FAQ
TL;DR: 1200–1600 rpm jerking on a Skoda Fabia 1.4 MPI (AZE); an electrician flagged a "vacuum problem." Triage fuel pressure, ignition, and LPG isolation first; leave software for last. [Elektroda, Łuki0601, #17053319]
Why it matters: This FAQ helps Fabia 1.4 MPI AZE owners—especially with LPG—quickly pinpoint low‑rpm bucking causes and prioritize fixes.
Quick Facts
- Symptom band: jerking at 1200–1600 rpm on Fabia 1.4 MPI (AZE) [Elektroda, Łuki0601, #17053319]
- Fuel pressure observed: ~3.0 bar without vacuum, ~2.5 bar with vacuum at idle [Elektroda, Łuki0601, #17053378]
- VAG negative pressure reported: ~360–390 (units as read by scan tool) [Elektroda, Łuki0601, #17053319]
- Compression reported: 12.5 bar on each cylinder after head-gasket work [Elektroda, Łuki0601, #17056012]
- 2001 car with KME LPG; same symptoms on petrol and LPG [Elektroda, Łuki0601, #17054669]
Whats the first thing to check for 12001600 rpm jerking on a Fabia 1.4 MPI?
Start by verifying fuel pressure during high demand. "Fuel pressure to check. It tugs when it has a high fuel demand." Log pressure while accelerating through 12001600 rpm. Compare requested vs actual using your scan tool or an inline gauge. This quickly separates fuel supply from other causes. [Elektroda, Pawel wawa, #17053336]
Are my fuel pressure numbers OK, and how should I test them?
Recorded values were ~3.0 bar without vacuum and ~2.5 bar with vacuum at idle. Recheck dynamically while reproducing the jerk. Watch pressure stability as load rises in the 12001600 rpm band. A sag under load supports a fuel supply issue; a steady line suggests looking elsewhere. [Elektroda, Łuki0601, #17053378]
How do I properly rule out LPG influence when diagnosing?
Edge case: residual LPG can be drawn in on petrol. "Because if not, it sucks." Do this:
- Close the LPG tank valve.
- Run on petrol until gas in lines burns out.
- Test drive on petrol through 12001600 rpm and observe.
If symptoms persist, focus on common systems. [Elektroda, Pawel wawa, #17056272]
Could the ignition system cause jerking on both petrol and LPG?
"The common part for these fuels is the ignition system." Misfire under load feels like bucking. Swap-test the coil module if possible and confirm plug condition and gap. Even new parts can be faulty, so verify under load. Keep the diagnosis focused on components common to both fuels. [Elektroda, hobbista, post #17056341]
Should I worry about piston rings with an LPG car?
A contributor advised a compression test because LPG can stress rings. If compression is low or uneven, mechanical wear is likely. If compression is even, direct attention to fuel, ignition, or vacuum issues. This simple test helps avoid unnecessary parts replacement. [Elektroda, szumi214, post #17055790]
What were the compression results in this case?
Compression measured 12.5 bar on each cylinder. The sleeves showed original honing, and no oil was consumed between changes. Those signs pointed away from ring wear or major mechanical faults here. With sealing confirmed, the focus returned to mixture and ignition control. [Elektroda, Łuki0601, #17056012]
What do VAG negative pressure readings of 360390 indicate here?
These values appeared along with the jerking. After a head-gasket replacement and head work, the behavior improved a little. That kept the vacuum suspicion alive for this case. Continue checking hoses, connections, and intake sealing during transitions. [Elektroda, Łuki0601, #17053319]
Do throttle body or pedal replacements rule out air-path faults?
No. The throttle body and gas pedal were replaced, yet the jerk remained. That outcome points back to fuel delivery, ignition quality, or vacuum behavior rather than those actuators. Avoid shotgun part replacement; test each system under the symptom conditions. [Elektroda, Łuki0601, #17053319]
How can I reproduce the symptom reliably for logging?
Slow the car into 12001600 rpm, then try to hold or raise speed. The jerk typically appears there. Pressing the clutch to drop rpm, or clutching and adding throttle, temporarily clears it. Use this pattern to capture data in your logger or VAG tool. [Elektroda, Łuki0601, #17053378]
Can LPG injectors or lines cause petrol-mode jerking?
The LPG injectors and lines were disconnected in this case. The problem persisted on petrol. That result suggests the root cause was not the LPG injection hardware alone. Keep attention on fuel pressure, ignition, or vacuum shared by both fuels. [Elektroda, Łuki0601, #17057983]
Is ECU software a likely cause of this symptom?
A service mentioned possible software issues. However, contributors focused on mechanical checks first. Rule out fuel pressure, ignition components, vacuum integrity, and LPG interference before pursuing software. Mechanical verification prevents unnecessary programming work. [Elektroda, Łuki0601, #17053319]
What vehicle and LPG details were involved here?
This was a 2001 Skoda Fabia 1.4 MPI (AZE) with KME LPG. The car jerked on petrol and on LPG. That points to systems common to both fuels rather than fuel-specific hardware. [Elektroda, Łuki0601, #17054669]
I already fitted a new coil and plugs. Should I still suspect ignition?
Yes. New parts were installed here, yet the jerk remained. Load-related faults can persist with new components. Swap-test with a known-good coil or recheck plug condition after driving. Confirm under the exact conditions where the symptom appears. [Elektroda, Łuki0601, #17057983]
What finally fixed the problem in this thread?
The original poster reported "Problem solved" but did not share the fix. Use the outlined diagnostic flow to reach a verified root cause on your car. [Elektroda, Łuki0601, #17110192]
