Horrible gas mileage (13 mpg) ..

[GoodOmens]

I've recently replaced my MAF and while the car is idling just fine (no longer stalling out or stuttering) it still doesn't seem to be running right.

Power seems slightly robbed and it makes a low rumble/vibration (from the engine) at around 40~55MPH. Not to mention bad gas mileage

Plugs (NGK), wires, air and fuel filter have around 8k miles on them... but I don't know when the last time the dist. cap was replaced.

You think the cap could be the problem? I want to avoid taking it to the mechanic if possible for something easy to replace.

When replacing the MAF (around 500 miles ago) I did notice the plugs had some carbon build up from when the car was running really bad ... you think the plugs might be bad? I would have assumed they would have self cleaned by now.

After I replaced the MAF I was throwing a bad knock sensor so I assume that is still the case (just as a FYI) but there are no other codes.

Any other suggestions?
Modified by GoodOmens at 12:01 AM 3/12/2008

[pathfinder_se_black]

direct injection clean? try that.

[GoodOmens]

I've run some cleaner through a bit ago (not recently ... maybe 4k ago) or do you mean taking it in for a proper cleaning at a shop?

The car has around 190k on it