E91 325i rough idle: diagnosing 2A98 after swapping cam sensors

An E91 325i that idles roughly, sometimes stalls when warm, and stores 2A98 after the intake and exhaust cam sensors have been swapped still needs a diagnosis. The unchanged fault does not clear every sensor or condemn the timing chain. It keeps the intake-side correlation problem in focus and makes the next tests more useful than another parts swap.

E91 325i rough idle after the cam-sensor swap

The owner report behind this test plan is specific. The fault is intermittent, the engine can stall when warm or after deceleration, restarting clears the bad running, and the only reported code is 2A98 - inlet camshaft reference. Swapping the inlet and exhaust cam sensors made no difference. The owner also saw VANOS and Valvetronic stop operating in live data when the fault occurred.

I would not treat those stopped values as proof that both systems failed. Once the DME rejects a timing reference and changes its control strategy, data seen after that moment may describe the fallback response rather than the first fault. The useful capture starts before the idle breaks down.

Before clearing anything, I save the exact BMW fault wording, status, frequency, mileage and freeze-frame values. I scan the other modules for voltage-related faults and record oil and coolant temperature. I also note whether the event follows a cold start, full warm-up, a long idle or closed-throttle deceleration. The reported restart reset makes a time-aligned capture especially important.

Pelican Parts lists 2A98 as crankshaft to intake-camshaft correlation outside the reference range. That wording describes a relationship, not a component. BMW training shows why several branches remain open: the DME uses the intake cam sensor to determine cam position relative to the crankshaft and as feedback for VANOS control. VANOS then depends on controlled engine-oil pressure to move the camshaft.

What the unchanged sensor swap proves

A controlled swap can be useful when both sensors are the correct interchangeable parts and the connectors are not damaged. If an intake-side fault becomes an exhaust-side fault under the same test condition, the moved sensor becomes a strong suspect. Here, 2A98 reportedly stayed put. That lowers confidence in a simple sensor-only explanation, but it proves less than it first appears.

The intake connector and its terminals did not move to the exhaust circuit. Neither did the intake harness, the DME input, the intake impulse wheel, the VANOS oil path or the mechanical relationship between the intake camshaft and crankshaft. A marginal sensor can also pass at one temperature and fail at another. I keep both sensors on the list until their signals and circuits have been checked.

With the ignition off and the engine cool, I remove each connector and inspect it under good light. I look for oil or moisture, corrosion, a broken lock, spread or backed-out terminals, pulled wires and rubbed insulation. I compare intake and exhaust connectors rather than judging one in isolation. Then I check harness routing around the front of the cylinder head and any area disturbed during the swap.

If a wiring diagram and suitable test equipment are available, I verify the circuit using the procedure for that exact DME and production date. I do not invent a pinout from a forum post or pierce sealed insulation. A static continuity check can miss a brief dropout, so an engineer should use terminal checks, loaded tests and a scope when the symptom is intermittent.

Test VANOS oil control before opening the engine

BMW's VANOS training says the N52 system uses engine-oil pressure, metered by two solenoid valves, to move the intake and exhaust adjusters. It also says oil condition, level and pressure affect operation. That makes the oil and control side a direct part of a 2A98 test, especially when the engine runs normally again after a restart.

I start with the oil level and service history. I inspect the oil-filter cap and filter installation because BMW's service information for related N52 VANOS faults warns that a missing filter-cap insert can affect oil supply and contaminate the solenoids. That bulletin addresses VANOS movement faults rather than 2A98, so I use it as an inspection lead, not as a diagnosis for this car.

Next I remove and inspect the VANOS solenoids on a cool engine using the correct repair procedure. Debris, damaged seals or an obviously contaminated valve deserves attention. Pelican Parts specifically recommends checking the VANOS solenoids when camshaft-position faults are present and includes 2A98 in its related list.

If the intake and exhaust solenoids are the same correct part, a marked swap is another controlled test. I clear the fault only after saving its details, then reproduce the same warm idle or deceleration event. An exhaust-side fault that appears after the swap supports the moved solenoid. If 2A98 remains on the intake side, I stay with intake oil delivery, wiring, feedback and mechanical timing. Cleaning or swapping a valve is not proof that it works correctly under hot oil pressure.

Capture the failure, then choose the branch

For the live-data run, I record engine speed, system voltage, intake and exhaust cam requested positions, intake and exhaust actual positions, and VANOS command values where the DME exposes them. I begin before start-up, let the engine reach the condition that usually causes trouble, and include the deceleration into idle. I keep all channels on the same timeline.

I am looking for sequence, not a universal angle limit. If intake actual position stops following its request before 2A98 sets while the exhaust side remains responsive, the intake VANOS branch moves up the list. If both reported cam values or several unrelated channels break at once with a voltage disturbance, power, ground and charging checks come first. If the rough idle begins before any visible cam disagreement, I preserve that clue and widen the engine-running diagnosis rather than forcing every symptom into 2A98.

When scan data cannot show which signal failed first, a simultaneous crankshaft and intake-camshaft oscilloscope capture is the next electrical test. The goal is to see whether either signal disappears, distorts or changes its relationship during the event. Mechanical correlation requires a known-good pattern and the correct BMW test conditions for that engine. I do not declare stretched timing from an unverified internet waveform.

Clean electrical signals with repeatable correlation loss justify mechanical inspection by an engineer. That can include cam timing, the impulse wheel, chain drive, VANOS adjuster and oil supply. There is also a separate BMW safety recall involving VANOS adjuster bolts on certain 2010 to 2013 vehicles with N51, N52-family and N55 engines. Applicability is VIN and market specific. I check the car's open campaigns through the official recall channel rather than assuming every E91 is covered or that the recall explains 2A98.

Prove the repair under the original condition

The repair is not proven by one smooth restart. I repeat the same warm idle, deceleration or driving condition that used to trigger the fault. I record the same channels, confirm that requested and actual cam behavior remains coherent, and rescan for pending and stored faults.

A connector repair needs a stable signal during a harness movement check. A solenoid or oil-service correction needs repeatable hot operation, not only a cold idle. Mechanical work needs a clean correlation test and a quiet, stable run through the original event. That matched before-and-after evidence is what turns an intermittent 2A98 from a parts guess into a finished diagnosis.