* Fuel Injector Problem (worsened by cleaning): While fuel injector cleaner is meant to improve performance, sometimes stubborn deposits break loose and temporarily clog injectors further, leading to misfires (jerking) and uneven engine operation. The "flapping metal" sound could be related to this if it's causing a component to vibrate unusually due to the inconsistent engine operation.
* Vacuum Leak: A vacuum leak can cause erratic engine behavior, including jerking and a possible metallic flapping sound (depending on what's vibrating due to the leak). The cleaner might have indirectly revealed the leak by changing air/fuel ratios, making the leak's impact more noticeable.
* Ignition System Issues: Worn spark plugs, bad ignition wires, or a failing ignition coil can also lead to misfires, resulting in jerking. The metallic sound could again be a secondary effect of the misfires causing vibration in something.
* Intake Manifold Problem: A loose or damaged intake manifold can cause vacuum leaks and air intake problems, leading to rough running and potentially a metallic sound from vibrating parts.
* Loose Heat Shield: A less likely culprit, but a heat shield that has become loose due to rust or vibration could be making the flapping sound. The jerking might be coincidental or related to a separate issue.
Why it happened *after* the cleaner:
The cleaner didn't *cause* the problem, but it likely brought a pre-existing, smaller issue to the forefront by:
* Dislodging debris: Cleaning might have dislodged deposits that were partially blocking injectors or passages, temporarily worsening the situation until they completely cleared.
* Exposing a weakness: A system already operating near its limit might have become visibly symptomatic after the improved (but still imperfect) fuel delivery.
What to do:
1. Check for obvious loose parts: Look around the engine bay for anything visibly loose or damaged, especially near the intake manifold or exhaust system. This could identify the source of the flapping sound quickly.
2. Diagnose misfires: Use an OBD-II scanner to read trouble codes. Misfires will often be indicated by specific codes. This will point to cylinders with problems.
3. Inspect the fuel injectors: This is more complex and may require professional help, but visually checking the injectors for clogging or leaks can be beneficial.
4. Listen carefully to the sound: Try to pinpoint the location of the "flapping" sound. This will help narrow down the potential sources.
5. Seek professional help: If you're not comfortable diagnosing engine issues yourself, take it to a mechanic. They have the tools and expertise to properly diagnose and repair the problem.
Don't continue driving the car with these symptoms, as continued operation could worsen the underlying problem and potentially cause further damage.
