* Incorrect Timing: When you replace the heads, you potentially disturb the timing chain or belt (depending on the engine). Even a slightly off timing can cause a misfire, leading to backfires that manifest as popping sounds from the exhaust. This is the most likely culprit.
* Ignition System Problems: A faulty distributor, worn spark plug wires, bad spark plugs, or a malfunctioning ignition coil can all contribute to misfires that cause exhaust popping. These issues might have been unrelated to the head replacement, but brought to light by it.
* Vacuum Leaks: While less likely to cause *popping*, significant vacuum leaks can disrupt the engine's smooth operation and contribute to erratic combustion that might present as popping. A vacuum leak wouldn't necessarily be directly caused by the head replacement, but it could be aggravated by it (e.g., accidentally damaging a vacuum hose during the process).
* Exhaust Restriction: Although less likely, a severely clogged catalytic converter or muffler could contribute to backpressure leading to popping, but it's more likely to present as a general performance issue rather than just a popping sound.
Troubleshooting Steps:
1. Verify Timing: This is the most crucial step. Use a timing light to check the ignition timing against the specifications for your engine. If it's off, adjust it accordingly. If you're unfamiliar with this, consult a repair manual or seek professional help.
2. Inspect Ignition System:
* Spark Plugs: Replace them with new, correctly gapped plugs.
* Spark Plug Wires: Check for cracks, damage, or loose connections.
* Distributor Cap and Rotor: Inspect for wear, cracks, or corrosion. Replace if necessary.
* Ignition Coil: Test the coil for proper resistance.
* Ignition Module (if applicable): Test the module according to your engine's specifications.
3. Check for Vacuum Leaks: Carefully inspect all vacuum lines for cracks, loose connections, or damage. Use a vacuum gauge to test the system's pressure if you have one.
4. Exhaust System: Inspect the catalytic converter and muffler to make sure there's no significant restriction. Though less probable, it's good to rule this out.
Important Note: If you're not comfortable working on your engine's ignition system or timing, it's strongly recommended that you take your vehicle to a qualified mechanic. Incorrect timing or ignition adjustments can cause serious engine damage.
