310S engine, fuel and emission systems practice questions.

Five free practice questions on engine, fuel and emission systems: block B of the Red Seal 310S Automotive Service Technician exam, about 22 of its 125 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Critical · Diagnoses gasoline electronic ignition systems

   A coil-on-plug engine sets P0303 (cylinder 3 misfire). The technician swaps the cylinder 3 coil with the cylinder 1 coil, clears the code and drives the vehicle. The code returns as P0301. What should be done?

   1. Replace injector 3
   2. Repair the cylinder 1 wiring, since the fault is now there
   3. Replace the coil moved from cylinder 3
   4. Replace the spark plug in cylinder 3
   Show the answer

   C. Correct. This is a swap test: the misfire followed the coil to cylinder 1, which proves the coil (or its boot/spring) is the cause. Had the code stayed P0303, the coil would be cleared and the diagnosis would move to the plug, injector, wiring or compression of cylinder 3.

   Why not the others

   A. The injector did not move and cylinder 3 is now fine. The fault followed the swapped component.

   B. The wiring for cylinder 1 did not change; only the coil did. Blaming the harness ignores what the swap test just demonstrated.

   D. The plug stayed in cylinder 3 and the misfire left cylinder 3. Whatever is failing moved with the coil, not the plug.

2. Critical · Diagnoses engine assembly

   A customer reports a knock on cold start that fades as the engine warms up. With a stethoscope the technician finds a hollow, rattling knock loudest at the side of the block that quiets after a few minutes of running. What is the most likely cause?

   1. Piston slap (excess skirt clearance)
   2. Worn connecting rod bearings
   3. Worn main bearings
   4. Excessive valve lash
   Show the answer

   A. Correct. A piston with too much skirt clearance rocks in the bore when cold. Aluminum pistons expand more than the iron or aluminum-lined block as the engine warms, taking up the clearance, so the noise fades. It is loudest at light load and confirmed by measuring piston and bore diameters.

   Why not the others

   B. Rod knock is a deeper, sharper knock that gets worse as the oil thins with heat and under load, and it changes or disappears when that cylinder is disabled. It does not fade as the engine warms.

   C. Main bearing noise is a deep, dull rumble from low in the block, worst under load and at higher speed, usually with low oil pressure. It does not go away with warm-up.

   D. Loose valve train produces a light, rapid tapping from the top of the engine at half crankshaft speed. It does not sound hollow from the block and typically persists when warm.

3. Procedural · Repairs gasoline intake and exhaust systems

   A turbocharger on a gasoline engine has failed from oil starvation. When installing the replacement, which step is essential?

   1. Coat the compressor wheel with assembly grease to reduce break-in friction
   2. Leave the air filter out for the first run so the new turbo breathes freely
   3. Run at high rpm immediately to seat the shaft seals
   4. Clean or replace the oil feed/drain lines, pre-lube, change oil and filter
   Show the answer

   D. Correct. Whatever starved the old turbo — a coked feed line, a kinked or sludged drain, contaminated oil — will kill the new one. Replace or clean the lines, prime the bearing housing with oil through the feed port, fit new oil and filter, inspect the intercooler and piping for oil and debris, then idle until pressure is established before loading the engine.

   Why not the others

   A. The compressor wheel touches nothing; grease on it is thrown into the intake. Lubrication is needed at the bearing housing, through the oil feed.

   B. A compressor wheel spinning at over 100 000 rpm is destroyed by the smallest ingested debris. Never run a turbo without its filter and sealed intake ducting.

   C. Loading the turbo before oil pressure reaches its bearings repeats the starvation that destroyed the last one. Turbo seals are not seated by revving.

4. Procedural · Repairs engine assembly

   A technician checks connecting rod bearing clearance with plastic precision clearance gauge (Plastigage). The crushed strip measures 0.09 mm (0.0035 in). The specification is 0.025–0.064 mm (0.001–0.0025 in). What is the correct next step?

   1. Mic the journal and choose undersize bearings or machining
   2. Rotate the crankshaft with the strip in place to average the reading
   3. Tighten the rod cap to a higher torque to close the clearance
   4. Accept it — the reading is within normal wear limits
   Show the answer

   A. Correct. Clearance is well beyond the limit and would give low oil pressure and rod knock. Measure the journal with a micrometer for wear, taper and out-of-round; if it is undersize or damaged, the crankshaft is reground and undersize bearings (e.g. 0.25 mm) fitted; if the journal is in spec, the bearings or rod bore are the problem. Then re-check with Plastigage.

   Why not the others

   B. The crankshaft must never be turned with Plastigage installed — the strip smears and the reading is destroyed. Cap on, torque, cap off, compare the width to the scale.

   C. Over-torquing distorts the rod bore and stretches the bolts; it does not shrink the bearing. Caps are always torqued to the specification when Plastigage is used, otherwise the reading is meaningless.

   D. 0.09 mm is about 40 % more than the maximum allowed. Excess clearance bleeds oil pressure at idle and lets the rod hammer the journal.

5. Critical · Diagnoses diesel intake and exhaust systems

   A turbo-diesel lacks power and produces black smoke under load. A manometer connected downstream of the air filter reads 8 kPa (32 in H2O) of restriction at full load. The manufacturer's maximum is 6.2 kPa (25 in H2O). What is the correct conclusion?

   1. The reading is normal for a turbocharged engine
   2. The intake is restricted — replace the filter and re-test
   3. The exhaust is restricted
   4. The turbocharger is over-boosting
   Show the answer

   B. Correct. Excess intake restriction starves the engine of air, so the mixture goes rich under load and produces black smoke and low power. Replace the filter, and inspect the ducting for a collapsed hose, debris, or snow and ice packed in the intake in winter, then re-measure to confirm the restriction is within limits.

   Why not the others

   A. The manufacturer's limit is the limit; 8 kPa is about 30 % over it. Restriction indicators exist precisely because this level starves the engine.

   C. Exhaust restriction is measured as back-pressure on the exhaust side. A manometer after the air filter measures how hard the engine is pulling air through the intake, and that is what is out of limits here.

   D. Over-boost gives too much air, not too little, and sets boost-related codes. Black smoke is the sign of too little air for the fuel.

02Answer key

03What block B covers

This block covers the engine and the systems that keep it running: cooling and lubrication, compression and leak-down testing, belts and accessory drives, gasoline fuel injection including GDI, ignition systems, intake and exhaust, EVAP and catalytic converters, and diesel fuel, turbocharger and emission systems such as glow plugs, DPF regeneration, DEF and SCR.

The Red Seal Occupational Standard names this block “Diagnoses and repairs engine and engine support systems”. It carries 17.6% of the exam, about 22 of the 125 questions, split across six tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Compression and leak-down clues

Two adjacent cylinders reading equally low on a compression test point to a head gasket breach between them. On a leak-down test, high leakage with air heard at the oil filler means the air is getting past the rings.

Fuel supply under load

Fuel pressure that is correct at idle but falls under load is a supply problem. On port injection, test pump volume and check the filter and strainer. On a common-rail diesel, check suction restriction and lift-pump delivery first.

Swap tests and scope patterns

If a misfire code follows a coil moved to another cylinder, the coil is the fault. On a secondary ignition pattern, a tall firing line with a short burn line points to extra resistance, such as a wide gap or a bad plug wire.

Diesel after-treatment

DEF is 32.5 % urea and freezes near -11 °C. A low refractometer reading means diluted fluid: drain, flush, refill and run the scan-tool reset. Once soot passes the automatic regeneration threshold, a scan-tool service regeneration is needed.

05The other blocks

The 310S exam has eight blocks. Each has its own page of free questions:

The 310S Automotive Service Technician practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.