Hi Scotty,
I have a 2000 grand marquis with about 110,000 miles on it I change the oil about every 3000 miles. it has new 02 sensors, cats, spark plugs & coil boots. compression is about 170 on all cylinders +- 4 psi. Had the valves resurfaced due to a bad tank of gas about 12 years ago that's what ruined the cats which held up till about 3 years ago. also gone through 2 plastic intake manifolds. gone through 2 fuel pumps too
Now for the problem the car runs great but for the last few years uses alot more gas than it should. one day i'm getting 14 mpg and another day only getting 8 mpg driving the same rout this is mixed highway & city driving. fuel trims are about +- 5 usually around 0. No misfire codes but mode 6 data shows enough cylinder events to fail but no actual misfires. The problem seems to be worse when the weather is hot and here in Las Vegas it gets HOT. also seems like the engine is a little low on power only in hot weather and ac on engine temp is normal, I can run the ac in cooler weather and engine power is normal I compared the cylinder head temp with engine temp before starting and they are within 3 degrees of each other.
I did try the MAF sensor on a mechanics advice and reset the computer it made everything worse hesitation & stalling. I put the old one back in and it runs good.
I do get a very slight vibration like a lean miss if i rev the engine some ware in the range of 1200 to 2000 rpm but nothing shows on the scan tool and it never does it at the mechanic.
I also have a small evap leak they said they can't fix because the tube that runs along the top of the fuel tank is not available and yes i searched high and low and could not find one. i don't think that will hurt my mileage like that. but it sure stinks when the temp is 110 or hotter.
over the years I had 4 mechanics look at it and no one knows why it's using gas like this. Well just like they kept telling me for 6 years my transmission leak is the pan gasket just a real slow leak well got lots of fresh transmission fluid in it.
well i finally got under there my self and found it the speed sensor was leaking through the wiring harness in to the protective jacket then down on to the flange and wetting the gasket leaving NO fluid on the outside of the sensor or transmission it self. shows how good the mechanics check it out.
Also when i pulled the plugs it looks like the engine is running rich that was 3 years ago as soon as the weather cools down here i'll pull them again and check.
Can you suggest where to look next?
Mike.
Well I can be an absolute stinker trying to find vacuum wakes so basically start with a mechanic with a smoke machine because smoke will come out of the leaking areas and a good smoke machine has a pressure gauge so he can see if they're actually is a vacuum leak
I did that and he said it's the tube that runs across the top of the fuel tank and the part is no longer available. I searched for weeks and it's discontinued by ford and no after market is available.
Scan tool says it's a small leak I don't think this is hurting my fuel economy that much is it?
Everyone out here wants $800 to drop the tank but they won't touch it because they don't want to make the problem worse.
What about the problem of getting 8 mpg one day and 14 mpg the next driving the same route and conditions? Engine is not running rough at all you hardly know it's running.
Mike.