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Very Very Slow Acceleration

I came off the turnpike the other day stopped at a light and my car, 2006 Hyundai Elantra 175000 miles, started rolling backwards. I put my foot on the gas and there was no movement. Put the car and park and it wouldn’t start. I pushed it into a gas station and the attendant (attendant, not mechanic there was no service station) changed the oil and added some coolant. The car then started up fine and I was able to drive a quarter mile to work. When I was driving home I immediately noticed that there was a change in acceleration. It takes forever for the car to speed up… Pedal to the floor and it is lagging greatly. It seems to be going up to 4500 rpms just to reach 40 mphs at a slow pace. Once the car gets up to 60 mph the rpms drop to normal. I haven’t gone faster than 60 mph in it since this happened, but it seems to handle fine at that speed. The issue is getting up to that speed. I don’t know if it is related, but the air conditioning is very weak since all this happened. I have to turn it off when I’m accelerating now. I’ve been planning on buying a new car soon, but I want this one to last just a little while longer. Any ideas would be greatly appreciated.

If your car has an automatic transmission, this is almost surely a case of a badly slipping transmission.
If it has a manual transmission, then it is a case of a badly slipping clutch.

The attendant’s actions–changing the oil and adding coolant–might have been necessary, but for the symptoms that you mentioned, those procedures would still be comparable to rearranging the deck chairs on The Titanic while it is sinking.

If you have an automatic trans, my advice is to first check the level, color, and odor of the trans fluid, and then report your findings to us. You may well need a trans overhaul, but checking the trans fluid is still an important first step.

If you have a manual trans, there is almost surely no alternative to replacing the clutch.

I expect VDC’s diagnosis above is probably the case. But be sure to check the battery and charging system for proper operation, especially if you have an automatic transmission. Modern automatics use electric activated solenoids on mode-changes and if the battery voltage is too low they won’t perform the needed action, and this could be a symptom. The first thing to do in any event is to read out all the stored diagnostic codes from the engine and transmission computers.

I agree with VDCdriver that this sounds like a slipping automatic trans or failing clutch if it is a manual.

If it’s an automatic you might pull the dipstick and look at the transmission fluid on the stick. If it’s black or dark brown that may answer the question.

I have to believe the check engine light is on and the system is in limp-in.

I fully thought VDC’s diagnosis was what was wrong with the vehicle, until Nevada’s comment. Now I see it is possible the high rpms are because of limp mode causing the trans to be in 2nd gear.

@MikeJones: Is the check engine light on?

I also found it strange that the vehicle died & wouldn’t start which I didn’t think would be related to a slipping clutch or slipping transmission . The input & output sensors are very close together & use identical connectors on a transmission I’m familiar with & I once connected them backwards by mistake & the transmission stayed in 2nd gear .
The rpm’s were high for the speed I was traveling but I wouldn’t say acceleration was very slow . It also didn’t cause the engine to die or not start .
Perhaps a low powered vehicle might be slow to accelerate from a stop if taking off in 2nd gear . Still doesn’t explain the vehicle dying & refusing to start .

The OP stated that he was putting the pedal to the floor to try to get more power. When you do that the throttle position sensor tells the transmission to downshift. I wouldn’t be surprised if he had a clogged catalytic converter.