Hi all,my stepdaughters totally stock 91 spirit 2.5 normally aspirated TBI auto trans is down on power and stumbles /shudders on light throttle cruising down the highway and nearly dies under load ,IE climbing a hill,if i floor the throttle it will usually kickdown the trans rev up and go but with lite throttle its dead,no fault codes other than occasional code 21 (oxygen sensor),has 165 000 kms on it(100 thousand miles).new spark plugs,air filter fine,good battery etc

In the spirit easy things first check for vacuum leaks. OK at WOT and rough at cruise suggests this. A flexible rubber air intake hose for example... or any other intake or vacuum line or gasket.

Check vacuum and be sure timing belt has not moved a tooth or two. One tooth will change vacuum enough that the map sensor starts pouring in the fuel.

When you put the plugs in, did you change the cap, rotor and ignition wires as well? If not, I would install those, and check the fuel filter and rubber hoses as well. If the timing belt has not been changed, its possible it has stretched and needs to be replaced.

There was an issue with the distributor pickup failing, but those usually just died whenever they felt like it, and sometimes would restart instantly, and sometimes would take hours before they would restart.

A stumble/hesitation sounds more like a wire/cap/timing issue, or a fuel flow issue. Those cars, with that motor, were pretty solid runners, and 100,000 miles is about time to replace original ordinary wear parts if they are still present or are of unknown condition. Gene

vacuum hoses etc all look fine and i did inspect the cap and rotor,cleaned them up with some emory cloth,havent checked timing yet.the car seems ok when cold but worse when at operating temp

You say it gets an 02 sensor code at times ? Well the 02 sensor can cause some bad drieability problems sometimes. When you floor the car the 02 is ignored and the car runs on programing from the other sensor readings (MAP , TPS , Coolant , RPM and others) as you said it runs good when floored and when the eng is cold the 02 is ignored at first until the eng hits about 120 I believe and the coolant sensor adds fuel like a choke which helps it run better cold. I would want to put a scanner on it and see what the 02 is reading when it runs bad. Ron

ron mentioned the coolant sensor. i ran into a deal one time where the coolant sensor had film around the end bulb [lime deposits and anti freeze film] where it was in contact with the coolant. this film tricked the coolant sensor into a false reading, thereby causing havoc with the fuel curve and severe driveability issues. swapped out the coolant sensor and all was well. remember that some applications have a separate coolant temperature sensor for the gage reading in addition to the coolant temperature sensor for the computer function. some are combo sensors. i don't know what you have, so if changing the sensor, make sure it is the one for the computer function.

Just one thing I ran into on a 2.5 take the cap and totor off and lift the plastic sensor plate and check to see if the plastic parts on main shaft of dist are still there and functioning I had to JB weld mine back together as mine was doing the very same thing { Timing was moving all over the place }

Check the cam and distributor timing and go over the basics (plugs, wires, etc) they are cheap, I'd just replace.

Pickups work or do not generally speaking, some are heat sensitive and will die when heated up.

The TBI fuel injectors are notorious for being junk but usually just run a little rich and fail smog.

Thanks for the help guys,changed the o2 sensor and a sketchy looking coil wire,plugs and new battery and its good to go now.i "flipped the plug wire terminals inside the cap as well and checked the shutter wheel,all is well!

X2

The shutter wheel in the dist is a known for coming loose from the plastic base.

I have seen that also and I agree that of course that can make it run bad or not at all but he said it ran good at wide open throttle and seemed to run good cold. If this was the problem I would think it would effect it cold or hot or at WOT which is why I did not suspect that. And he did say it had set an 02 sensor code. But lets hope he has it fixed as it sounds like he does have it fixed now since he replaced some parts. Ron