ok guys i've got the classic (my gf car is fucked up and i have no clue what it is?) the car starts bucking every once in awhile usually in lower rpms, no CEL nothing. finally a CEL came on, i checked it and got a vehicle speed sensor malfunction, after replacing that today it still does it. this is on a 96 nissan sentra obd2 so i thought it would have to throw me a code but nothing. what could cause this w/ out a CEL please share ur ideas i'm stumped.

 

Dirty fuel filter?

 

just replaced that also. also had the battery and alternator tested both fine. looked at cap and rotor=fine
changed spark plugs and wires look great.

 

sounds like maybe tranny? is it auto or manual?

 

its a 5 speed manual. the weird thing is that her speedometer sometimes got stuck temporarily at around 40mph so i thought it had to be the VSS, but when i replaced it it still stumbled. now i'm thinking its the wires themselves and not the sensor. thanx for ure input guys. lmk if u have any more suggestions

 

Have you checked the fuel pressure out. Have you actually looked at the plugs to see if they are cracking anywhere near the insulation or if you see any carbon tracking. Sounds like an ignition problem in the secondary part of it. Did it just do it all the sudden or did it just creep up on ya.

 

yeah i replaced the plugs first thing the old ones looked fine.i figured that if it was something electrical then it would throw a cel? when it starts to happen your foot will be on the gas and then suddenly theres no power, then the power will come back. i was also thinking possibly a fuel pump malfuncion. the problem only started happening when it got real cold out but it doesn't happen every time the car is driven.

 

The only thing i can recommend and hopefully you know someone that works at a mechanic shop with a snapon scantool and will look at the datastream from the scantool and see he sees anything weird on the values that the computer is seeing.

 

I don't know exactly what your describing as "bucking" but my honda started doing something similar there for a while. It started sputtering and missing. The tack and speedo was jumping around and acting all stupid and it ended up being a loose ground. May not be whats up with yours but its a thought.

 

does the symptoms go away at higher rpm's? when its cold? how cold? maybe frozen gas lines?