Piston Slap: The Panther's 20 MPG Fuel-Economy Barrier?

TTAC commentator VolandoBajo writes:

Sajeev, my worthy and esteemed fellow Panther defender,

I acquired my ’97 Mercury Grand Marquis LS about six months ago and have enjoyed everything about it. I’m hoping to find a good source for a dual exhaust that doesn’t cost more than the book value of the car, and to convince my wife that the mileage increase will pay for the mod over time.

But my present problem is baseline fuel economy. I see repeated references to a 20 miles per gallon highway figure, but I can only manage 17 mpg at the best of times.

I’ve driven just a limited number of highway runs, but even under controlled conditions, I have gotten at best 17 mpg or so. Around town, whether I punch it at lights, or drive it like the old man who I am not (on the inside), I cannot get more than a hair above 14 mpg.

The car had accumulated 185,000 miles when I acquired it from its original owners (the best provenance for any older car, I’m sure you’ll agree), and I am going on 196,000 miles now. I try not to make really short hops, and don’t run in dusty environments.

I’m hoping that you might suggest some routine maintenance items I should consider, especially ones I can do in my driveway, that might net me better mileage. I have considered things like a new EGR valve, but not sure where to start or what’s worth the effort.

Thanks in advance. You and Jack are the twin bright spots of TTAC. I enjoy a lot of the rest and many of the B&B, but it is you and Jack who really make me want to waste, er … spend, lots more time reading TTAC than I probably should.

Sajeev answers:

On behalf of myself and Jack, thank you for the kind words! And yes, you should easily break 20 mpg while cruising in the 65-70 mph range. I’ve done it in several 4.6-liter Panthers on highway runs, even high-mileage examples like yours.

Fuel economy issues are usually from common wear items needing attention, especially at your Grand Marquis’ age and mileage.

Did you do a tune up after buying? You should replace spark plugs, wires, PCV, air and fuel filter, then inspect all vacuum lines for leaks (i.e. gooey or brittle rubber hoses), and don’t forget the rubber elbows/T-fittings. Pop off the EGR valve and clean the carbon off both the valve and the corresponding intake manifold orifices. It’s also possible your fuel injectors are toast. You can get an inspected/cleaned set for $100-ish on eBay. One or all of these things will likely solve the problem.

And just to make sure we overlooked nothing, have you (or the previous owner) done the plastic intake manifold replacement? If not, do yourself a solid and do all the above when you yank off the original intake.

[Image: Shutterstock user iQoncept]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.