My (miss) all started with the CEL coming on.Can't remember codes but were lean bank one and two.replaced fuel filter and clear codes.Light back on after 100 miles.Replace plugs and wires with platinum plugs as recomended,that's when I started to get the (miss) runs great down the road but when I sit @ an idle in gear I can feel a sputter sometimes I can feel it during deceleration also and my MPG has gone down.cleared codes and about 100 miles CEL on again.Take back to mechanic to scan again same lean codes.He suggests and replaces upper intake gasket and CEL has not come back on BUT this idle thing and mpg are bumming me any suggestions anybody?

 

I'd next clean the fuel injectors.

 

How many miles on the engine? When was the last time the oxygen sensor was replaced? It's cheap enough, and if you have a lot of miles, you should probably replace it.

 

Your mechanic has fixed a probable vac leak.

Now it's time for an "Italian tune-up".
First add a double shot of injector cleaner to the gas.
The find some open road and red line it a few times.

 

The most effective way to clean the injectors is with one of those machines that runs solvents through them. But will require you removing them from the engine.

I've done the next best thing. i got a can of carb/injector cleaner spray with a nozzle that will fit the cap/hose of one of those tire inflater/sealer spray cans. First, look into the cap that screws onto the tire valve to see if there is a rubber gasket. If so, remove it, as it can not withstand the cleaner fluid.

Disable the fuel pump, either by disconnecting the pump relay, or the inertial shutoff switch. Then try to run the engine until it stalls. Without the fuel pump, it may not even start, but the point here is to depressurize the fuel lines.

Find the pressure test port on the fuel rail; it should be a Schrader valve like a tire valve, with a cap on it. Remove the cap and valve core with a core removal tool, and screw the tire inflater hose onto it, making sure it seals tightly. Attach the hose cap onto the carb/injector cleaner can, and start pressurizing the fuel rail with the cleaner.

Start the engine, and let it idle on the cleaner fluid for a couple minutes. You will need to keep pushing the spray head on the cleaner to keep the fuel rail supplied. After a couple minutes, the rail should be full of cleaner, and you can let it sit for a while to let the cleaner do its thing. You can repeat the procedure a couple of times.

When you're done, remove the inflater cap, re-install the valve core, and replace the cap. Now re-connect the fuel pump and start the engine. If your engine was running poorly due to dirty injectors, it should run better now.

I've used this trick to fix at least 2 cars successfully. Another car had a stuck injector that had to be replaced, so no amount of cleaning could help it.