1. Vacuum Leaks: This is the most common cause of rough idling after engine work. Even a tiny leak can significantly affect the engine's performance.
* Check all vacuum lines: Inspect every vacuum line connected to the intake manifold, throttle body, brake booster, etc. Look for cracks, loose connections, or lines that weren't properly reconnected. Pay close attention to the PCV (Positive Crankcase Ventilation) system.
* Spray carb cleaner (or starting fluid – use cautiously!) around suspected leak areas: With the engine idling rough, spray a small amount of carb cleaner around suspected vacuum connections. If the idle changes (gets worse or better), you've found a leak. *Use extreme caution with starting fluid – it's flammable and should only be used in a well-ventilated area, and only small amounts.*
2. Throttle Body:
* Clean the throttle body: Accumulated grime can interfere with the throttle plate's smooth operation. Remove the throttle body, clean it thoroughly with throttle body cleaner, and ensure the throttle plate moves freely.
* Throttle Position Sensor (TPS): Even if you replaced it, double-check the TPS connection and ensure it's properly calibrated (some require specific procedures). A faulty TPS can cause a rough idle.
3. IAC (Idle Air Control) Valve: This valve regulates air flow at idle.
* Clean the IAC valve: Similar to the throttle body, dirt buildup can cause issues. Carefully clean it with appropriate cleaner, paying attention not to damage the delicate internal parts.
* Check the IAC valve's electrical connection: Make sure it's secure and free of corrosion.
4. Air Filter: A clogged air filter restricts airflow and can cause rough idling. Replace it with a new one.
5. Mass Airflow Sensor (MAF) Sensor: While you say you replaced almost all sensors, double-check this one. A faulty MAF sensor provides inaccurate air flow readings to the computer, leading to poor idle. Make sure it's clean (use MAF sensor cleaner, never anything else).
6. Fuel System:
* Fuel pressure: Low fuel pressure can contribute to a rough idle. A fuel pressure gauge test is needed to check fuel pressure.
* Fuel injectors: Clogged injectors can also lead to rough idling, though less likely right after a top-end rebuild unless something happened during the repair.
7. Spark Plugs and Wires: If you didn't replace these during the gasket/sensor work, they are prime suspects. Even new plugs can be faulty.
8. PCV Valve: A malfunctioning PCV valve can cause vacuum leaks and lead to a rough idle. It's a relatively inexpensive part to replace.
9. Incorrectly Installed Parts: Double-check *everything* you touched during the repair. A misplaced hose, improperly seated gasket, or loose connection is a real possibility.
Troubleshooting Steps:
1. Start with the easiest and cheapest: Check vacuum lines, air filter, and clean the throttle body and IAC valve.
2. Then, move to the TPS and MAF sensor checks.
3. If the problem persists, consider a fuel pressure test.
4. If you have a code reader (OBD-II scanner), read the trouble codes. This can provide valuable clues.
If you're not comfortable performing these checks yourself, take the vehicle to a qualified mechanic. A rough idle can be caused by many things, and a professional diagnosis is often the most efficient way to solve the problem. Describing the specific symptoms (e.g., "rough idle at all RPMs," "stalling at stoplights," etc.) to the mechanic will help them narrow down the possibilities.
