1992 Buick Lesabre Idle Problem/ Bad gas mileage

Check to see if your third break light is working, usually cruise control and third break light are on the same circuit. Bad gas mileage could be numerous. Does it smell like raw gas out the exhaust? Could be O2 sensor going bad.

First thing, fix leaking exhaust, this can cause erratic idling...second thing, remove and clean IAC valve and MAF sensor..dry and replace, also try putting some SeaFoam in the petrol, engine oil and air intake, let sit for 10 minutes then go for 10-15 min drive...suggest you change engine oil shortly after driving about 5000kms, if the oil is old..

Did you get any codes on the misfire? If you did not, test ignition coils, test vacuum, test fuel injectors, verify correct plugs..That's right, anything from a fouled spark-plugs to burned cylinder head valves can cause a misfire. Whatever the root cause of the misfire, the bottom line is that one or more cylinders are not contributing to total engine power output. This leads to power loss, rough idle, unstable RPM and bad gas mileage...cheers

You first need to have a mechanic attach an oil pressure gauge to the engine to check oil pressure at various engine revs to see what is actually happening.

This is a very old car with no doubt a very high mileage and a worn engine. A very worn engine even with fresh engine oil just will not hold oil pressure whether the oil pump is running well or not.
Furthermore, if the oil in the engine is old and diluted and needs changing you will typically get a lower oil pressure reading at idle.
As a first step you should change the oil with a quality brand(only use the correct grade and type) and the oil filter. Then re-check for the problem.

The idle speeds software driven But vacuum leaks can raise idle speeds or weak mounts cause the shift in position giving you the jolt.

REMOVE UPSTREAM 02 SENSOR INSTALL VACCUM/PRESURE GAUGE START VEHICLE IF THERE IS MORE THAN 1 PSI EXHAUST HAS A RESTRICTION, CAT/MUFFLER/COLASPED IN EXHAUST PIPE INTERNALLY

well 3.8 engine is not suppose to burn much gas.check for low tire pressure.if engine got many miles on it it will burn more gas,if you live in city making short stops burn more gas,if you driving a lot of roads with steep hills and mountains car will burn more gas.lead foot quick acceleration speed driving burn lots of gas.

Could be two things, your oil pump pick tube is clog, oil pan would have to be drop to check, or your camshaft bearing worn causing low oil pressure,I would not say piston rod bearing or main bearing, cause there's no knock noise. You will have better oil pressure when cold due to oil much thicker. so I would forget about a bad oil pump.

Valve covers are leaking on to the exhaust manifolds. Check for a vacumn leak

Sounds like you might have an oxygen sensor problem. Your catalytic converter(s) maybe defective or dirty. That in turn can dirty up the oxygen sensor. A defective oxygen sensor will send faulty info to the computer, interpreting the fumes as cold (even when the engine warms up). That causes the computer to adjust the fuel mixture to very rich, giving you poor gas mileage and the fuel smell. Hope it helps.

i heard of a cam position sensor going bad causing really bad gas mileage--5 mpg,or it could be the fuel regulator bad.i wouldnt run it like it is,change parts ,the the oil before trying to start it to avoid a fire,be really careful!!