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'99 Grand Marquis MPG down from 28 to 18

My MIL has a '99 Grand Marquis with about 112K on it. Don’t know which engine, but she’s complaining that in the last year or so it’s gone from about 28 MPG on the open road to about 18.

My first thought was that it wasn’t going into overdrive, but I’ve checked, and it is. Transmission operates normally. Engine runs flawlessy. No brakes dragging, tire pressure’s normal. Plugs and wires are new. As I said, it runs flawlessly. No CEL.

It’s the perfect platform question for Caddyman and ok4450. Any thoughts on where to look for the missing MPG?

Thanks in advance, JayWB

Assuming the tire pressure is ok maybe the thermostat is stuck open, or partially open.
It wouldn’t be a bad idea to scan the car for codes as they may exist even when the CEL is not illuminated.

Something like a leaky fuel pressure regulator could cause a mileage drop like this but this should cause it to run a bit ragged, set the CEL, or belch a little smoke.
That’s just a wild guess. Hope it helps.

(I saw a show on TV over the weekend about the Clunker Car program and they were about to crush a slick Grand Marquis of this vintage. It’s a rotten shame a vehicle like this would be considered a gas guzzler.)

Thanks ok,

I’ve got an infrared temp sensor; I’ll check the coolant temp with it. I’ll have a look at the vacuum hose working the fuel pressure regulator too.

It just occurred to me: as I recall, shortly before this started the heater hose fitting blew out of the intake manifold. I believe she had the manifold replaced by a local shop. I wonder now if everything got put back together right …

ok4450 is dead on about a code existing even when cel is not lit up.

so many variances. what are her driving habits? people do change. i used to drive like h*ll and now i drive like an old man and I’m far from it, almost 30.

could be that the car is struggling to breathe, check air filter? fuel filter? any hint of smoke out the tail pipe?

let us know of your findings please.
Good luck, partner!

You might want to have that manifold checked by a good mechanic. If it’s leaking the computer is going to have a hard time trying to keep the engine running in tune. Fuel economy will suffer as a result.

One thing I’d try is cleaning off the MAF sensor. These cars are sensitive to how dirty they get, and it doesn’t take much. It’s located in the air intake box between the filter and the flexible tube to the intake. Spray that down good with several coats of MAF sensor cleaner (can be had at any parts store), then hook it up and see how she runs. It’s also recommended to unhook the battery and let the PCM relearn after this.

Yes, clean the MAF and perhaps it’s time to replace the FRONT oxygen sensors…They have a design life of 100K miles. These are somewhat difficult to change, the sensors themselves, but even worse, the wiring connectors which are secured to the top of the transmission are almost impossible to get to…Successful mechanics remove a few things by the firewall then, in the prone position over the top of the engine, reach down the bell-housing and disconnect them by FEEL…

It is the oxygen sensors that send the signal to the ECM that controls the fuel mixture and thus the mileage…if they get out of calibration (from age) the fuel mixture will not be correct…For the CEL to come on, they must fail completely…Your car has 4 (ouch!) of them…Bosch makes exact replacements…Again, the front ones are the critical ones…


Thanks, C-man. I’d considered the oxygen sensors, but they always get a bad rap. A lot get changed for no good reason, and I suppose a lot of bad ones don’t get changed soon enough. It’s always a guess with them until they crap out completely.

I’ll save that for a lst resort, and I’ll give the MAF a cleaning.