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VW Lupo 1.0 MPI - 4th Cylinder Ignition Loss Issue: Engine Error & Troubleshooting Steps

gogus_92 10971 19
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Why does a VW Lupo 1.0 MPI show an ignition/misfire fault on cylinder 4 only after long low-speed driving, even after swapping the spark plug and HT lead?

Najprawdopodobniej przyczyną jest zbyt niska kompresja na 4. cylindrze, a nie same świece czy przewody [#16799948][#16801031] Po pomiarze wyszło 11 bar na 1. cylindrze, 13,5 bar na 2. i 3., oraz 9 bar na 4., czyli 4. cylinder odstaje od pozostałych [#16815795] Po próbie olejowej kompresja wzrosła do 18 bar na 1–3 cylindrze i 10 bar na 4., co wskazuje na problem mechaniczny w silniku [#16859471] Mechanik zasugerował naprawę głowicy i pierścieni, bo ciśnienie na 4. cylindrze jest za niskie [#16816407] W wątku padła też wskazówka, że przy prawidłowej kompresji trzeba jeszcze sprawdzić cewkę i wtryskiwacz [#16799948]
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  • #1 16799599
    gogus_92
    Level 7  
    The problem concerns the VW Lupo 1.0 MPI car. The car shows an engine error - ignition of the fourth cylinder. After clearing the error, it switches on only when the car is running for a long time at low speed. The error does not turn on during normal driving. I bet that they were candles or cables but for checking they were replaced with a candle 4 out of 3 but the error still pointed to the 4th cylinder. After that, the cables were changed from 4 to 3 and further shows the error of the fourth cylinder. What could be the reason? Please help :)
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  • #2 16799686
    moto-kord
    Level 28  
    gogus_92 wrote:
    What could be the reason?

    What the candle on the 4th cylinder looks like is clean or dirty e.g. with oil.
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  • #3 16799948
    przemek25l
    Level 29  
    Compression measurement at the beginning. If the compression is correct, check the coil and the injector
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  • #4 16799966
    gogus_92
    Level 7  
    moto-kord wrote:
    gogus_92 wrote:
    What could be the reason?

    What the candle on the 4th cylinder looks like is clean or dirty e.g. with oil.


    The candle is clean

    Added after 1 [minutes]:

    przemek25l wrote:
    Compression measurement at the beginning. If the compression is correct, check the coil and the injector


    And what should the compression value be?
  • #5 16800159
    Anonymous
    Level 1  
  • #6 16800175
    gogus_92
    Level 7  
    Bieda z nędzą wrote:
    Confident air leakage through leaking servo hose

    The vacuum servo hose was replaced so it falls off.
  • #7 16800624
    przemek25l
    Level 29  
    Check what it is and write the results
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  • #8 16801031
    moto-kord
    Level 28  
    gogus_92 wrote:
    And what should the compression value be?

    Normal 10 - 12 (bar)
    7 (bar) minimum
    The engine is warm.
  • #9 16804410
    gogus_92
    Level 7  
    I will check only at the end of the week because I have no way so far. And please tell me if these methods, which I checked if candles and cables are good, make sense? After such a check that the error "does not go" behind the candle or behind the cable can be ruled out that they are the cause of this error? I am asking, because a familiar mechanic claims that it is from replacing candles and wires that you need to start.
  • #10 16805715
    moto-kord
    Level 28  
    gogus_92 wrote:
    I am asking, because a familiar mechanic claims that it is from replacing candles and wires that you need to start.

    Candles and HV cables have been checked yet the ignition coil.
  • #11 16805871
    gogus_92
    Level 7  
    moto-kord wrote:

    Candles and HV cables have been checked yet the ignition coil.


    Compression and coil will be checked on Friday :) I also noticed that the car smokes more :(
  • #12 16815795
    gogus_92
    Level 7  
    Compression on the first piston 11, on the second and third 13.5 and on the fourth 9.
  • #13 16816106
    moto-kord
    Level 28  
    gogus_92 wrote:
    Compression on the first piston 11, on the second and third 13.5 and on the fourth 9.

    What mechanic opinion after measuring the compression ?.
  • #14 16816407
    gogus_92
    Level 7  
    The mechanic claims that the head and rings should be done. He says the pressure on the 4th cylinder is too low compared to the others. What is your opinion?
  • #15 16816462
    andrzej20001
    Level 43  
    To get detailed information you really need knowledge and you will not go without an oil test
  • #16 16816465
    gogus_92
    Level 7  
    And what is the oil test?
  • #19 16859471
    gogus_92
    Level 7  
    After the oil test carried out on the 1.2 and 3 cylinder compression 18, and on the fourth 10. Could someone help interpret these results? :)
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Topic summary

✨ The discussion revolves around an ignition loss issue in a VW Lupo 1.0 MPI, specifically concerning the fourth cylinder. The user reports an engine error that activates after prolonged low-speed driving, despite replacing spark plugs and cables. Responses suggest checking the condition of the spark plug, measuring compression, and inspecting the ignition coil and injector. The user finds the spark plug clean but later measures compression values indicating a significant drop in the fourth cylinder (9 bar compared to 11 and 13.5 in others). A mechanic advises that the low pressure in the fourth cylinder may require head and ring repairs. Further discussions include the need for an oil test to diagnose the issue more accurately.
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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

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?

  1. 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]
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