91 240sx : Premium gas causing knock to be erratic???

[Sygerwulf]

As I posted a couple weeks ago my car developed a knock about 30k miles ago (roughly a year). since gas prices have been rather high and I've had job problems I've been running regular in it for a while. Now that gas has gone down some and I've got a better job I've been running premium in it for about a week and a half.

Anyway two days after I started running premium in it I noticed something funny in the knock. It would go from the normal sound to be almost completely unaudioable and could stay like that for an indefinate amount of time, then for no apparent reason will start knocking again. I've also noticed that when you give it a steady rev (the knock starts at around 2500rpm) that it isn't constant, but rather irregular, like *ticktickticktick...tick tick..tick....ticktick.....tickticktick* and the volume of it varies during this period too.

Now I was pretty much convinced that it was either a rod bearing or wrist pin, but now I'm just damn confused. Why would 93-95 octane make any difference in a knock from a bearing or wrist pin? Anyone with any ideas would be great. If this is just another effect of said problems then oh well, I'm about to buy a new car anyway but I would like it to last until then.

[NISTECH]

year/model of car?

[Sygerwulf]

it's a 91 240, sorry I keep forgetting this is a general post.

[180fan]

betchu it's the timing chain guides.

[Sygerwulf]

Can't be, both top guides are gone, it could only be the lower chain guide. But besides that I can kill spark and fuel to #2 cylinder and it stops knocking so I know it's got something to do with the #2 cylinder, the question is why would a change in octane make any difference for something like that?

[s13sr20chris]

spark knock. you said irregular rattley type of sound? your pinging. you should ping more on regular and less on premium. heres a good decarb method(in case its because of carbon on pistons). pull plugs.pour in 2 or 3 teaspoons of top engine cleaner or seafoam. put rags over plug holes and let sit over night. in morning, crank over and let carbon blast out of plug holes. install plugs. start and drive around. she should smoke and stuff. when shes good and warm, unplug a vacuum hose close to the throttle body but not in front of it. that insures you are getting to all cyls equally. let it suck up some top engine cleaner or seafoam. not too fast though. she could stall or lock up. the more she gets teh better. pop in one can of high quality injector cleaner like bg 44k or chevron techron and go to gas station. fill up with premium. that should help out. if not repeat. when done, pour a quarter quart of atf and and equal amount of kerosene into your oil filler. let engine idle for 30min. do oil change. now she should be really clean. all this info came from a proffessional chemist and oil guru.

[Sygerwulf]

2 or 3 teaspoons for each cylinder or total? I'm guessing each cylinder

[s13sr20chris]

each.