I posted this a while ago. My Dad's Blazer has a NASTY start-up knock. It started probably last year, only did it in the winter, but now it does it every time you start it. It sounds exactly like bearing knock. It goes away completely after about 2-3 minutes of idling, but if you try and drive it before that, it sounds HORRIBLE. Somebody in my previous post thought it may be the thrust bearings on the crankshaft. Does anyone have a clue what this might be? Truck has 160K now (1998 4.3L Blazer).
4.3L GM start-up KNOCK
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Propflux01
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Rod knock will usually only last (continuously) a few seconds during start up, however, piston slap will last the 2-3 minutes your speaking of. Just to make sure, hook an oil pressure guage up to the tap and get a pressure reading. If the pressure is good and constant, and the noise stops after the 2-3 minutes, then I would go with teh slap, if it is low at start, and gradually goes up, in the 2-3 minutes, I would say Clogging oil pickup tube. If it starts low, and rises within a few seconds, then I would suspect bearing clearances. I am also assuming that you feel it is louder than 'valve clatter', or that you have localised it to the lower end of the engine, and not at the valve cover area.
Oil pressure has been checked recently, and it's A-OK. The noise is for sure not piston slap. It's in the lower end of the motor, doesn't sound like the valvetrain at all. It sounds identical to rod knock, and it's pretty darn loud sometimes, especially when you give it the least bit of throttle.
A few other people I know say it couldn't be rod knock because it would knock all the time. It runs freakin good for the mileage; the oil pressure is like what it was when brand new. Internally, the motor is spotless upon inspecting it while the intake gaskets were replaced.
I doubt its rod knock either because rod knock generally doesn't go away as the engine warms up. My 98 5.7L has a pretty loud startup knock and its been doing it since about 36,000 miles. I'm at 71,500 miles now. Mine goes away after a minute or so. Mine is a pretty loud tapping noise but I'm not sure where its coming from. I'm not sure what his could be either.
Moms and Pops have a 96 Monte Carlo w/ 3.1L, bought it brand new. It makes that slap noise when you start it cold. It sounds horrible. Give it 2 mins and it goes away. They took it to the gargage under warranty and they said "its normal". We have tried different oils and it does not make a difference.
Eric
Eric
Rod knock WILL NOT go away after an engine warms up, it actually gets LOUDER as the oil warms and thins.
I would look at the facts that the noise originally only occurred when cold outside, and also goes away upon engine warm-up. two possible situations come to mind; a bad piston pin/piston pin bore or a collapsed piston skirt/piston slap. As miles continue to accumulate either condition will gradually worsen, thus explaining the start-up noise in warm weather now also.
Just two possible theories.
I would look at the facts that the noise originally only occurred when cold outside, and also goes away upon engine warm-up. two possible situations come to mind; a bad piston pin/piston pin bore or a collapsed piston skirt/piston slap. As miles continue to accumulate either condition will gradually worsen, thus explaining the start-up noise in warm weather now also.
Just two possible theories.
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