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1993 engine knock

10-30-2010, 11:32 AM
I need help!! My 16 year old son just bought a 1993 GMC Jimmy. 2 door sport. 6 cyl 4.1? Vortec. This car has been sitting for about 3-4 years. It has 92,000 orig miles 2 owners and has well over 3000.00 worth of custom body work and looks amazing. the interior is spotless... so far everything works... brand new tires..brakes..exhaust.. really mint cond. he only paid 2,000.00. Soooooo... heres the problem. the engine has a HUGE knock. the seller said it never did it before he parked it, but when he started it up last year he noticed it so didnt drive it. it has been stored inside. So.. now that my son drained his bank account I thought someone here could steer me in the right direction. The car starts and runs really smooth... but the knock is pretty loud. it comes from the front end of the engine and gets worse when you rev it up. did an oil change and replaced oil with 10-50. son looked on youtube and found a video of a bad timing chain.. sounded exactly like the noise... dont know where to start and am hoping its not too major. he gets his license next week and is so worried about this. he cant drive it the way it is.. afraid to do more damage. any ideas??? i've always had good luck here.. thanks.

10-30-2010, 01:15 PM
Timing chains are not a common problem on the 4.3L especially with such low mileage. I service a fleet of 4 of them ranging from 285,000 miles up to 356,000 and none of them have ever had the chain replaced. The timing chain typically doesn't make noise when it wears. Usually it stretches and jumps a tooth or so on the camshaft. When that happens, the engine runs very poorly, if at all. The 10W50 oil is not recommended, it will do more damage. Rod bearings and crank bearings do not contact the journals on the crankshaft. A "cushion" of oil separates them. If oil can't get in there, the bearing contacts the journal and it knocks. When the oil pump circulates oil, it is pumped into the center of the crankshaft. Each journal has an oiling hole that the oil passes through to lubricate the bearing surface. The oil between the bearing and the journal creates a "cushion". If the oil is too thick, it can't "fit" between the bearing and the journal causing a knock. Years ago, when engine bearing tolerance was larger, thicker oil was used to quiet down noisy bearings. Thicker oil causes oil pressure to increase simply because the pump has to work harder to force the oil through. As a result, the volume of oil that is pumped decreases. With newer engines, the manufacturers recommend 5W20 or 5W30.

While the engine is at idle, remove one plug wire at a time and listen for the knock to change or stop completely. If/when the knock changes, you'll know what cylinder is causing it and the bearing and or crankshaft needs replacement. If it doesn't change, it's not a bearing.

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