FAQ
TL;DR: VW Lupo 1.0 MPI misfire on cylinder 4? Start with a compression test: "Normal 10–12 bar; 7 bar minimum." Low, uneven compression points to top-end work. [Elektroda, moto-kord, post #16801031]
Why it matters: Quick compression data helps you decide between ignition/injector checks and removing the cylinder head.
Quick Facts
- Spec: Warm-engine compression is typically 10–12 bar; 7 bar is the floor for serviceability. [Elektroda, moto-kord, post #16801031]
- Reported readings: cyl 1=11 bar, cyl 2–3=13.5 bar, cyl 4=9 bar. [Elektroda, gogus_92, post #16815795]
- After oil test: cyl 1–3=18 bar, cyl 4=10 bar; imbalance stayed on cyl 4. [Elektroda, gogus_92, post #16859471]
- Symptom pattern: fault appears after long low-speed idling; not during normal driving. [Elektroda, gogus_92, post #16799599]
- Next checks after compression: ignition coil and injector for the affected cylinder. [Elektroda, przemek25l, post #16799948]
Why do I get a P0304-style misfire on cylinder 4 mainly at idle?
At idle the mixture and burn speed are less forgiving. Any air leak or poor sealing pushes the mixture beyond the flammability limit, causing misses. One expert noted a leaking servo hose can create a too-lean mix that won’t ignite reliably. [Elektroda, Anonymous, post #16800159]
What compression should a warm VW Lupo 1.0 MPI show?
Target 10–12 bar on a warm engine. Treat 7 bar as the minimum threshold. Large variation between cylinders, even when above 7 bar, can still trigger rough idle and misfire detection on a single cylinder. “Normal 10–12 (bar). 7 (bar) minimum. The engine is warm.” [Elektroda, moto-kord, post #16801031]
My results are 11/13.5/13.5/9 bar. Is that a problem?
Yes. Cylinder 4 at 9 bar is below the others by 2–4.5 bar. That imbalance exceeds typical tolerance and aligns with a cylinder-specific misfire. Plan further diagnosis to isolate whether rings, valves, or head gasket are responsible for the loss. [Elektroda, gogus_92, post #16815795]
What is an oil test (wet compression test) and why perform it?
An oil test adds a small amount of engine oil to the cylinder, then rechecks compression. A big jump suggests worn rings; little or no change points to valves or head sealing. An expert insisted on this step to pinpoint the source before teardown. [Elektroda, andrzej20001, post #16816462]
How do I read these oil-test results: 1–3 at 18 bar, 4 at 10 bar?
Cylinders 1–3 improved strongly, but cylinder 4 stayed low at 10 bar. That pattern indicates a top-end sealing fault on cylinder 4. The forum expert’s succinct guidance after these numbers: “Head up.” Plan for head removal and inspection. [Elektroda, moto-kord, post #16859568]
Does swapping spark plugs and HT leads rule them out?
If you swap plugs and leads between cylinders and the misfire code stays on cylinder 4, the fault likely isn’t the plug or lead. That A/B test narrows the issue to compression, coil output on that channel, or fueling on cylinder 4. [Elektroda, gogus_92, post #16799599]
Could the ignition coil or injector cause a single-cylinder miss?
Yes. After verifying compression, test the coil channel and the injector for cylinder 4. A weak coil output or clogged injector can mimic mechanical issues. Perform output tests and swap components to see if the fault follows the part. [Elektroda, przemek25l, post #16799948]
Can a brake servo hose leak trigger cylinder 4 misfire?
A vacuum leak can lean the mixture and cause misfire at idle. One expert highlighted the servo hose as a leak source that can push mixtures beyond flammability. Fix leaks before deeper engine work. [Elektroda, Anonymous, post #16800159]
I already replaced the vacuum servo hose. What then?
If you replaced the servo hose and the issue persists, revisit compression, then ignition and injection on cylinder 4. With leaks addressed, focus on mechanical sealing or cylinder-specific hardware. [Elektroda, gogus_92, post #16800175]
Quick 3-step: how do I structure diagnosis on this Lupo misfire?
- Measure warm compression on all cylinders and note spread. 2. Do an oil test to separate rings from top-end faults. 3. If compression is low or uneven, plan head removal; if OK, test coil and injector on cylinder 4. [Elektroda, moto-kord, post #16801031]
Is 9–10 bar on one cylinder still drivable?
The engine may run, but imbalance versus 10–12 bar normal causes rough idle and misfire detection. You risk catalyst damage from unburned fuel if you continue. Bring compression back within spec to restore smooth operation. Minimum is 7 bar, but balance matters. [Elektroda, moto-kord, post #16801031]
The car started to smoke more—could it be related?
In the thread, increased smoke appeared alongside the misfire. That symptom often accompanies oil entry or incomplete combustion on the weak cylinder. Plan inspection during head removal to check valves, guides, and gasket sealing around cylinder 4. [Elektroda, gogus_92, post #16805871]
What does the advice “Head up” actually mean?
It means remove the cylinder head for inspection and repair. Check valve faces and seats, head gasket, and potential head cracks. “Head up.” is concise expert shorthand for proceeding to top-end teardown when compression evidence warrants it. [Elektroda, moto-kord, post #16859568]
Should I start by replacing plugs and wires anyway?
Basic service items are worth checking, but your swap test showed the fault stayed on cylinder 4. Prioritize compression and cylinder-specific checks instead of parts tossing. Use the initial swaps as proof to focus the next steps. [Elektroda, gogus_92, post #16804410]
