My Corrolla Won't Reverse!

So, I don’t know if any of these problems are related but a few months ago, I accidentally put diesel fuel in my car. We got it flushed out (used some fuel injector cleaner and filled the tank up to dilute what fuel was left after i siphoned it) and my car was doing fine. Then my car started running really shitty, just taking forever to accelerate, revving really hard when I gassed, but not going fast at all, so we changed the spark plugs, oil, and transmission fluid and now it drives forward just fine, but will not reverse. all it does is move forward a bit then stop. Oh, and it will drive in neutral up to 20-25 mph. Anyone have the slightest idea of what might be wrong??

What year? How many miles?

“it will drive in neutral up to 20-25 mph”

I’m having a hard time understanding what the above statement actually means.
Do you mean that you can shift from Drive to Neutral, and coast at those speeds?
Or does it mean something else?

In any event, based on, “taking forever to accelerate, revving really hard when gassed, but not going fast at all”, it sounds to me like a case of slippage in the transmission. If this Corolla (of unknown vintage and unknown odometer mileage) has more than 90k miles on the odometer, I believe that a rebuilt transmission is in the OP’s immediate future.

Just for future reference, the time to change transmission fluid is not when trans trouble appears. Instead, your transmission needs preventive maintenance, i.e.–changing the fluid & filter every 3 yrs or 30k miles (whichever comes first). I have always followed this plan, and my cars have never had a transmission problem, even up to 14 years/180k miles.

Sorry for the bad–and expensive–news.

If I were to bet on which clutch(es) is/are damaged I would choose the high and/or forward clutches. You let the transmission slip to long probably because of low fluid level or clogged fluid strainer/filter.

All that slipping has warped the steel and friction plates so that these clutches no longer disengage even though the hydraulic pistons are retracting(?). Heat does wonders to precision steel and aluminum parts.

You are probably looking at a rebuild at a minimum. However, you might be better off going with a recycled (used) transmission. If you do, drop the pan, change the filter, drain the torque converter, flush the transmission fluid cooler, and add the correct amount and type of fluid. Should get you a few more miles down the road.