F31 335d: 240 PSI, then zero compression. Any ideas anyone? Thank you

This F31 335d starts quickly, runs smoothly for 30 to 60 seconds, then shakes at idle, becomes smoother around 1,200 to 1,500 rpm, and does not change when injector 1 is unplugged. The owner asks, “Any ideas anyone? Thank you”; my answer is to settle the contradictory compression results first: repeat a controlled test on all six cylinders, confirm any near-zero result, and use leak-down at true compression TDC before naming a failed part.

Why 240 PSI and zero PSI cannot both guide the repair

The earlier cylinder 1 reading was about 240 psi. A later cold test showed cylinder 1 at 0 psi and cylinder 2 at about 250 psi. Those numbers do not establish a trend because the test conditions were not shown to be equivalent. Temperature, battery speed, gauge and adapter sealing, disabled fuel delivery, the number of compression strokes, and even the point at which the reading is recorded can move the result.

A genuine fall from 240 psi to zero would demand attention, but zero is also the reading most likely to expose a setup problem. A leaking adapter, a valve held open, a gauge fault, or testing the wrong cylinder can all send the diagnosis in the wrong direction. I would not buy an injector, remove the cylinder head, or call the engine damaged from that comparison alone.

BMW published a specific N57T compression procedure in a service bulletin. It calls for testing with the engine at operating temperature and using the same number of crankshaft rotations for every cylinder. In that bulletin's procedure, compression should be greater than 16 bar, with no more than 2 bar difference between cylinders. Those figures are useful as a procedural reference, but they are not automatically the VIN-specific specification for every N57D30B. The repair information and test plan for the exact car remain the authority.

The practical goal is repeatability. If cylinder 1 remains drastically lower while the other five readings group together under the same conditions, the low reading becomes evidence rather than a surprise number.

Repeat the compression test before taking anything apart

I would record one complete six-cylinder test, not another isolated comparison between cylinders 1 and 2. Use the correct diesel compression adapter, verify its seal, keep the battery supported, and follow the applicable BMW instructions for preventing fuel injection during cranking.

The test sheet should record:

1. Engine temperature at the start.
2. Battery voltage or evidence that cranking speed stayed stable.
3. The same crank rotations or compression events per cylinder.
4. Each cylinder's first meaningful rise and final reading.
5. Adapter, gauge, and any retest performed after reseating the tool.

If cylinder 1 shows zero again, swap or independently verify the measuring equipment before opening the engine. If the equipment allows it, confirm that the gauge responds on another cylinder, then reinstall it on cylinder 1. A second near-zero result under controlled conditions is the point where leak-down earns its place.

Cold numbers can still reveal a large outlier, but the owner should not compare them directly with warm numbers as though the conditions were identical. The BMW bulletin and the Engine Australia compression bulletin both reinforce why procedure and comparison matter.

Use leak-down to locate the lost seal

Compression testing says a cylinder cannot build pressure. Leak-down testing helps identify where that pressure goes. Bring cylinder 1 to top dead center on its compression stroke, make sure both valves should be closed, secure the crankshaft as required, and introduce regulated air with suitable equipment. Diesel compression makes correct tooling and safe restraint important.

Listen and observe instead of jumping to a part:

• Air at the intake points toward an intake-side sealing path.
• Air at the exhaust points toward an exhaust-side sealing path.
• Air heard through the crankcase points toward piston, ring, or bore leakage.
• Coolant disturbance can indicate a path into the cooling system.
• Little or no leakage after a near-zero compression result sends attention back to the compression setup, valve actuation during cranking, and cylinder identification.

The ALLDATA leak-down reference explains the basic method and the need to place the piston correctly. The result should narrow the next inspection. It should not be stretched into certainty beyond what was heard, measured, and reproduced.

Treat injector 1 as evidence, not a verdict

No change when injector 1 is unplugged means cylinder 1 appeared to contribute little or nothing during that check. It does not prove injector 1 itself has failed. A cylinder with poor compression also will not respond normally to injected fuel, and an electrical, connector, harness, control, or cylinder-identification problem can imitate a bad injector.

I would first confirm that “injector 1” in the test and the physical connector both refer to the same cylinder. Then inspect the connector and harness, retain the fault context attached to 2A6C00 and 2A6A00, and follow the BMW test plans that apply to the exact DDE and VIN. I would compare smooth-running or cylinder-correction data only as supporting evidence, not as a substitute for the compression and leak-down results.

Injector swapping or replacement adds risk and can muddy the evidence if identity, configuration, cleanliness, and sealing work are not controlled. The order matters: prove the mechanical result, verify the circuit and identity, then decide whether an injector-specific test is justified.

Keep intake and EGR checks running in parallel

The timing of the symptom matters. A quick start and 30 to 60 seconds of smooth running do not clear cylinder 1, but they also leave room for an air-path or EGR change as the engine settles into idle control. The improvement at 1,200 to 1,500 rpm can fit several causes because airflow, EGR command, fueling, and engine speed all change together.

I would review commanded and measured air-path data, inspect accessible intake and EGR hardware for sticking or restriction, and note exactly when the shake begins. I would not use a forum pattern to declare an EGR valve, intake fault, or injector failed. The BMW N57TU training document is useful for understanding the engine's systems, while owner discussions remain leads rather than specifications.

The shortest defensible path is: standardize all six compression readings, reproduce cylinder 1 if it is low, leak-test it at compression TDC, verify injector 1 identity and circuit, then compare the result with intake and EGR behavior. If cylinder 1 truly reads near zero twice, I would stop prolonged running until the leak path is understood. If compression and leak-down are sound, the diagnosis can move toward injector control and the air path without an unsupported internal-engine conclusion.