The engine runs much better since taking off the Delteq system and putting it back to the stock coil. Had no error codes when checked. I just finished running the first tank of gas through and my average was just over 20mpg. This is a '93 LT1 6 speed coupe. Opti spark is only 4000 miles on it, I put new plugs and wires on, OEM AC Delco. Previous owner had put new OEM injectors in and he put in a new fuel pump with a higher pressures and E3 spark plugs. Getting Black exhaust on the tips of exhaust system. Not getting enough oxygen for a clean burn, at leat that would be if it were a diesel. 80,800 total miles. Friend here at work has a '93 6spd and gets over 30 on the hwy, so he is pretty surprised at how low mine is. I have tried driving at several speeds on the hwy. and gears, 4th, 5th and 6th to see how it is doing. usually 18-22 on the level. Tire pressure set at 35psi. Stock exhaust system. Using Shell V-Power premium fuel. about a 1000 miles on new fuel filter and cleaning of K&N air filter.

Some ideas I will be checking even though no codes have popped up are:

1. OS2 sensors
2. Throttle Position Sensor
3. Idle Air Control Sensor
4. Vaccum leak.
5. Fuel Pressure Regulator
6. Put new paper OEM air filter back in

I am wondering if the fuel pump he put on is to high of pressure forcing excess fuel pass regulator and injectors. I don't smell excess fuel from the exhaust just see excess carbon. If the ECM is compensating for excess fuel to burn it may not show a code if it is still in range I suppose.

So any other obvious ideas that I may be overlooking to check? I have the shop manuals and my neighbor works for a GM dealer and can bring home that Tech 4 diagnostic tool, but if I can figure out some of this on my own to better understand the car.

scott
Charlotte, MI