Excessive fuel consumption

Two years ago I had the heads replaced on the engine which are a known bad part. After I got it back from the mechanic I could feel some thing with the engine every once and a while, like a stumble or a stutter. I had a 12,000 mile/one year warranty with the mechanic so I took it back to him. He kept it for a few weeks driving it 350 plus miles and said he did not find anything. It kept doing it so I took it back a few months later. Again, he kept it for three to four weeks driving it 350 plus miles. The mechanic said he replaced the thermostat because he had a message the engine was not reaching operating temperature but did not see or find anything else.

After the warranty ran out the stumble did not get better and gas mileage was horrible. This mechanic has since closed his shop is working for a municipality in their garage.

I purchased a good scan tool and started trying to figure out what was happening. I also enlisted a mechanic friend to help but did not do much. So far this is what has been replaced and why:

Spark plugs (Initial shot in the dark).

Spark plug wires (Initial shot in the dark).

Coil (Concern the coil had been tracked plus engine was getting louder. New coil engine was smooth and quiet.)

Idle Air Control valve (Engine seemed to never idle down. Valve had heavy deposits and gumming so it was cleaned first. Cleaning did not seem to help as well as the valve could not be opened using advanced scan tool.)

Upper Oxygen sensors (Sensors were reading "Lean" - using the actual word on the scan tool - even though the engine was using more gas. When changed engine responded the most at the time.)

Lower Oxygen sensors (Because Upper sensors were changed.)

Thermostat (Engine was running around 171-176 degrees Fahrenheit according to the scan tool. Installed 195, had 192.)

Engine Coolant Temperature sensor (After changing thermostat engine operated 182-186 degrees Fahrenheit according to the scan tool. After changing sensor engine operates within the correct temperature range as read by the scan tool. Largest difference in engine performance.)

MAF sensor (Shot in the dark since the problem is present at specific engine RPM.)


Best Description:
The stumble is most felt between 50-60 MPH in 5th gear. The engine also bogs down at times in this speed range.
If pulling a hill or air conditioner is on it is the most evident.
Gas mileage stays around 18 mpg +/-. At the worst it was around 16 mpg. There has never been a smell of sulfur such as gas hitting the catalytic converters.

The best I can figure is the engine is calling for more fuel than what is needed so the engine is essentially flooding itself. When parts have been replaced and the battery cables touched the engine runs great for a day or two then it goes back.

I have thought about spraying some starting fluid around the intake and heads to see if the engine changes speed but have not done it.

Any assistance and/or guidance would be appreciated.


Chris