1997 Pontiac Sunfire surging at highway speeds

Tiny
MPUETZ84
  • MEMBER
  • 1997 PONTIAC SUNFIRE
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 112,000 MILES
Ok I bought this car for my little sister had to put a new transmission in the car the case was cracked and locked up. I have been trying to figure out its poor performance for the last month. The car gets about 13mpg which I know is no where near what it should. I have a 96 cavalier that gets almost 30 and its in alot worse condition. The problem is that at 55 and higher the motor loses rpm and reves up and down constanly which makes it almost worthless to drive on the freeway. I have removed the converter to it dident look the best untill I have this figured out I took it out and straight piped it to see if it was the problem. I am so lost on what to try I am hopeing that someone will have had this problem and know exactally what to do to fix it.
Thursday, September 2nd, 2010 AT 10:32 PM

2 Replies

Tiny
MADMIKE1735
  • MECHANIC
  • 951 POSTS
Almost sounds like a TPS acting up. While your driving highway speeds, your foot isnt really moving on the accelerator. If you floor it does it still fluctuate rpms? Also, did it do this prior to the tranny being replaced? May be a speed sensor issue. Also, when was the last tune up done? Plugs, wires, fuel filter play a big act in how a car performs. Is the check engine light on? If so, have it scanned by your local parts store, and let me know what codes if any show up.
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Thursday, September 2nd, 2010 AT 10:39 PM
Tiny
MPUETZ84
  • MEMBER
  • 1 POST
Ok I thought it might be the TPS but wasent sure dident want to replace the sensor if that wasent the problem. Yes keeping the same pressure the rpm will varry by up to 500rpm. I dont know weather or not the problem was there before the trans swap I bought the car with a bad trans thought ok quick I can change the trans and she will have a good car. Tune up was done same time as the transmission plugs, wires, pcv valve, fuel filter. The check engian light is on the codes are circut fault coolant temp sensor, circut fault cooling fan, mutipul cylinder miss fire, and bank one sensor one lean condition. The car develped a stalling problem I changed the TPS with one that I had off a 97 caviler same motor it fixed the stalling problem and seemed to help the stutter but dident fix it. The bigest problem is that my sister is not wanting to put any more money it to the car untill I am pritty sure that will fix the problem. Thanks for your help
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Friday, September 3rd, 2010 AT 7:58 PM

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