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

Sajeev Mehta
by Sajeev Mehta
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.

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4 of 48 comments
  • Jagboi Jagboi on Apr 14, 2016

    I'd want to check the thermostat to make sure it's not stuck open, and change air filter and O2 sensors. It should be able to get better mileage than it is.

    • See 1 previous
    • Jimbob457 Jimbob457 on Apr 18, 2016

      @Scoutdude Check your odometer against a measured mile.

  • Compaq Deskpro Compaq Deskpro on Apr 20, 2016

    I got a ~24 MPG highway, 27 if I stuck religiously to 55, out of my 03.

  • Jeff S If AM went away I would listen to FM but since it is insignificant in the cost to the car and in an emergency broadcast it is good to have. I agree with some of the others its another way to collect money with a subscription. AM is most likely to go away in the future but I will use AM as long as its around.
  • BEPLA I think it's cool the way it is.If I had the money, time and space - I'd buy it, clean it up, and just do enough to get it running properly.Then take it to Cars and Coffee and park it next to all the newer Mustangs.
  • Dave M. I suppose Jethro’s farm report comes via AM, but there’s a ton of alternative ways to get that info. Move forward people. Progress is never easy.
  • BEPLA For anything but the base model, I'd rather have a pre-owned Polestar 2.
  • BEPLA "Quality is Job........well, it's someone's job, but it's not our job.Neither is building vehicles that people actually want or need.We only build what's most profitable. If only someone would buy our 97 day supply of SuperDutys."