Not sure if you mean 3rd to OD (upshift) or OD to 3rd (downshift, more common).
Upshifting - Do you have all your gears or will it try to shift to OD but not engage? Premature engaging? This could be a bad 3rd-OD clutch/band (no engage). It could be too much fluid or an over-adjusted band. Either way, it doesn't sound like the tranny shop did much more than a fluid change, maybe a flush.
Downshifting - If this is what you meant, your transmission may be fine. You could have a bad seal, clutch, too little/much fluid, and/or a bad torque convertver (doesn't convert enough torque). This would also mean that the tranny shop did little to nothing for your car.
Most likely (and easiest/cheapest to start with) your bad spark plugs/wires, fuel filter, etc. Are tryin to tell you that the previous owner neglected the car. My '94 Bonneville came to SoCal from Pennsylvania by way of Ohio and is pushing 165,000mi w/no rebuild and all my engine has is a minor leaky valve cover gasket. These engines last as long as they're taken care of and you can usually tell pretty quickly by how much you spend on repairs. To get a peek at how the car was treated replace your valve cover gaskets. Look for build-up, sludge, etc. And also smell around. A "burnt oil" smell and/or a "rich-burning-fuel" kinda smell is bad. Sludge/gunk = insufficient oil changes = major problems in your future and at least part of a reason for a lagging engine.
Otherwise, and possibly additionally, if your fuel filter was so crudded up I'd take a look at the rest of your fuel system to see what else has crud in it. When you're going 45-55mph you're probably just starting 3rd gear, no? If you are then the engine -not transmission- seems like the likely culprit as any sluggishness will show itself mostly in the low RPMs of the upper gears. If this is the case you should see even more sluggishness around 65-70mph or whenever your car shifts to 4th. But if the sluggishness is too severe it will simply slip out of this gear cuz the engine is unable to produce the power needed. Try to accelerate harder so your RPMs go higher than usual in 3rd before switching to OD. This should get you going in OD and high enough on the power band to sustain it if there are no tranny problems.
Hope i've been helpful (and not too confusing or long-winded :)
Friday, June 13th, 2008 AT 5:09 AM