Engine Knock


Asked by Feb 25, 2014 at 12:08 AM about the 1990 GMC S-15

Question type: Maintenance & Repair

My uncle has a 96 GMC s-15 2wd 4.3l vortec, and has 199k miles, and it just recently
started making a terrible knocking noise. almost sounds as if something is loose, but,
heres the scary part, the other day he came home and pulled in the driveway and the
truck was idling rough and nearly died out on him, and steam was pouring from the
motor, i pop the hood and his brake master cylinder lid was loose and brake fluid was
everywhere, it knocks on startup, and when its warm. but its not a loud knock until you
give it gas. my uncle, bless his dumb lil heart, "SAID" he changed the oil and put
synthetic oil in it after it had regular 10w-30, im thinking he didnt change the oil, and
kept the same ooil in it and just threw synthetic in it, but would it make that noise? idk
im no mechanic, or could the brake fluid maybe have gotten down inside the cylinder or
whatever and is making that noise. his oil pressure lights reads a little high also and he
also stated that he has stalled out coming up the road, but would the engine knock
make it stall out? i feel bad for him bc now he has no vehicle, and id like to help him out
by figuring this out, hes ready to scrap it but i just want to make sure its not a simple
fix, or if he needs a whole new motor? please help and thanks in the advance.

2 Answers


Knocking and loss of power. I would lean towards a broken rod. It would cause the knocking and the one pistion is redered stationary, hence the loss of power. Drain the oil and check for metallic residue. Caution if it is a broken rod it most likely sheared close to the crank and the crank is hitting the broken tail, it will eventually settle and the crank will put it through the block. If there is no metallic residue then it may just be bearings you can pull the pan and check. I don't know what the used/wrecker supply of those trucks are where you live but the 4.3's are everywhere and cheap here. Scrap or fix?

2 out of 2 people think this is helpful.

ok i will have him check that. so difficult fix? if it is that, would he be better off buying another motor? or can he possibly maybe rebuild it? thanks so much man

1 out of 1 people think this is helpful.

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