Fuel Delivery Issues (Most Likely):
* Fuel Injectors: A clogged or malfunctioning fuel injector(s) could be preventing fuel from reaching the cylinder(s). If gas is pooling in the intake, it's *probably* not a complete fuel pump failure (though a weak pump is possible), as some fuel is getting there. The injectors are the more likely culprits.
* Fuel Pressure Regulator: A faulty fuel pressure regulator could be allowing too much fuel pressure to leak back into the return line, preventing adequate pressure at the injectors.
* Fuel Filter: A clogged fuel filter will restrict fuel flow to the injectors.
* Fuel Pump (Less Likely but Possible): While you see gas in the intake, a weak fuel pump might not be providing sufficient pressure to overcome injector resistance. This is less likely given the pooling, but worth considering.
Ignition Timing Issues (Less Likely but Still Possible):
* Distributor Issues (if applicable): If your Pontiac Sunbird has a distributor, the rotor could be worn or the distributor cap and/or wires could be cracked or corroded, leading to misfiring or lack of consistent spark delivery. Even if you have spark at the plugs, it might not be happening at the right time. Check the timing chain/belt to ensure it's not slipped. This can cause extreme mis-timing.
* Camshaft Position Sensor (CMP) or Crankshaft Position Sensor (CKP): These sensors tell the computer the engine's rotational position for accurate timing of spark and fuel injection. A faulty sensor will result in erratic or absent ignition.
* Ignition Control Module (ICM): This component controls the ignition timing and firing of the spark plugs; a failing ICM can cause intermittent or complete spark failure. Although you're reporting spark, the timing could be radically off.
Other Possible, Though Less Probable, Causes:
* Vacuum Leaks: Large vacuum leaks can significantly disrupt the fuel/air mixture, hindering combustion.
* Compression Test Accuracy: While you say you have compression, double-check the compression test. Inconsistent compression across cylinders points towards a problem, such as a blown head gasket or valve issues.
* Engine Sensors: A multitude of sensors (e.g., MAF sensor, oxygen sensor) provide input to the engine's computer. A faulty sensor can cause a lean or rich fuel mixture, affecting starting.
Troubleshooting Steps:
1. Fuel Pressure Test: This is the most crucial step. Measure fuel pressure at the fuel rail to confirm whether sufficient pressure is reaching the injectors.
2. Injector Test: Check the injectors for proper operation. This often involves a resistance test and/or a visual inspection to see if they're spraying fuel correctly. A fuel pressure gauge helps here. Spraying directly into the intake manifold is one diagnostic technique.
3. Visual Inspection of Wiring: Check for any obvious signs of damage, corrosion, or loose connections in the ignition system and fuel system wiring.
4. Check Fuel Filter: Replace this if it hasn't been done recently, it's a cheap and easy check.
5. Timing Check (if applicable): If you have a distributor, verify timing and condition of distributor cap and rotor.
6. Vacuum Leak Check: Listen for hissing sounds around the intake manifold and vacuum hoses.
7. Scan Tool: A scan tool can read diagnostic trouble codes (DTCs) from the engine's computer, which can pinpoint the problem.
It's highly recommended you start with a fuel pressure test. If that's low or erratic, it will strongly point to the fuel system as the source of the issue. A mechanic specializing in older vehicles might be the best bet for diagnosis and repair since this sounds like multiple potential issues.
