Knock Knock

[JOHNSUAVE]

My car rides excellent but the last couple of days she has been knocking when I start her up and when I cut her off....I put some STP fuel injector and carb treatment in there and it has been doing it since could this be the cause

 

[68 Coop]

Sorry, but could you be a little more precise??? Are we talking about the heads, or internally??? Just want to be sure before I look like a dumbass, that happens enough as it is.

 

[1966conv]

Hate to sound like a butthead, but give some info about your car, motor, tranny, any modifications from standard.

It could be anything from bad gas to throwing a rod.

Basically most(not all) of the stuff you add to gas is alcohol based. It's gonna mix with any moisture and then mix with the gas to burn. You could have gotten a slug of water in at one time.

 

[JOHNSUAVE]

everything is standard just bought the car about two months ago it has a rebuilt standard 200 6c, automatic trans, I havent made any mods yet....Im thinking its either bad gas as well as the addition of the fuel injector I use STP gas treatment on the regular it never gave me a problem

 

[Pakrat]

I tend do believe that as long as it was a fuel injector/carb treatment that the probelm is only a coincidence. Are you buying the cheap 87 octane? You do know that the original leaded gas these cars took was even higher than the best 94 available today right? While each car is different and any car new or old should try all at one time or another to determine which octane it runs on best, most of the older cars do not like the cheap stuff. Also, if when rebuilt you car did not have hardened seals put in then you may want to add some lead substitute at least once a year, just to keep things moving smoothly.

It is highly possible that the knock does come from a loose rod or even an exhaust leak as well. Without a way to hear it for ourselves, it's hard to say for sure.

My vote is either try better gas if you aren't using it or even try adding some octane boost to what you have left and see if it goes away.