Fuel, ignition, emissions and driveability review

Marty opened the driveability session by putting a scan tool screenshot on the lounge television: short-term fuel trim +3 percent, long-term +18 percent on bank 1, short-term +2 and long-term +4 on bank 2, at idle; at 2,500 rpm steady, bank 1 long-term fell to +7. "What is it?" Kiran answered before Theo had finished reading: a lean condition on one bank that gets better with airflow, so unmetered air on bank 1, probably an intake or exhaust manifold leak upstream of the sensor. It was, almost exactly, Gord's pickup from the AUTO-230 capstone. "Good," Marty said. "Now tell me three things that would look the same and how you'd rule each one out." That is a driveability question.

Engine Performance, A8, has 50 scored questions on general diagnosis, ignition systems, fuel, air induction and exhaust systems, emission control systems, and computerized engine controls. Its stems are often long, with scan data or a waveform, so pacing matters. The advanced L1 test goes further into the same material for experienced technicians.

The review follows AUTO-230 (Fuel, Ignition and Emission Systems) and AUTO-330 (Engine Performance and Diagnostics).

Combustion and air-fuel ratio

Gasoline burns completely at a stoichiometric air-fuel ratio of about 14.7:1 by mass, which is lambda 1.0. Richer than that, lambda below 1, raises carbon monoxide and hydrocarbons; leaner, lambda above 1, raises oxygen and, with high combustion temperatures, oxides of nitrogen. The three-way catalyst works best in a narrow window around lambda 1, so the engine control module constantly corrects the mixture using oxygen or air-fuel ratio sensors. Exam questions use gas readings: high hydrocarbons with normal carbon monoxide suggest a misfire; high carbon monoxide suggests a rich mixture; high oxygen suggests a lean mixture or an exhaust leak.

Fuel trims

Fuel trims are the module's corrections. Short-term trim reacts quickly to the oxygen sensor; long-term trim learns the steady correction. Positive trims mean the module is adding fuel because it sees lean; negative trims mean it is removing fuel because it sees rich. Total trim, short-term plus long-term, beyond about plus or minus 10 percent deserves attention, and beyond about 20 to 25 percent usually sets a code. The pattern of trims tells you where to look: lean at idle that improves with airflow suggests a vacuum leak, because a fixed leak is a smaller share of a larger airflow; lean everywhere that gets worse under load suggests low fuel delivery; one bank lean points to that bank, such as a leak or a sensor on that side; both banks lean together point to something shared, like the fuel supply or the mass airflow sensor.

# Fuel trim totals and what the pattern suggests
cases = {"idle": ((3, 18), (2, 4)), "2,500 rpm steady": ((1, 7), (1, 3))}
for cond, (b1, b2) in cases.items():
    t1, t2 = sum(b1), sum(b2)
    print(f"{cond:<18} bank 1 total {t1:+3d}%   bank 2 total {t2:+3d}%")
idle1, cruise1 = sum(cases["idle"][0]), sum(cases["2,500 rpm steady"][0])
if idle1 > 10 and cruise1 < idle1 / 2:
    print("bank 1 lean at idle, better with airflow: suspect unmetered air on bank 1")
elif idle1 > 10:
    print("lean at all loads: suspect fuel delivery or airflow measurement")

idle               bank 1 total +21%   bank 2 total  +6%
2,500 rpm steady   bank 1 total  +8%   bank 2 total  +4%
bank 1 lean at idle, better with airflow: suspect unmetered air on bank 1

Sensors that set the mixture

The mass airflow sensor measures incoming air; a contaminated sensor reads low, so the engine runs lean and the module adds fuel, often with the trims climbing most at high airflow. Harold's over-oiled air filter coating his MAF in AUTO-230 was the example. A manifold absolute pressure sensor reads intake pressure; at idle on a healthy engine at sea level it reads low, about 9 inHg absolute, and with the key on and the engine off it reads barometric pressure, which at Ridgeline's altitude is noticeably below sea level. Upstream oxygen or air-fuel ratio sensors control fueling; the downstream sensor monitors the catalyst. A narrowband oxygen sensor switches between about 0.1 and 0.9 V; a wide-band sensor reports lambda or a current. Coolant and intake air temperature sensors, throttle position and the crank and cam sensors complete the picture.

Ignition and misfires

Coil-on-plug ignition dominates current vehicles. A misfire is detected by the crankshaft speed dropping on that cylinder's power stroke. Diagnosis isolates the cause by moving parts: swap the coil to another cylinder and see if the misfire follows; if it follows the coil, the coil is at fault; if it stays, test the plug, the injector, and compression. An ignition scope shows the firing line, spark line and coil oscillations; a spark line that is short and high suggests high resistance in the secondary, such as a wide plug gap or a cracked boot; a long, low spark line suggests a fouled plug or low compression. Harold's coil with shorted turns in AUTO-330 showed as a weak spark and a short ringing pattern.

Spark plugs are inspected for gap, wear and deposits, and replaced at the maker's interval with the specified heat range and gap; a carbon-fouled plug points to a rich mixture or weak spark, an oil-fouled plug to oil entering the cylinder, and a blistered white insulator to overheating.

Emission controls and OBD

Exhaust gas recirculation lowers combustion temperature and oxides of nitrogen; a stuck-open valve causes rough idle and stalling, a stuck-closed one causes detonation and high oxides of nitrogen. The evaporative system stores fuel vapor in a charcoal canister and purges it into the engine; a stuck-open purge valve causes rich trims at idle and hard starting after refueling; leak tests find holes down to 0.020 in. Catalysts are monitored by comparing the upstream and downstream oxygen sensors: a good catalyst stores oxygen and smooths the downstream signal. Diesel and some gasoline vehicles add particulate filters and, on diesels, urea injection.

OBD-II monitors test each system and report readiness. A trouble code tells you which test failed, not which part is bad. Freeze frame records the conditions when the code set. Mode 06 shows the test results and limits, which can reveal a marginal component before a code sets, as on courier van 1's cylinder 5 in AUTO-330. Codes follow the SAE J2012 structure: a P for powertrain, a 0 for generic or 1 for maker-specific, then the system and the specific fault.

In the United States, the Clean Air Act prohibits removing or disabling emission controls and selling defeat devices, and the EPA enforces it. Many areas, including Colorado's Front Range, require periodic emissions inspection, which for most newer vehicles means an OBD check of codes and readiness monitors. Removing or disabling emission equipment is never legitimate repair.

Driveability strategy

AUTO-330 lesson 1 set out the strategy the exam rewards: verify the complaint and the conditions, check bulletins and history, scan all modules, analyze data and freeze frame, form a theory, test the theory with the right tool, repair, and verify the repair under the conditions that caused the fault. No-code complaints, like the hesitation in AUTO-330's capstone, are found by recording data during the fault and looking for what changes first.

More practice in this area

A8 and the engine block of the Red Seal exam both like questions about fuel delivery: a fuel pressure that is correct at idle but falls under load points to a weak pump, a restricted filter or a voltage drop to the pump; pressure that bleeds down quickly after shutdown points to a leaking injector, a check valve or a regulator, and causes hard starting after a hot soak. Direct injection adds a high-pressure pump driven by the camshaft, with its own pressure sensor and its own codes, and its rail pressure runs many times higher than port injection.

Kiran's driveability errors were almost all reasoning errors: she knew the facts but chose an unusual cause. Her habit for the last weeks was to ask, before choosing, what a working technician would see most often. The exam is written for the common case.

Questions on ignition also test timing on older systems and the crankshaft and camshaft sensors on current ones. A crank sensor fault that cuts the engine out when hot and lets it restart after cooling is a classic pattern, and a scope captures the signal dropping out at the moment it happens, as on Gord's hot stall in AUTO-330. A cam sensor fault usually lets the engine run but sets a correlation or circuit code and may lengthen cranking while the module works out the engine's position.

On the emissions side, catalyst questions ask what destroys a catalyst: misfires that send unburned fuel into it and melt the substrate, oil or coolant contamination, and rich running. Replacing a failed catalyst without fixing the cause sends the new one the same way. The exam's answer for a repeat catalyst failure is always to find the misfire, the oil consumption or the rich condition first.

Exhaust gas recirculation questions often hinge on when the symptom appears. EGR flow is not wanted at idle, so a valve stuck open shows as a rough idle or stalling that disappears when the valve is blocked off for a test; a valve stuck closed or passages blocked with carbon show as detonation under load and a flow code, with no idle symptom at all.