 6 stores manufacturer-defined test results including misfire counts per cylinder. Many scan tools also offer enhanced or proprietary data that displays misfire events as a live bar graph or count per 200-revolution and 1,000-revolution windows. Retrieve both current and pending codes and then navigate to the misfire data. Record the count for every cylinder on your worksheet.

Freeze-frame. When a misfire trips a pending or confirmed code, the PCM captures a freeze-frame snapshot of key parameters at that moment: engine load, coolant temperature, RPM, and fuel trim. A freeze-frame showing high long-term fuel trim on one bank combined with a misfire on a cylinder of that bank points toward a lean condition — weak injector or air leak — rather than an ignition fault.

Ranking suspects. Sort your cylinder misfire counts from highest to lowest. The cylinder with the highest count is your primary suspect. Note whether the misfires cluster on one bank (fuel or air management issue) or appear on a single isolated cylinder (spark, compression, or injector problem specific to that bore).

Step 2 — Cylinder Contribution and Relative Compression Test

Cylinder kill / contribution test. Using the scan tool's bi-directional controls or a shorting method specified in service information, disable one cylinder at a time and observe the drop in RPM or torque. A healthy cylinder produces a noticeable, consistent RPM drop when killed. A cylinder already contributing little or nothing shows almost no drop — confirming it as a weak contributor.

Relative compression with an inductive amp clamp. Clamp an inductive amp clamp around the battery negative or positive cable and crank the engine with ignition disabled. Each cylinder's compression stroke places a load on the starter motor, causing a current rise. Record the peak current for each cylinder. Equal peaks indicate even compression. A cylinder with a lower peak current draws less from the starter, indicating lower compression. This test is non-invasive and extremely fast.

Dry compression test. Remove the spark plug from the suspect cylinder (and ideally from all cylinders for comparison). Disable fuel and ignition per service information. Thread in the compression tester and crank the engine for four to five compression strokes. Record the reading. Compare to the manufacturer's specification and to adjacent cylinders. A reading more than 10–15% below the highest cylinder, or below the minimum specification, is significant.

Wet compression test. If dry compression is low, add approximately one tablespoon (roughly 15 mL) of clean engine oil through the spark plug hole, wait 30 seconds, and repeat the test. If compression rises significantly (typically 20 psi or more), the rings are worn and allowing blow-by. If compression does not rise, suspect a burned valve or head gasket leak. Record both the dry and wet values.

Step 3 — Injector Balance and Circuit Test

Injector resistance check. With the key off and the injector electrical connector unplugged, use a DVOM set to ohms and measure resistance across the injector terminals. Compare the reading to the specification in the service information. Most port fuel injectors are in the range of 11–17 ohms for high-impedance designs, though values vary by manufacturer and design type. A reading that is open, shorted, or significantly out of range confirms an electrical fault in the injector itself.

Noid light / pulse test. Plug a noid light matched to the connector style into the injector harness connector and crank or run the engine. A steady flash confirms the PCM is commanding the injector. No flash points to an open power feed, a failed injector driver circuit, or a missing crankshaft position signal. Compare the suspect cylinder's pattern to a known-good cylinder.

DVOM voltage drop. With the connector plugged back in and the engine running, a DVOM can measure voltage drop across the injector power feed. Excessive voltage drop indicates high resistance in the supply circuit — corroded connector, damaged wiring, or a faulty fuel injector relay.

Writing the repair recommendation. Your recommendation must cite specific measured values. Example: "Cylinder 3 misfire count: 847 events. Dry compression 95 psi (spec 175 psi minimum). Wet compression 96 psi — no rise. Injector resistance 14.2 ohms (within spec). Root cause: burned exhaust valve. Recommend cylinder head inspection and valve replacement." Specific numbers are what separate a professional repair order from a guess.

Putting It All Together

Misfire diagnosis follows a logical funnel: data narrows the field, contribution testing confirms which cylinder is weak, compression testing separates mechanical from ignition or fuel causes, and injector testing isolates the fuel delivery circuit. Never replace parts before completing the sequence — a fouled plug may be a symptom of low compression, not the cause.

Key terms10

Misfire monitorAn OBD-II system that counts combustion failures per cylinder over defined crankshaft revolution windows and stores the counts as diagnostic data accessible through Mode 6 or enhanced scan tool data.
Mode 6An OBD-II communication mode that provides access to on-board diagnostic test results, including cylinder-specific misfire event counts, which the technician can read with a compatible scan tool.
Freeze-frameA snapshot of key engine operating parameters captured and stored by the PCM at the moment a diagnostic trouble code is set, used to reconstruct the conditions under which a fault occurred.
Cylinder contribution testA test in which each cylinder is disabled in sequence while the scan tool monitors RPM or torque output, revealing which cylinder produces little or no power contribution when killed.
Relative compression testA non-invasive test using an inductive amp clamp on the starter circuit during cranking; differences in peak current draw per cylinder indicate differences in compression pressure.
Wet compression testA follow-up compression test performed after adding a small amount of engine oil to a cylinder with low dry compression; a significant pressure increase indicates worn piston rings, while no increase points to a valve or head gasket fault.
Noid lightA small test lamp designed to plug into a fuel injector connector in place of the injector; it flashes when the PCM sends an injector pulse, confirming the injector driver circuit is functioning.
High-impedance injectorA fuel injector design with relatively high electrical resistance (commonly in the range of 11–17 ohms) that limits current without requiring an external resistor; the most common type on modern gasoline port and direct injection engines.
Fuel trimA PCM correction factor, expressed as a percentage, that adds or subtracts fuel delivery to maintain the target air-fuel ratio; high positive long-term fuel trim on a bank suggests a lean condition such as a weak injector or vacuum leak.
Blow-byCombustion gas that escapes past worn or damaged piston rings into the crankcase, resulting in low compression that improves when engine oil is added during a wet compression test.

Worked examples4

1. A technician records dry compression values for a four-cylinder engine. Cylinders 1, 2, and 4 read 175 psi each. Cylinder 3 reads 140 psi. The manufacturer specifies a minimum of 150 psi and a maximum variation of 15% between cylinders. Is cylinder 3 within specification?
  1. Step 1 — Check against the minimum spec: 140 psi is below the stated minimum of 150 psi. This alone is a failure.
  2. Step 2 — Calculate the allowable variation: 15% of the highest reading (175 psi) = 175 × 0.15 = 26.25 psi.
  3. Step 3 — Determine the lowest acceptable value based on variation: 175 − 26.25 = 148.75 psi. Cylinder 3 at 140 psi is also below this threshold.
  4. Step 4 — Conclusion: Cylinder 3 fails both the minimum pressure test and the maximum variation test. A wet compression test is the next indicated step.
Answer: Cylinder 3 is out of specification on both criteria: it is below the 150 psi minimum and more than 15% below the highest cylinder reading. A wet compression test should be performed next to distinguish ring wear from a valve or head gasket fault.
2. After the dry compression test in the example above, the technician adds oil to cylinder 3 and retests. The wet reading is 142 psi — only 2 psi higher. What does this indicate and what is the likely root cause?
  1. Step 1 — Calculate the pressure rise: 142 − 140 = 2 psi increase.
  2. Step 2 — Interpret the result: A rise of only 2 psi is not significant. When rings are the cause of low compression, oil seals the ring gap and typically produces a rise of 20 psi or more.
  3. Step 3 — Apply the diagnostic rule: If compression does not rise significantly with oil added, the sealing problem is not at the rings. The likely locations are a burned or bent valve, a leaking head gasket, or a cracked cylinder head.
  4. Step 4 — Recommended next step: Perform a cylinder leakdown test to confirm the leak path and determine whether air escapes at the exhaust pipe (exhaust valve), intake (intake valve), coolant reservoir (head gasket), or adjacent cylinder (head gasket between cylinders).
Answer: A 2 psi rise is not significant and rules out worn rings as the primary cause. The low compression in cylinder 3 most likely results from a burned or damaged valve, or a head gasket failure. A leakdown test is the next diagnostic step.
3. A technician measures injector resistance on a suspect cylinder and reads 4.2 ohms. The service information specifies 11–17 ohms for this high-impedance injector. A known-good cylinder injector reads 14.6 ohms. Is the suspect injector within specification, and what fault does this suggest?
  1. Step 1 — Compare the reading to the specification range: 4.2 ohms is well below the specified minimum of 11 ohms.
  2. Step 2 — Compare to the known-good reading: 14.6 ohms is within the 11–17 ohm range and is normal. The suspect injector at 4.2 ohms is 10.4 ohms lower.
  3. Step 3 — Interpret the low resistance reading: Resistance below specification on an injector indicates an internal short — windings inside the injector coil are shorted together, reducing total resistance.
  4. Step 4 — State the conclusion: A shorted injector may cause the PCM's injector driver circuit to see excessive current, potentially damaging the driver or causing the PCM to shut off that injector to protect the circuit — resulting in a misfire.
Answer: At 4.2 ohms, the suspect injector is far below the 11–17 ohm specification and is internally shorted. Replacement of the injector is indicated. The technician should also inspect the injector driver circuit and PCM driver for heat or current damage before installing the new injector.
4. During a relative compression test, the technician records starter current peaks for each cylinder of a four-cylinder engine: Cylinder 1 = 210 A, Cylinder 2 = 208 A, Cylinder 3 = 175 A, Cylinder 4 = 211 A. Calculate the percentage difference between cylinder 3 and the average of the other three cylinders.
  1. Step 1 — Calculate the average of the three good cylinders: (210 + 208 + 211) ÷ 3 = 629 ÷ 3 = 209.67 A.
  2. Step 2 — Find the difference between cylinder 3 and that average: 209.67 − 175 = 34.67 A.
  3. Step 3 — Calculate the percentage difference: (34.67 ÷ 209.67) × 100 = 16.54%, approximately 16.5%.
  4. Step 4 — Interpret the result: A drop of roughly 16.5% in starter current demand indicates that cylinder 3 is placing significantly less load on the starter during its compression stroke, pointing to meaningfully lower compression in that cylinder compared to the others.
Answer: Cylinder 3 draws approximately 16.5% less starter current than the average of the other three cylinders, which is a significant difference indicating low compression. This cylinder should be followed up with a dry compression test and, if low, a wet compression test.

The math in this lesson2

Learners compute each cylinder's compression as a percent of the highest and relative starter current peaks.
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Brush up in the math halls at newSchoolWorks.com:
In class: Scan tool misfire data review: student retrieves misfire counts by cylinder from Mode 6 or enhanced data, records freeze-frame at the misfire event, and uses the data to rank suspect cylinders before touching the engine. · Cylinder contribution and compression test lab: student performs a cylinder kill or relative compression test using scan tool and an inductive amp clamp on the starter circuit, then follows up with a wet and dry compression test on the suspect cylinder, recording all values on a structured worksheet. · Injector balance and circuit test: using a noid light or injector pulse tester and a DVOM, student checks injector resistance and pulse on the suspect cylinder and one known-good cylinder, compares results to spec, and writes a repair recommendation citing specific measured values.
Materials: Shop vehicles with seeded misfire faults (rotated: fouled plug, weak injector, low compression), scan tool with Mode 6 and graphing, inductive amp clamp, DSO, compression tester, noid light set, injector resistance specs from service information, DVOM, structured misfire lab worksheet, safety glasses, nitrile gloves
Check: Student submits completed worksheet with misfire counts, compression values (dry and wet where applicable), injector resistance readings, and a written repair recommendation with supporting data. Instructor confirms the identified cylinder and root cause match the seeded fault. Pass mark: correct cylinder and cause identification plus 75% of recorded values accurate.

Practice check8

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

Engine Repair (A1) / Diagnostics
1. A technician retrieves OBD-II Mode 6 data and finds that cylinder 4 has 912 misfire events recorded over the last drive cycle while all other cylinders show fewer than 20. Which is the best immediate next diagnostic step?
Engine Repair (A1)
2. During a wet compression test on cylinder 2, the pressure rises from 110 psi dry to 155 psi wet. What does this result most likely indicate?
Engine Repair (A1)
3. What is the purpose of adding oil to the cylinder before performing a wet compression test?
Engine Repair (A1) / Diagnostics
4. A technician uses an inductive amp clamp on the battery cable during cranking with ignition disabled. Cylinders 1, 2, and 4 show current peaks of approximately 215 A; cylinder 3 shows a peak of 182 A. What is the most accurate interpretation of this data?
Fuel and Induction (A8) / Engine Performance
5. A noid light plugged into the cylinder 3 injector connector does not flash during cranking, while noid lights on cylinders 1, 2, and 4 flash normally. Which component or circuit should the technician investigate first?
Fuel and Induction (A8) / Engine Performance
6. When checking injector resistance with a DVOM, a technician measures 3.1 ohms on the suspect injector. Service information specifies 11–17 ohms. What fault does this most likely indicate?
Engine Performance / Fuel and Induction (A8)
7. Freeze-frame data captured with a cylinder 5 misfire code shows long-term fuel trim (LTFT) at +22% on bank 2. Cylinder 5 is on bank 2. What does this fuel trim value most likely suggest about the root cause?
Safety / Shop Practices
8. Which personal protective equipment (PPE) is required as a minimum before performing a compression test on a warm engine?
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