MPG Dropping rapidly!

[MarkV]

My friend's 1986 Oldsmobile 98 Regency with a 3.8 6 cyl has a problem. The MPG is dropping rapidly! We have no idea as to the problem, it is now down to 16 mpg which is about 4 mpg lower than it should be. What is the problem?

[MCHinson]

There is no way to diagnose it with the information given. I would start by checking the computer for stored codes. There is a good chance that there is a sensor that is bad. A plugged catalytic converter could be another potential source of such a problem. I doubt that underinflated tires would cause that much of a decrease, but while your friend is at it, check the tires for proper inflation too.

[Guest Mighty Mouse]

A clogged air filter would be another possibility

[Bill Boudway]

I doubt very much that the ethanol in today's gas has anything to do with this.

[jrbartlett]

Sounds like about the right amount of drop for an inoperative vacuum advance unit, assuming this car has one. The engine would still run smoothly, and the drop in power might not be noticed. I had a '73 Riviera on which the vacuum advance would fail about once a year. I would only became aware of it by checking fuel mileage.

[avgwarhawk]

I don't believe the HEI on this olds has a vacuum advance.

[rocketraider]

Yup- completely electronic distributor. All advance and spark functions computer controlled. See if it has set any codes- O2 sensors are notorious for failure and causing the electronic brain to go into limp-home mode. You may have a slightly misfiring plug too.

Have his driving habits changed any?

Try changing brands of fuel. Any gasoline hot with ethanol is going to drop nearly 25% fuel mileage, which is consistent with what has happened. Though I think that car should be getting up around 23-24 mpg easy enough, upwards of 30 mpg in steady-state highway driving.

[joe_padavano]

There are dozens of possible causes, and most of them will NOT set a code in one of these older OBD I computer systems. The factory service manual has a well detailed section on driveability issues and what to check.

[old car fan]

The foot?

[NTX5467]

Look over by the power brake booster and find the vacuum hose junction in the line that is the check valve AND vacuum tee for the vacuum going to the hvac system. As the main vacuum feed is from the intake manifold, the minor crack these items tend to develop will cause a vacuum leak into the intake manifold. It's not a big one, which the ECM can cover, but one none-the-less, which can affect MPGs. A main item is that as the vacuum to the hvac system is decreased, the system goes into the "default mode" of air coming out from the floor and defroster vents ONLY, regardless of where you'd like it to come out. The idle might be just a hair rougher, but not enough to really notice (as the ECM is trying to cover the additional leanness with more fuel. This was a common failure point as the cars aged . . . really, almost every other GM car which used this check valve back then. It was a dealership item until the HELP! vendor picked it up. Now, you'll most probably find it on the HELP! rack at the auto supply. Not terribly expensive.

Looks like a plastic vacuum line with a check valve (about the size of a dime, and a slight cone shape) with another vacuum line which tees into it on the longer side of the line.

Usually, a hair line crack where the two vacuum lines come together.

Just some thoughts,

NTX5467

[Rusty_OToole]

Probably something the matter with the engine but could also be low air pressure in the tires, brakes stuck, different driving habits etc.

This seems to be the day for questions the oldest man in the world could not answer without a crystal ball. But the questioner could figure out for himself with a repair manual and some common sense.

[Larry Schramm]

As Joe Padavano says, there are a number of items that can cause this type of mileage drop off.

For a few of the culprits, it might be the engine coolant sensor, dirty air filter, mass air flow sensor or something in the air stream, leaky injector(s), bad spark plug(s), bad spark plug wire, intermittent bad coil, bad fuel pressure regulator on the rail, ambient air temp sensor, and the list goes on. Depending on the severity of the problem, the check engine light might not come on.

[61polara]

Wes,

I would start with the basics. Spark plugs and wires and clean the injectors. Check the air filter. I'm assuming the check engine light is not on, since you didn't mention it. If no improvement from these, then move on to the more advanced areas.

[Dave@Moon]

There are dozens of possible causes, and most of them will NOT set a code in one of these older OBD I computer systems. The factory service manual has a well detailed section on driveability issues and what to check.

I had this happen to my 1982 Subaru BRAT (luckily the thread http://forums.aaca.org/f169/photo-without-caption-4-5-2012-a-325808.html reminded me of it). My gas mileage dropped suddenly about 20% with no other symptoms (driveability, performance, codes, state of tune, idle characteristics, etc.).

In my case it turned out to be a stuck exhaust gas recirculation valve. A friend who's business worked on a lot of 1980s Subarus at the time told me to check it based on his experience. I guess they were prone to it. As soon as I replaced the valve the problem vanished.

Be sure to find the problem and fix it ASAP before the car is driven much like it is. Running that much extra fuel through the system is likely overheating and/or clogging the catalytic converter rapidly.