GM-GMC: 1997 Gmc 3500 van Hard clunk into OD, high octane gas, gmc 3500

Question
Hello,

 I just purchased last night a 1997 GMC 3500 Cargo van 5.7L 106,000 miles . I took it to get gas, and at highway speeds it would intermittently slam into and out of Overdrive. I called the previous owner and he stated that it would only do this if low octane gas was being burned and not 89 or 93.  Any help with this would be greatly appreciated. also all other gears shift very smooth no problems there. Thanks again

Answer
i have never heard of shifting issues due to gas octane to be completely honest. if you want to test that theory by putting either high octane gas or octane booster i suppose you could do that.

i personally have seen this when the engine has low power or is gunked up and having trouble pulling up hills or inclines mine did this until i replaced the knock sensors because they had gone bad. when the knock sensors are bad the engine goes into a stale mode and has no power and has to work hard to get up inclines.

given that its only od with the issue i wouldnt think its the ujoint but you could still have it checked.

clunks in gm trucks and vans are not uncommon but it shouldnt be enough of a kick to feel it.
you should ask the previous owner if the trans fluid has ever been changed and when and if its never been get it done this might help with that issue. or maybe the fluid is low.

i wish i was closer i could probably help more


2005 gmc sierra z71 4x4 145000 miles is what i have