Got a difficult one for ya. Just recently purchased a 94 s10 blazer 4dr 4.3l that didn't knock with 225,000 miles (A lot I know), but intrestingly did not smoke a bit. And the previous owner had replaced the coil, plug wires, and all of the plugs except the #3 plug.

Got it for a very good deal. Had been sittin up for about 2 years, and the whole fuel system had gone to crap and wouldn't star Had to replace the fuel pump assembly and fuel injection spider. Fired it up and ran like a champ.

Afterwhich I immedietly had the oil changed. Used Castrol GTX 10W-30. Ran it for an hour or two, rechecked it no problems. Only a very minor oil leak and trans leak.

A couple of days later on the way home from work I purchased a bottle of fuel injection cleaner.( I know that I have new injectors but I imagined that the valves and inside the combustion chambers were probably pretty carbon coated.) And went to my local perferred gas station(Mobile) and purchased 3/4 of a tank of 97 octane gas, just after adding the cleaner. I then made a few stops (about 20 min travel time) and went home.

About 4 hours later, I started it back up and noticed a minor vibrating sound like a heat sheild, but dissapperd before I could investigate. Proceeded down the street and it very quickly started to make a minor but omnious knocking sound, oddly i had normal oil pressure (40-60 psi) during this moment and normal pressure for the past few days. Parked it and shut it off for a minute to asess the situation. Started it back up to turn it around and proceed to get my butt back home.

When I did, the knocking sound got a lot louder what seemed like immedietely upon startup. The knock is not uniform and fades in and out, but mostly is very noticable. And is mostly present on deceleration and idle. The engine also seems to vibrate a little excessively at ilde while accompanying the rather loud knocking sound.

So in a nut shell the knocking: seems to fade out as the engine revs up, is loudest at idle, and is a very distinct knocking sound. Could this be a spun rod bearing?

I dunno what it could be. The harmonic balancer seems secure. Do you think that combining the injection cleaner and 97 octane fuel with a fouled out #3 plug could be the culprit? I wouldn't think so. Did some investigating on the search. Nothing difinitive. Any suggestions?

 

97 octane? You should be using regular gas, thats 87 octane here. 91 or 93 is premium where I come from - I imagine thats a typo on your part as I havent heard of 97 on pumps anywhere. And that #3 plug is really easy to get out from the drivers front wheelwell with a long extention. You PO didnt figure that one out I guess. Have you ruled out wore sway bar bushings, or something knocking in the suspention instead?

 

Whoops, I mean 93 octane. The knocking sound is definetly associated with the engine rpm. I checked the bushings on the sway bar and what I could see pretaining to the suspension. Everything seems to check out.

 

After firing it up and listening for a minute. I have a better understanding of the symptoms. All observations are made while in Park gear.

1) This knock/tapping sound is definetly coming from inside the motor.
2) It is not constant, and fades in and out at idle. Usually starts as a
minor tap and suddenly turns into a kinda light-medium knock. I then
tap the gas to get it to go away for a few seconds.
3) Knock usually dissapates upon application of throtle, but may still be
present.
4) Fairly rough idle compared to past few days
5) Exhaust is fairly rich smelling

 

Sounds too me you either have a worn cam lobe (high milage) worn or cracked bottom of a hydraulic lifter (high milage) a collapsed lifter (more likely the problem) a bent push rod, a cracked rocker arm or a rocker arm that is out of tolerance on the rocker arm shaft. Would explain the different sounds at diff rpms, rough idle, rich exhaust and normal oil pressure. The cheapest fix is a product that will clean and clear a lifter but at that high milage, I doubt that will work. Use a long alum tube or long screwdriver, put one end on the engine and an ear on the other and isolate where the knock is coming from and go from there. Good luck with it...

 

Mines got a leaking fuel system, and it knocks and runs rough because of it, and gradually gets worse. I dont run it much now though. May want to do a fuel pressure test as part of your diagnosis. Do the screwdriver method with your ear to try and determine what exact area that sound comes from.

 

Sounds good Magmotor and Ohsofly. That sounds like a logical possiblility. Got an automotive stethoscope here at work. Gonna bring it home and see what I can find out.

Have lots of expericence with automechanics, and have never heard or seen anything like this in my life. Will keep you all posted on my results as I find them..

 

Just went out and fired it up. Immediately it started to make that knocking sound and then it quickly dissapated (within 10 seconds after engine startup). Ran it in park for about 3-5 minutes and it hasen't come back. Perhaps a stuck lifter?

Very odd that it would go away so abruptly. Gonna keep a close eye on it for a while.

 

Are you sure it isn't that extra premium gas you put in there... The experts say its only best to run the octane for it. The higher octane takes more heat to ignite and since it started right after filling with premium I'd say run that tank of gas out, then refill with OE reccomendations and check back into it.

 

That was a thought as well. Drove it to and from work (40 minutes round trip) and nothing seems wrong. Got just under half a tank. Gonna try it and see what happens.