By on April 14, 2016


fuel economy. shutterstock user iQoncept

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 [email protected]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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48 Comments on “Piston Slap: The Panther’s 20 MPG Fuel-Economy Barrier?...”

  • avatar

    I got the same mpg hway on a GMarq I rented in FL in 2010.

    I couldn’t believe it considering the power was not great. I rented a Genesis V8 the previous trip and got 24 overall w/ incredible pickup.

  • avatar
    Chets Jalopy

    Bring it by the White House garage. Obama will properly inflate your tires. That’s 5 mpg right there.

    • 0 avatar

      But that can be tricky cuase the numbers explained and how they are measured has changed. after adjusting for inflation, the actual pounds of air in the tire and the retired air amount no longer even applying pressure and the air that is only part time…these are all measured differently from when this car was sold new.
      adding just 3 pounds might be looked upon as full when 15 is really needed.

      • 0 avatar

        Run the tires 35-40psi.

        Also, check alignment – if the tow-in is off, this can cause a huge mileage decrease.

        Also check for dragging brakes – easily could be a sticky caliper.

        Does the car have a tachometer? Verify that the transmission is going into overdrive, and that the torque converter is also locking up. You can watch the tachometer and see the RPM drop once when it goes into overdrive, and then again when the torque converter locks up (not necessarily in that order).

        • 0 avatar

          If you run the tires at that high of a pressure you will wear out the center of the tread pretty quickly, particularly on the rear. So that is a false economy if you plan to keep the car around for awhile and may need to spend several hundred dollars to replace the tires sooner than you should have.

          Tachs didn’t make in into Panthers, other than the Marauders until near the end. So unless someone has added one it doesn’t have a tach.

        • 0 avatar

          Tach came for the 06 model year.

        • 0 avatar

          Sajeev, thanks. I can tell that it goes into OD by cycling it with the OD switch on the stalk.

          To the other person said that the intake manifold problem was the gasket, no there was actually a plastic area in the manifold itself. That was checked before I bought the

          As I am retired now, I have been holding off on doing a major tuneup. But my wife just finally settled an accident case so the time has come to do much or all of the maintenance suggested by Sajeev, especially EGR, wires, injectors, fuel filter and plugs plus checking the vacuum lines.

          I have run measured miles and it is not an odometer problem. And I run 40 psi all around, the max recommended. No signs of cupping or other wear.

  • avatar

    If you’re willing to spend a bit of money, a scanguage (or similar) OBD2 scanner will be some of the best money you ever spent as someone who drives and works on older vehicles. It is possible to program in long term and short term fuel trims and monitor these values. In a properly running car, over an average drive cycle the two values should roughly add up to zero. If not, you can start looking at MAF sensors, oxygen sensors, etc.

    I’m always one to ‘baseline’ a used car when I buy it: replace ALL fluids, filters, belts/hoses if they look old, sparkplugs and wires, make sure the alignment is good (monitor tire wear, inflation too). I also check suspension for wear at this time in terms of loose balljoints/tire rod ends, worn shocks, and worn/misadjusted brakes. Sometimes this upfront maintenance can either solve an issue before it leaves you stranded or in disrepair, or else alert you to an oil leak or something else while you’re crawling around the vehicle working on it.

  • avatar

    Also, while duals look cool, don’t be expecting any fuel economy gain from the changeover.

  • avatar
    Felix Hoenikker

    I have a 97 Crown Vic with the single exhaust and 4.6L V8. I have no trouble getting 25 mpg highway at 65-70 mph. City mileage is much more variable with the seasons due the long warm up times for that the cast iron engine.
    My Panther has 89k miles. My mother passed it to me five years ago. I twas the last car my late father bought so it’s essentially a one owner car.
    I have replaced all of the ignition components – plugs, coils and spark plug wires, and noticed an improvement in mileage as well as performance. You can get all these parts from Rock Auto for stupid cheap prices. I spent about $100 for everything. I still have the original plastic manifold, and won’t replace it until it starts to leak as it has no bearing on performance.
    Now the braking system is another story for another time.

  • avatar

    i think a cost/benefit analysis is in order.

    Unless these are all going to be DIY repairs (you mention driveway repairs), you are gonna run up quite a bill fixing a relative minor problem; particularly at current fuel prices.

    Also, with nearly 200k miles, you are gonna be spending money on parts that, while not working optimally, are at least working. While you are doing that there are going to be parts that are probably just going to stop working… at all… and will cost money to repair.

    Preventative maintenance is a good thing, but I would be judicious about it and go in with a plan. Cheap stuff first.

    Also, as I re-read the poster’s “letter”… the more I feel this is a hopeless case. For example:

    11k miles and only a handful of highway runs? That tells me you do a crap ton of city driving. Let’s be honest… you will never get decent gas mileage in the city with that car. You will only ever get 14-15 mpg. Anyone that tells you otherwise is blowing more smoke than a ’99 Crown Vic with bad rings (and that’s a lot of smoke… I know). I drove a handful of Crown Vics in the city for 13 years, 8 hrs a day. Trust me. Give up that hope.

    You mention wanting to better MPG but then can’t help but bring up the desire for duals? Sounds to me like you like the car, you want to make it better, but the MPG is dragging you down.

    “…and have enjoyed everything about it.” Well… not quite, otherwise we wouldn’t be here talking about this. It is what it is… better accept it or get rid of it.

    • 0 avatar

      I agree with you. I think if OP DIYs the work maybe it is worth his time, but if a shop is doing it… I wouldn’t go too into all of these repairs unless necessary.

  • avatar

    Swap out the tires for low-rolling resistance tires if you don’t already have them. That should give you a small boost in economy.

  • avatar

    O2 sensors should be changed as well if they have not been at that age.

  • avatar

    I had a MY01 Marquis. On one tank, heading south from Michigan, I managed to pull almost 29mpg in Ohio. Most other situations, I got around 25.

    This was with single exhaust and 2.73 rear gears.

  • avatar

    Steady speed cruising fuel consumption is a deceiving measure, but just checking up on European drivers of this car, 20mpg seems to be among the better ratings. This one is pretty spot on:
    How about an engine swap? Would that be blasphemy?

  • avatar

    replace the thermostat

  • avatar

    Holy crap! The alternator, with all its belt tension, attached to the plastic intake manifold? Now there’s some f’ing brilliant engineering…

  • avatar

    I had a 1997 Town car 4.6l single exhaust. Bought it with 118k miles on it. I changed spark plugs and wires shortly after I bought it. I regularly got in the low 20’s. Only time mpg would dip slightly under 20 was in winter. I didn’t keep the mpg log, but I know my average for the 65k miles I owned car was 23 mpg.

  • avatar

    I had a 1986 Lincoln Town Car (loved it but had to choose between 45 mpg car and the Lincoln when I downsized my life). I had no problem getting 28 mpgs on the highway with that car and averaged 20 mpgs overall.

    The trick to that car, thanks to being anally retentive myself, was learning that that car needed to be kicked up to overdrive as quickly as possible. Once there that car could get about 30 mpgs at 45mph quite easily. Thanks to the trip computer in my Lincoln, I spent a summer learning how to drive the car and it was a great product save for some cheap interior vinyls. I ended up making custom door panels and dash cover (for top and glove box) so that everything looked the same and it held up well – made the car look new.

    Wish I still had that beast – was like driving a living room – but it was the most comfortable and quiet car I’ve ever owned. That 302 with FI and 4 speed overdrive transmission were dependable.

    Mileage was also checked at fill up and manually computed. Trip computer was surprisingly accurate.

    • 0 avatar

      Was going to comment on my 86 and this seems a good place. That was a year that had big improvement on the prior year. Had one of each and the 85 couldn’t break 20. The 86 routinely broke 20 mixed driving, 24 on the highway and once or twice hit almost 30. Great car and my ex drove it till it had almost 300k.

      All the advice sounds knowledgeable and frankly I don’t know the answer. I do know that there isn’t any reason that a 4.6 shouldn’t equal or improve on the 5.0 of twenty years earlier. Seems well worth keeping and playing with. I think I know that I read they stopped testing for timing chain wear on the 4.6 at something like a half million miles. That’s a very very hardy engine.

  • avatar

    There is certainly a problem with your car if you can’t break over the 20mpg barrier in hwy driving.

    I’ve had lots of planters ranging from a 92 with the HPP gearing to a Marauder and my current daily driver an 05 P71. My daughter currently drives a 03 P71 and my son a 01 GM.

    Before E10 I would regularly get 26.5 mpg on long hwy trips with the 92 HPP car and it would pull down 19.5 mpg in my daily mixed driving. My current 05 that sees some heavy traffic this time of the year regularly gets 19mpg and will break into the low 20’s when I do more hwy than average. My son who’s commutes is almost all hwy and almost no time sitting at stop lights will get 22 mpg. I would get 24 on E10 on long trips when I drove that one. My daughter who most of the time does a lot of short in town trips will drop to 17 mpg or so and when she mixes in driving home from college will get 21 or so on those mixed tanks.

    The main sensors in the fueling algorithm are the front O2 and the MAF. With that many miles the first thing to do is clean the MAF. Most auto parts stores will carry the CRC brand of MAF cleaner. It will do wonders for the driveability and MPG, though you’ll need a security torx bit to remove it from the housing to properly clean it, if it is an OE unit still.

    Plugged up EGR passages and any exhaust leak before the front O2 sensors will cause poor MPG, including the tube to the EGR.

    Dual exhaust won’t see a real difference in MPG, but with a nice set of mufflers it will give a boost in power and sound better. You do not have to by a “system to do so. Just order a H pipe from that year car that came with duals and the tail pipes w/o the resonator. I like the DynoMax mufflers but their cheaper Thrush line will save you a couple of bucks. Both lines have direct fit replacements. You do want the non resonator tail pipes to keep it sounding good and save your pennies. Theh only issue is that you’ll need to add a hanger for the driver’s side tail pipe and muffler.

  • avatar

    14 mpg in the city sounds about right, but 17 on the highway definitely does not.

    Everyone here knows my feelings about Panthers, and that the best way to spend a few hundred bucks would be to put it in the Rusty Prius fund, or maybe on blow, rather than funding a Panther. But if you’re determined to make the Panther a 100 percent Panther, Sajeev’s advice is right on the money.

  • avatar

    We took a ’96 Vic from 22K to 170K before we C4C’d it and it always got 23-25 on the highway.

    What is suspicious here is that the OP is not posting any CEL being turned on.

    Was it disabled by the previous owner?

    if you have broken O2 sensors or plugged EGR ports as ours did, the CEL should have been on most of the time. It certainly was on ours, one of the many reasons I wasn’t sorry to see it go.

    Pro tip to the OP: If you really have to have a Panther, trade it off for a ’98 or newer and get the better brakes that come with it. the ’96 brakes were way undersized and were always warping.

    • 0 avatar

      The first reaction I have re: C4C is: “you monster!”

      Second, changing to bigger brakes isn’t too onerous. and are good resources for many owners who have done the “Big Brake” upgrade.

      Then you get a nice “fat” Panther with the added stopping power. If this gentleman has a HPP model then he already has the required 16 in. wheels.

      • 0 avatar

        Monster? Me??
        Naahh, the car was the monster.

        It ate 2 heater cores and was working on a third. have you ever replaced one in one of these? Get ready to either drop near a grand to have it done, tear the dash and front seats out yourself and eat up a weekend, or bypass it. None of these solutions were acceptable anymore.

        Clearcoat was failing, interior door cards were split at the armrest from poor design, tranny was getting dodgy.

        It was worth more to me dead with 3 grand in my pocket than it was alive.

        I DON’T miss it.
        At all.

        • 0 avatar

          Guess I had a good one… yikes. I sold my ’92 P74 last fall with a little more miles than what you had on yours, and it had mostly original parts. Definitely orig. h. core. I would’ve taken the “government’s” money and run, too.

          I saw too many perfectly good cars being euthanized to ever have a net-positive outlook towards C4C. Still have nightmares about a small-block in a Roadmaster slowly choking to death…

          • 0 avatar

            Glad you didn’t have to do the dance with the HC.

            Oh yeah, and the intake manifold. Even though the alternator bolted to the block, the upper connection point was thru a steel bracket that bolted directly to the top wall of the coolant passage that ran left to right across the intake manifold.

            When mine blew, it was during the 2-3 shift near redline when I was passing another car in a 2 lane road.

            Stress was most definitely passed to the manifold thru the alternator. Coolant blew everywhere as the top coolant runner catastrophically failed.

            Ford’s fine print ruled out responsibility for covering it in the recall, so I was on the hook for a 500 dollar replacement.

            Another nail in the coffin.

    • 0 avatar

      Yeah, yours was the first year with that senseless plastic intake gasket.

      I also had no problems with the 02 sensors. I believe in life we’re all unlucky in different ways.

      Mine had a common early Mod motor issue with burning oil, but it wasn’t anything that our forebears didn’t have to deal with. I figured for what I paid for the car in 2007 I made out like gangbusters.

  • avatar
    formula m

    These cars do not get 25mpg+

    20-22mpg when new, maybe…

    • 0 avatar

      Eh,the EPA numbers are the participation ribbon of fuel economy. A careful driver can exceed those numbers,if they’re willing to be a rolling road block (think 50-60mph, exclusively highway).

      From personal experience, I got about 23 (verified with mileage vs fuel used, not just the trip computer) out of a rental ’10 Town Car, and that included a bit of in town driving, and I mostly do 70+ on the highway. That said, you still don’t buy one to save at the pump.

    • 0 avatar

      Yes they do get 25mpg at least if you can get pure gas in your area. What you see on the EPA’s site today are the twice revised numbers vs what was originally on the sticker

      I’ve had 8 Panthers over the last 20+ years and it was normal to get 26.5mpg on pure gas with the cruise set at then extra legal speeds. 75-80 all day long. Now that was in the best possible combo, a aero nose 6 window with the HPP package. Doing similar driving in the formal roofed early GM with the “economy gearing” 25.5 was the norm on hwy.

      Mixed everyday driving would return somewhere in the mid 19s week in week out.

      Now with the fact that I can only get E10 save a couple of places that have prices that are significantly higher than averaging 23-24 is certainly doable on hwy trips.

      My son’s GM manages to consistently pull down 22 or better in his more hwy oriented mixed use.

  • avatar

    Something ain’t right with this car. Do all the cheap PM items suggested above and you’ll probably hit the jackpot. If not, am wondering if these crates still had a radiator fan driven directly by the engine rather the modern electric radiator fan approach. Making said swap in a similar vintage V8 F-150 took my bro from 14 to 18 mpg highway. Good luck with your land yacht.

  • avatar

    I’ve seen no evidence that a performance exhaust has ever improved fuel economy. In fact, the smaller diameter exhausts are actually tuned to help the engine run more efficiently under typical driving conditions by maintaining adequate exhaust gas velocity at lower engine speeds. It would only cost a manufacturer a few dollars in steel to make a vehicle’s exhaust pipes larger. They’d do it if it yielded significant benefits to a typical driver.

    It might be worthwhile to check the exhaust backpressure though. Catalytic converters can fail internally and restrict flow. It’s just a pressure gauge that attaches to the O2 sensor port. If nothing shows up there, the exhaust is not a contributing factor to the poor fuel economy.

  • avatar
    Dan R

    Install an electric radiator fan.

  • avatar

    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.

    • 0 avatar

      Yes making sure that the thermostat allows the engine to get up to proper operating temp and do so quickly is important to good fuel economy. If the temp drops below 150ish it won’t be in closed loop and it may still have a little warm up enrichment pumping extra fuel in that is totally wasted.

  • avatar
    Compaq Deskpro

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

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