My Bonne idles very rough, and gets about 14 mpg. My LeSabre gets at least 30, and has 60k more miles. I swapped out all of the usual sensors between the 2 cars and no difference. Runs like a car with a normal dizzy would if it was was off.
Any suggestions?

'89 LE.

when is the last time you have done a tuneup?

plugs
wires
PCV valve
fuel filter
air filter

please put your year and trim model in your signature so we know what car you have

• Idling rough points to worn out ignition parts, plug wires most likely. You can't tell if the wires are bad just by looking at them. Okay, sometimes you can tell if they are bad just by looking at them, but you can't always tell they are good just by looking. If they haven't been changed recently, that is where I would start. If they have been changed, test them to be sure they are good.
• Tire pressure is the easiest thing to check and the thing most often neglected.
• Replace the rubber vacuum lines including the ninety degree elbows and all the connectors. If they are original, they should be turning to dust by now. A vacuum leak in one hose can affect everything else that uses vacuum.
• Swap your coils and ignition control module (ICM) to the later model ones.
• Check the mass air flow sensor (MAF) to be sure the little wires are intact. These can also fail internally, but I've never experienced that personally. Mine seem to break off the wires before that happens.
• While you are checking the air filter, make sure the intake air temperature sensor (IAT) is still connected and is not corroded.
• Dragging brakes will obviously make the engine work harder. Calipers and hoses should be checked and replaced or maintained as necessary.

After the routine tune up, if mileage is still less than it should be, consider the oxygen sensor. These need to be replaced at about 60K miles. They cause poor mileage when they wear out.

Also, the gearing may be different between the Bonneville and the LeSabre.

Thanks, I have changed all of those sensors, will try plug wires and PCv valve next. Would the PCV make that much of a difference?

PCV will not make a whole lot of difference unless it is totally plugged.

the type / heat range of spark plug will and if the injectors are clean and firing properly. These cars were qualified by the EPA with standard plugs and I find I get the best mileage with a copper plug going one heat range hotter.

Also on our cars the front disc calipers tend to stick and kill mileage.

Yeah I pulled some blackened plati'* out of it. Got plain old copper now. Idle is still horrible, I'm still troubleshooting.

The rough idle is telling you something is amiss. Either a vacuum leak problem, EGR,
IAC, or MAF, PCV malfunction.
Also check the fuel pressure at the rail and run a bottle of injector cleaner.

I'm betting it'* the plug wires or the coils.

thats a good point. coils. do you know if all plugs are firing? coils could be checked with a standard ohm meter, and you could test the wires with a pencil type spark tester at each plug to see if it flashes.