I've got a 98 Concours with 104,000 on it. I changed the plugs, wires and fuel filter about 15k miles ago. I changed the air filter within the past 2 weeks. Also added injector cleaner about once a month. I'm only getting about 14-14.7 MPG in the city with AC on auto. No codes have come in. I get about 17-18 MPG on the hwy.:disappoin
Anyone have any suggestions on what might be causing my poor fuel mileage?
The fuel mileage is actual...not what the dash info says.

dunno...my 98 deville (93,000miles) gets about 14 mpg in the city so it sounds about right.
I think your highway should be above 20 mpg though....setting your cruise at 75.

First of all, stop using so much fuel injector cleaner. That stuff can destroy the insulation on the fuel pump and injector windings.

Check the FPR. It is a small canister on the right bank fuel rail with a vacuum hose on it. While the engine is running, pull the vacuum hose off and see if any fuel is leaking from the nipple. If there is any replace the FPR. Any fuel leaking is unmetered fuel going directly into the intake manifold and reducing fuel milage.

I have a 99 Concours also, Only had it a week or so now. It's averaging around 15.5 city, and I don't get out on the hwy much to know how that would be. It only has 67k miles on it, and runs flawlessly so you're about right. If I drive with a heavy foot it is much lower, this is 15.5 driving like a grandpa.

Actually I was wondering if it were possible to chip it down to standard deville performance by replacing the ECM chip or something for better mileage while prices are high. Would that help or is it possible?

Troy

I was wondering when I made my post if your milage is normal or not. I don't to any city driving so I can't say. I was assuming you saw a drop. 17-18 is low for highway driving unless you are not zeroing it out at the start.

Pccinema,
The answer to your question is no. You can't replace a PCM chip.

Check your coolant temperature sensor. If bad, it tells the ECM that your engine is "cool" and sends a high fuel/air ratio. Also, insure that the flap is "opening" in the cold air intake duct.

Drury

Running the A/C will also drop your mileage.

My '95 concours (66,XXX miles) gets about 14 1/2 in town.
I've installed a new air filter,fuel filter and AC plat plugs, but noticed no
difference at all in the mileage. (a friend of mine has a '94 Concours
and he claims about 18 MPG in town)

Ed

Fuel filter will not affect milage. Neither will an air filter on a fuel injected engine. It will on a carburated engine.

My '95 concours (66,XXX miles) gets about 14 1/2 in town.
I've installed a new air filter,fuel filter and AC plat plugs, but noticed no
difference at all in the mileage. (a friend of mine has a '94 Concours
and he claims about 18 MPG in town)

Ed

Hows about wires.
Was changing my cap and rotor last spring and found a split in a wire.
WOW what a difference that made.

Fuel filter will not affect milage. Neither will an air filter on a fuel injected engine. It will on a carburated engine.

Im sorry but I dont get that. Im no guru by any means, so I need to ask as to why a clogged air filter would not affect mileage on a fuel injected engine.

No disrespect intended.
But if that was the case, then what would be the purpose of even changing it?
Other than the fact that the fuel needs sufficiant amount of air to burn properly?????

Like said, no disrespect intended. Just cant seem to wrap my noodle around that one.

You don't need to change it as often as we (old schoolers) were use to. Old habits are hard to break. I think the owners manual says 30K. Even then it is probably still good under normal conditions. Here is what Bbob had to say on the matter. Not surprisingly, he can explain it much better than I.

" A dirty air filter will cause a carburetor to run richer and richer the more restriction builds up with dirt accumulation. This is because a carburetor is sensitive to the pressure differential between the pressure in the float bowls and the pressure in the venturi area. Fundamentally, this is how the "choke" on a carburetor works. By making the throttle or choke ahead of the venturi of the carb the carb will deliver more fuel when the choke is employed so this is used for cold start enrichment. Any restriction in the inlet of the induction system will change the carb calibration and cause it to run rich, hurting fuel economy. This is because a carb is a mechanical device that is calibrated for a specific set of performance parameters and if pressures change due to restriction, altitude, etc...the carb calibration will be affected.

Fuel injection systems deliver fuel based on measuring the manifold pressure and/or inlet air flow (mass air flow rate) and calculating the correct amount of fuel. A fuel injection system typically will not care where the throttle in the system is at. It just looks at the conditions in the manifold that result and deliver that much fuel. So...if the air cleaner gets so dirty that it becomes a restriction the engine performance would decrease (since the air cleaner is doing the throttling now) but the air fuel ratio would not be affected thus , theoretically, the fuel economy would not change.

The exception to this would be if the air cleaner on a fuel injected engine got SO dirty that it restricted flow to the extent that the driver was forced to go to a very deep throttle opening or full throttle to maintain progress the system would see the high throttle opening and engage the power enrichment fueling and thus could get poorer fuel economy due to the added richness. The air cleaner would have to be so dirty in this case that it is almost inconceivable....but possible I guess.

There is no question that fuel injection systems are much more tolerant of a partially clogged/dirty aircleaner than carbureted engines.

Since most people consider "fuel economy" to be measured under driving conditions that are typically not very aggressive the issue of air cleaners, cold air induction systems, etc... does not come into play. At "fuel economy" conditions, cruising down the highway, the engine is heavily throttled and is operating at 10% of it's possible power or 10% of it's maximum airflow capacity. The air cleaner is typically sized so that it can flow the maximum possible air flow at full power/max RPM plus an allowance for dirt accumulation. So, a well designed and properly sized air cleaner system and air cleaner element will offer little or no restriction even at full power, 6500 RPM and a very visible level of dirt accumulation on an engine like the Northstar. Therefore, at the low air flow levels of fuel economy conditions there is absolutely no restriction from the air cleaner and thus no way possible for an air cleaner element or inlet air change to improve fuel economy.

I think the idea of a clean air cleaner element "helping" fuel economy comes from 30 or more years ago when air cleaner elements might have been undersized and carburetors were common. Then, heavy accumulations of dirt could cause the carb to run rich and deliver fuel economy.

Keep in mind, too, that we are talking passenger cars here. Truck engines that operate at very heavy throttle openings can run into excess fuel consumtion with dirt accumulation since the engine is pumping a great deal of air. Similarily, stationary power plants that are designed to run at max output can run into excess fuel consumption due to a dirty air cleaner element...i..e...your lawnmower will start to run rich with a dirty air cleaner element much sooner than your car will. The idea is the same but the relative magnitude of the problem is drastically different. If you look at the air cleaner systems on large trucks and industrial equipment you will often see a little pressure indicator that pops up when the air cleaner restriction reaches a certain point indicating that the element needs replacing or the system needs servicing.

One thing to consider is that a "dirty" air cleaner is very relative depending on who is evaluating it. To an enthusiast a slight dirty smudge on the paper element might be reason for replacement. To an air cleaner element engineer the element is fine until it is completely coated with a visible layer of dust/debris to the point that it is almost caked onto the element and continuous around the entire element surface area. I have seen elements flowed on a flow bench that were indeed caked with dust and they flow almost the same as a brand new element..... A slight smudge of dirt is absolutely NOTHING to the element. I would guess that 90% of the aircleaner elements replaced are done so needlessly. An aircleaner element filtration capability (it's ability to capture and hold smaller and smaller particles) actually improves as the filter traps more and more dirt. So...it is often hard to understand the claims of the aftermarket air cleaner marketeers regarding flow improvements, fuel economy, etc..... I think that they must run a hopelessly clogged, undersized element on an engine with a carburetor and then put one of their brand new elements in its place and base their claims on the fuel economy improvement seen in that single case.

If you are interesting in this at all you can prove it yourself very easily with a simple vacuum gauge. Plumb the gauge into the inlet system so that any restriction from the air cleaner system will show as vacuum on the gauge. If there is no vacuum then there is no restriction....period. Start taping up the element with duct tape and see how much area you can tape up before you start seeing restriction. It will surprise you.

On the other hand.....as high performance OEM engines reach 400 and 500 HP (and more) the amount of air that they can flow is very substantial so the air cleaner and inlet systems become more and more critical to the maximum power potential...and so is the dependence on a clean element."

Wow,Cool
I just keep learning more and more.
Thanx

You might toss in a new oxygen sensor. If it's bad the fuel/air ratio is thrown off, and not in your favor. If it's been bad for a while, the extra fuel going into the catalytic convertor might have caused it become overheated or obstructed as well.

You might toss in a new oxygen sensor. If it's bad the fuel/air ratio is thrown off, and not in your favor. If it's been bad for a while, the extra fuel going into the catalytic convertor might have caused it become overheated or obstructed as well.
It has 4 O2 sensors. That could get expensive.

It has 4 O2 sensors. That could get expensive.


As expensive as gas? I changed mine out and got 4 extra m.p.g. Didn't take long for a payback, really.

I don't know how expensive as I have never had to change one. I want to say I have heard someone mention $80 or so. That's $320. Better to diagnose it than to throw parts at it is all I am saying.