FAQ
TL;DR: Warm misfire on Mini 1.6 (120 HP) traced to low hot compression: "there is only 3 bars" after unplugging the phase adjuster motor. [Elektroda, mxas123, post #16856241]
Why it matters: This FAQ helps DIYers quickly isolate heat-related cylinder 3 misfires without shotgun parts swaps.
Quick Facts
- Heat-soak misfire ended up mechanical, not ignition or fuel: hot compression dropped to ~3 bar on cyl. 3. [Elektroda, mxas123, post #16856241]
- ECU may disable the injector on a detected misfire to protect the catalyst (fuel cut strategy). [Elektroda, carrot, post #16856155]
- Briefly unplugging a sensor can force group injection, masking a sequential-injection misfire. [Elektroda, carrot, post #16854360]
- Always confirm compression on a fully hot engine; cold-only tests can miss faults. [Elektroda, sanfran, post #16853233]
- Symptom here: warm engine, cylinder 3 goes dead with zero response at idle/load. [Elektroda, mxas123, post #16853734]
What caused the warm-engine cylinder 3 misfire in this Mini?
Low hot compression on cylinder 3 was the root cause. With the cam phase adjuster motor unplugged, compression measured only ~3 bar. That mechanical loss created the misfire, and the ECU then halted fuel on that cylinder. "There is only 3 bars" confirmed the diagnosis. [Elektroda, mxas123, post #16856241]
How can I tell if the issue is fuel cut or lost spark?
Observe live data or use a noid light and spark tester during the fault. In this thread, the owner confirmed the ECU cut fuel to the third cylinder when the misfire occurred on a warm engine. That distinguishes a commanded fuel shutoff from ignition failure. [Elektroda, mxas123, post #16854015]
What does code P0303 mean on this car?
P0303 flags a misfire detected on cylinder 3. The ECU uses crank acceleration to spot combustion loss. When it counts enough misses, it sets P0303 and may protect the catalyst by cutting fuel on that cylinder. "Error P0303?" was the clue in the thread. [Elektroda, ladamaniac, post #16854162]
Why does unplugging a sensor make all cylinders run briefly?
Unplugging forced the ECU from sequential to group injection. Group injection can temporarily mask a misfire pattern caused by timing or per-cylinder control. Once the ECU returns to normal strategy, the misfire shows again. "The controller switches... to group injection," explained one contributor. [Elektroda, carrot, post #16854360]
Should I perform compression testing hot or cold for this symptom?
Test on a fully hot engine. Some mechanical issues appear only with heat, so cold compression can look normal while hot compression reveals the fault. This thread’s guidance started with, "check the compression pressure on a hot engine." [Elektroda, sanfran, post #16853233]
Cold and hot compression both looked equal—am I done?
No. Re-test with the cam phase adjuster actuator disconnected if symptoms persist. In this case, repeating the test hot with the phase adjuster motor unplugged exposed only ~3 bar on cylinder 3. That pinpointed a cam-timing or head-related issue hidden in earlier checks. [Elektroda, mxas123, post #16856241]
What ultimately resolved the misfire for the poster?
Identifying the mechanical cause: hot compression dropped to ~3 bar on cylinder 3 when the phase adjuster motor was unplugged. The poster reported the problem solved after acting on that finding. This directs repairs toward cam phasing or head work instead of fuel or ignition parts. [Elektroda, mxas123, post #16856241]
How do I reproduce the winning diagnostic steps from this thread?
- Warm the engine until the misfire on cylinder 3 appears.
- Perform a hot compression test.
- Repeat the test with the cam phase adjuster motor unplugged; compare readings.
The owner found only ~3 bar on cylinder 3 under step 3, confirming a mechanical/timing fault. [Elektroda, mxas123, post #16856241]
Is swapping injectors or coils enough to fix this warm misfire?
No. The ECU can cut fuel after it detects misfire, so injector swaps won’t help if the root cause is mechanical. In this case, ignition and injection components had been replaced already, but the ECU’s catalyst-protection strategy kept fuel off the failing cylinder. [Elektroda, carrot, post #16856155]
Can a cylinder really go completely dead only when warm?
Yes. The poster confirmed the third cylinder had "totally zero reaction" once warm. Heat can worsen sealing or timing issues, dropping effective compression. That turns a marginal cylinder into a dead one at idle and low load. [Elektroda, mxas123, post #16853734]
What is sequential vs. group injection in this context?
Sequential injection times each injector to its intake stroke. Group injection fires multiple injectors together. When a sensor is unplugged, the ECU can fall back to group injection, briefly smoothing symptoms but not fixing the cause. "Switches... to group injection" captured this behavior. [Elektroda, carrot, post #16854360]
When should I consider cylinder head work on a warm misfire?
Consider head work if hot testing shows compression loss or timing-related issues. A contributor noted, with this engine, a head job might be required when misfire data trends with temperature rise. That aligns with the final low-compression finding on cylinder 3. [Elektroda, mario 77, post #16854332]
