Motor sluggish and missing

Backfiring is usually associated with a lean condition. Is fuel pressure correct? Is this vehicle TBI and are both injectors working properly? If timing related it should spark rattle under load and the timing would be way out and not run/start well at idle

Hi,there are lots of hand computers on e-bay that can read the ECU and you can cleer old faults that are stored on the ECU.
hope this was some help.
good luck.

oxygen sensor

this could be a problem with engine managment you can get it pluged in to a diagnostic tool and they will give a fault code these can be purchesed from well known aution sites for 30 pounds

Well code 306 is for a miss on cylinder six and code 316 is for a miss detected on startup, which you probably already know.
Since I assume you have replaced the ignition parts for that cylinder,
the next component to check is the injector. You may want to check fuel pressure too.

I doubt the problem is timing. If that were the case, it would have the same miss at road speed.
Is the check engine light on ?

Most Lesabre have the 3.8 engine. The engine has a device called an automatic idel speeed motor or AIS for short. It's held on with two torx 20 screws and a 4 wire electrical connector on the side of the Throttle body. BUT BEFORE YOU GO REPLACING THIS DEVICE THAT CAN COST OVER $50. Try this first. Don't drive the car around this way just test it! Remove the big vacume hose from the power brake booster and put your finger over the end of the hose going to the motor. Have an asstant start the car and hold the gas so it'll keep running, trying to go as slowly as they can. Slowly remove your finger from the end of the hose and the motor should pick up speed. you should be able to get a good idle with your helper completly of the gas. if you do the idel motor is'nt working ( right) so your not done checking. Let's say you can't even get a fast idel that stays. The timing chain needs checked and other vacume or engine control codes need checked. Has the check engine light been on? ( with the engine running) . If it has the idel motor will set a code as will a cam senser and all the other sensers on the car. Follow code charts if a code is found. Most auto parts stores will check your car for free for codes but their recomendations very with the experience for the operator of the tool, and if they were good enough to make the mechanics money why are they in a parts store? If you can acheive a good idel playing around with the vacume hose then put it back on and go to the next step. Remove the Idel motor and clean it and the hole it came out of with oven cleaner. Just like cleaning an oven don't get it on you or the wiring part of the motor. With an OHM meter the pins will show two different pins hooked together ( two Pair) If you get only one it's bad. Every pin is connected to just one other pin. If not it's bad. You can get new cheap ones that are bad. I use cheap ones and cross my fingers. If they work the first time their good for a long time. Then you have to learn the idel. Try to learn the idel with the old one cleaned first. Start theengine and hold the idel with the gas pedel. after holding the speed at 2000 Rpm for two minutes ( a good engine will do this easy) let off slowly it should idel down slowly then (catch) and speed back up on it's own. If it does your fixed and go on your merry way.

Vacume leak or bad idle motor. With the car in park and running with the hood open listen for leaks moving your hand around the different hoses looking for a change in the sounds that you hear. The idel motor also makes the car idel up to 1500 RPM or so right at start-up then idel down, If it doesn;t the idel motor is likly. Check engine light is on when running? Test for codes, Follow code test charts don't skip any steps!.

weird must be a ecu problem

I can almost guarantee you that the gear position sensor is bad. I've replace 30 or 40 of these on 960's for these very same complaints! If you check the codes for the transmission, it will give a 3-1-3 or P0705 code. You will however need a code reader to check this on a 1997 model.
Check page 33 in this PDF for an explanation of the fault

Bret
RepairPal.com