CMR B6.7 & L9 Mid-Range — practice test

High-yield CMR topics

The material that shows up year after year. If you're short on time, start here.

• Valve lash, both engines, cold, at the valve-set marks. B6.7 is 0.010 in intake / 0.020 in exhaust. L9 is 0.014 in / 0.027 in. Exhaust is always the looser of the two because the exhaust valve runs hotter and grows more. They will put both spec sets in the same answer list and see if you know which engine is in front of you.
• Firing order 1-5-3-6-2-4, cylinders numbered from the gear (front) end. This is not trivia - the cylinder-cutout test in INSITE only means something if you know which cylinder the ECM just shut off and what a dead hole should do to fueling and idle quality.
• Aftertreatment flow and how the ECM sees it: DOC, then DPF, then the DEF doser and decomposition tube, then SCR. The delta-P sensor bridges the DPF inlet and outlet so the ECM can infer soot load. Two NOx sensors bracket the SCR and the ECM divides inlet by outlet to compute conversion efficiency. DEF is 32.5 percent urea / 67.5 percent deionized water and it slushes at about 12 F (-11 C), which is why the tank and lines are heated.
• Lubrication numbers and Fault Code 415. Minimum oil pressure at hot low idle is roughly 10 psi, climbing to about 30-40 psi at rated. Code 415 (oil pressure below normal, most severe) triggers a protection derate. Oil is API CK-4 to Cummins CES 20086; coolant must meet the Cummins CES spec, fully formulated with SCA (test and adjust it) or the OAT extended-life.
• Holset VGT with an electric smart actuator - no vacuum, no wastegate. Closing the vanes raises backpressure for boost response, exhaust braking, and regen heat. Soot builds on the sliding nozzle and sticks it: vanes stuck open gives you low boost, low power, black smoke, and lazy regen temperatures, plus a commanded-versus-actual position mismatch on the scan tool.
• Cold-start and coolant-loss patterns. There are no glow plugs on these - it is an intake grid heater, and a grid heater circuit fault shows up as hard cold starts and white smoke. Coolant loss with no external leak, sweet white smoke, and bubbles in the surge tank points at a cracked EGR cooler or head gasket, not at 'it must be evaporating.'

Where techs lose points on CMR

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• Fuel system identity. Know that the B6.7 runs a Bosch-supplied high-pressure common rail and the L9 is spec'd with the Cummins XPI - and that XPI is the heavy-duty X12/X15 system, so 'XPI on the B6.7' is a distractor. What both share is the part they actually test: the accumulator holds pressure (up to roughly 1,800-2,200 bar, call it 26,000-32,000 psi) essentially independent of engine speed, and it does NOT collapse to zero between injection events. That last one is the throwaway wrong answer they hide in the EXCEPT questions.
• Low oil pressure at hot idle. The reflex answer is worn bearings or a tired pump, and sometimes it is - but read the stem. If a master mechanical gauge reads normal and the engine is quiet with no knock, the ECM is lying and it is the oil pressure sensor or its circuit. If the mechanical gauge agrees and pressure comes back up with rpm, then it is bearing clearance. Same complaint, two answers, decided entirely by whether they handed you a gauge reading.
• SCR conversion-efficiency fault (SPN 4364) with a full DEF tank. The obvious pick is 'add DEF' or 'replace the doser,' and both are wrong when the stem says dosing quantity and aftertreatment temps look normal. That combination means off-spec, diluted, or contaminated DEF, or a drifted outlet NOx sensor. Confirm the DEF with a refractometer at 32.5 percent before you touch a part.
• Regen burns soot, not ash. A DPF with high delta-P right after a successful regen is ash-loaded and needs to come off for cleaning or replacement - it does not need another regen or a new delta-P sensor. Same family of trap: pulling the thermostat is never the fix for an overheat, because coolant that moves too fast through the radiator sheds less heat and the engine never warms properly.

A study plan that works for CMR

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-3: Brute-force the numbers until they are automatic. Both lash specs and which engine owns which, firing order 1-5-3-6-2-4, thermostat cracking near 180 F (82 C) and full open near 200 F (93 C), 10 psi idle / 30-40 psi rated oil pressure, DEF at 32.5 percent freezing near 12 F, CK-4 / CES 20086 oil. Write them on a card. These are the questions you cannot afford to think about on test day.
2. Days 4-6: Fuel and air. Walk the low-pressure side (lift pump, Code 2265, filter/water separator, priming after a filter change) into the high-pressure side (pump, fuel metering actuator, rail pressure sensor, relief valve, injector return-flow test). Then do the same for the VGT and cooled EGR - commanded versus actual position, what stuck-open versus stuck-closed each look like.
3. Days 7-9: Aftertreatment end to end. Trace DOC, DPF, doser, SCR on a real truck or a diagram until you can name every sensor and what it feeds the ECM. Learn the inducement ladder: warning, then torque derate, then the roughly 5 mph speed limit for an empty tank, repeated bad DEF quality, or tampering. Know why a parked regen aborts.
4. Days 10-12: Practice questions only, and be strict with yourself on the Technician A/B and EXCEPT formats - those two formats are where good techs give away points. Every miss, write one sentence on why the wrong answer looked right. Re-drill your number card the morning of the test.

Sample CMR questions

Straight from the bank — answers highlighted, with the explanation underneath.