I have a 2000 Gmc Savannah 1500 4.3l. The van was shutting off and the check engine light was on. After scanning it said a bad down stream 02 sensor and bad Cat. So I changed the cat and 02 sensor. Now the light is off but the van dosen't run or shift right. The fuel gauge vibrates, the oil pressure gauge jumps and the speedometer dosen't work. It runs fine in park and neutral. When you drive it no matter how much gas you give it it just shuts on and off like you were turning the key on and off. I changed the battery, vehicle speed sensor and it runs the same, the fuel pressure is around 60lbs?
I did some testing and the fuel pressure remains the same as well as the spark when the van dies. It shifts strange, it does'nt downshift or up shift properly. The Abs light is on. It runs fine in park and nuetral. It has the 4L60E transmission in it. Any help would be great I am out of Ideas.