I recently took my 97 (3.8) in to have the fuel injectors cleaned. In the process I was informed that the intake manifold gasket was on its way out and should be replaced. I did so but now my gas mileage is less. The engine seems to run smooth but am wondering why my gas mileage dropped instead of getting better. Is there a chance that during the process of injection cleaning the throttle sensor could have be affected? Any suggestions?

 

The manifold wasnt installed correctly. You might have a small vacuum leak over one or more cylinder. Check for misfire.

 

One thing that I noticed after I posted was when I am in 3rd gear doing about 40mph I am getting an intermittent surge of power. Does this fall in line with a vacuum leak or is it possible the cleaning as affected one of the sensors. The mechanic is telling me he thinks it might be the oxygen sensor but I seem to recall when they are going the engine runs really rough or quits when you idle.

 

When you have a vacuum leak, it makes the engine think it needs more fuel. So when you are in high rpms you will get a surge of power as more fuel is being dumped to compensate.

 

Just so I know what needs to be done how do they test for an intake manifold vacumn leak?

 

The way I do it, while the car is hooked to a scanner, I watch the fuel trim and the O2 sensors while spraying throttle body cleaner around the intake manifold gasket. I watch for rich/ lean conditions in the cylinders.

 

by fuel injectors cleaned, do you mean taken off and flow matched?

 

They were cleaned on the car. Removed only when they changed the intake gasket.

 

Thanks to everyone who got back to me. I had the mechanic check it out today. No vacumn leaks but found that the egr valve was bad (code 9). Everything else seemed to check out or be within specs. No misfires on the plugs but the wires may need to be replaced as they are about 4 years old. Hopefully my gas milage will go back up.