86 Gli starts and runs rough for three minutes with black smoke if kept above 1200-1500 then slowly dies out and won't restart for 30 minutes. Seems to run extremly rich. Changed fuel pump relay with no change. What could be the problem? Would an O2 sensor cause such a condition light was on for months?

Sound like an airflow issue. But not sure if that car has a mass air flow sensor. Clean intake with carb cleaner or seafoam. I use seafoam for these issues. on throttle body unplug largest hose that has suction when running pour very slowly 6 oz seafoam into funnel connected with hose then shut car off let sit 15-30 minutes then in an open area run the car for 10 minutes revving engine to clean out the carbon. Then drive high acceleration 55+ for 10-20 miles or until smoke clears from tail pipe.

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If it's blue smoke, your burning oil. Get the head gasket checked. If that's ok your looking at something more serious, (more expensive), inside the engine.

change the fuel filter and check the fuel pressure

CRANKSHAFT SENSOR.

Check for compression on your cylinders. That would tell you if you have a bad head gasket or a bad head.check your distributer and see if is good, i seen that happend too

Check your fuel injectors

I just recently have been fighting the same problem with the same make/model/year vehicle. For myself, I would drive off and on through the day and it would run fine. Then suddenly after a start it would run rough and die. I would have to wait 20 to 60 mins before it would start again. I had it checked and fuel pump was only putting out around 19 psi. It should be around 41 psi. Had fuel pump replaced and no more problems. Furthermore, now it runs so much better. It's funny how over time I didn't notice how the performance was slowly degrading. Unfortunaley fuel pumps for this car run around $300.

1988 ranger 2.9 runs great cold then after a few minutes when warmed up hesitates with intermittent black smoke from the tail pipe until i hit the the throttle then clears and will repeat this behavior...