* Compression Test: A low compression reading in cylinder 4 strongly suggests a problem with the engine itself, such as:
* Worn valve(s): A burnt or sticking valve could cause inconsistent combustion.
* Damaged piston rings: Worn or broken rings allow combustion gases to escape, resulting in low compression.
* Head gasket leak: A leak between the cylinder and coolant jacket can also manifest as low compression.
* Valve Train Issues: Even with new parts, there's a small chance of a problem in the valve train:
* Bent pushrod: A bent pushrod in cylinder 4 can prevent the valve from operating correctly.
* Faulty rocker arm: A cracked or worn rocker arm can also impact valve operation.
* Ignition System (despite replacements): Although you replaced parts, there could still be a subtle issue:
* Faulty crankshaft position sensor (CKP): While unlikely after a PCM replacement, a faulty CKP sensor can cause misfires, especially under light load.
* Wiring harness issue: A damaged or corroded wire within the harness could be affecting cylinder 4 specifically. Carefully inspect the wiring to cylinder 4 for breaks, shorts, or corrosion.
* Fuel Delivery (despite injector replacement): There might be a problem even with a new injector:
* Fuel pressure regulator: A faulty regulator could deliver insufficient fuel pressure to cylinder 4.
* Fuel filter clog (unlikely, but possible): A partially clogged filter could restrict fuel flow to just one injector.
* Exhaust Restriction: While less likely, a severe clog in the exhaust system could affect backpressure and lead to a misfire.
Troubleshooting Steps:
1. Compression Test: This is the most crucial step. A low reading in cylinder 4 confirms internal engine problems.
2. Visual Inspection: Carefully examine the cylinder 4 area for any visible signs of damage or loose connections.
3. Wiring Harness Inspection: Trace the wiring for cylinder 4 meticulously, looking for damage, corrosion, or poor connections.
4. Fuel Pressure Test: Check the fuel pressure to ensure it's within specification.
If you're not comfortable performing these checks yourself, take your vehicle to a qualified mechanic. The problem is likely more involved than simple component replacement. Providing the compression test results will significantly narrow down the possibilities.
