RPMS keeps shifting during drive in 1.5k-3k range

[v01dy]

I just bought an auto stock 97 Spyder GST recently and I've noticed that quite often, when I'm in either 2nd or 3rd gear, and maintaining a light but consistent pressure on my acc pedal, the RPM shifts up and down quite often, as if the ECU was trying to decide on a gear, or whether it should shut off the O/D. I've noticed that this occurs most often when I'm travelling downhill, but it has also happened on level roads and uphill. The transmission is smooth during acceleration and the problem only occurs when I'm trying to maintain a constant speed. I've recently replaced a faulty catalytic oxygen sensor after driving around with it for almost a week. I'm thinking that this may be just the ECU re-adjusting itself after a period of bad inputs from the oxygen censor but I'm afraid it might be a problem with some other part of the vehicle. Can anyone shed any light on my problem? Thanks in advance.

 

[blueberry_dsm]

mine does something like this, but on mine after a little bit of this, i will get a check engine light, and i wil have a misfire. when it happens, does your car kinda jerk when this happens?

 

[v01dy]

yeah it jerks abit when the rpm increases. I've just had the car for 2 weeks, and the check engine light came on during the first one. Sent it to Mistubishi for a diagnosis and it turned out to be just a faulty oxygen sensor. I had that fixed but the problem still remains so I have no clue what the problem is. What's a misfire by the way? I don't think I've experienced anything like that.

 

[blueberry_dsm]

sounds like its a problem with the EGR system. it probably is clogged passages from the egr to the intake manifold. it could cause the the car to run lean, and which would definitaly trigger a faulty O2 sensor code. just replacing the 02 sensor wouldnt fix it. u need to have the EGR passages cleaned out, they r plugged with carbon build up. i would say im 95% sure this is the problem

 

[spyder97gs]

What you are describing is nothing out of the ordinary. You might as well get use to, because you won't be able to stop it. That is typical of a DSM transmission, and although as nerve-wrecking as it may be, it's just doing it's job. When going downhill in my auto and it shifts out of OD, I just pop it in neutral and back into D and it shifts back into OD. May not be the best thing for it, but I don't mind now see'ing as i'm swapping a MT in. Good luck.