Engine Knock Right at Engine Start only!!

I'm trying to get as much research for this as I possibly can before taking action. Any info you guys could provide would be greatly appreciated!
My Situation:
Sometimes when I go to start my truck, I will turn the engine over and when it fires up there are a series of 4 knocks and then it goes away. Each and every time it does it, it's four knocks. It usually happens after the engine has been sitting for the night. I took my truck to a mechanic concerning this "knock". He said it sounded like front/rear main bearings or possibly the crank shaft, but he could not tell which area it came from. What puzzled him is that it only "knocked" four times and then went away. After that, everything sounded perfect. Absolutely NO ENGINE NOISE WHAT SO EVER. He commented that the truck ran remarkably well.
Info I recieved from another forum:
this is most likely a problem with the crankshaft main bearings. general motors had a bulletin on this when these engines were new and the fix was to measure the main bearings and install oversized bearings on the mains. they also note that this will not affect the longevity of the engine. bottom line is leave it alone.
Do you guys concur with this response?
Thank you for your input!
Phil
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Sounds like classic bearing rattle to me. You get it at startup till the pressure builds up.
Longevity? Hard to say. Were it me, Id think about using a heavier oil and see if that helps any. Maybe a can of oil treatment. You want to prevent the rattle. I have to assume when it rattles it is potentially scarring the bearing. For putting around town you may get by for a long time, Id be afraid to make any long distance trips.
Id also think twice before I gunned it to get around grandpa.
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Thanks a lot! I appreciate the info!!
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Thanks a lot! I appreciate the info!!
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I know this sounds kind of weird----but have you had any work done on the AC? If there is a bit too much oil in the AC system, it will settle in the pump lower cylinder overnight, and knock a few times on start-up. Had it happen on my 94 K1500.
Bill
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No work done to the A/C. As a matter of fact my air hasn't worked in a long time. I'm just to lazy to replace the sensor on the drier unit.
BillM wrote:

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It is simply the hydraulic lifters pumping back up after leaking down overnight. Engines have been doing this ever since hydraulic lifters were invented. There is nothing wrong, they are working like they are supposed to. See http://en.wikipedia.org/wiki/Hydraulic_lifters if you don't understand.
Randal
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Well...now theres "rattle" and there's "tick tick tick". If its rattle rattle rattle, Im leaning towards rod/crank bearing. If its "tick tick tick" then definately lifters. This has been my experience anyway.
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How about the year and engine?????, some 4.8 and 5.3 engones had an issue with the oil pump to block seal which caused some start up issues

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It's a 95 K1500 w/ 5.7 350 engine. 203K miles on the truck, but I put in a new "crate" engine about 8 years ago. I think it MAY have 50K on it. The issue is that I bought the truck when I was a young punk and only had one thing on my mind and it wasn't the truck. So, the truck has been neglected in the past. Now I'm trying to bring the truck back up to its former glory. I want to get everything under the hood where it needs to be to get optimum efficiency and functionality before I start working on the outside of the truck.
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May not make a difference, but just for kicks, Id remove the serpentine belt and crank the engine cold, and see if you get any rattle. You never know, sometimes idler bearings, alternators, etc, make odd noises. I just did this with my step sons Jimmy, to try and isolate noise. Didnt help, rod /crank bearing for sure. Off topic, but they only want 1800 bucks for a 4.3 long block!! (Napa and the Zone) Maybe yours wont be so bad? Ill cross my fingers.
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Not a "tick" or "rattle". If you knock on a desk or door with your knuckle, that's the sound. I spoke with an old timer up in North Ga today that has been the lead technician at his chevy dealership for forty years. I described the noise and circumstances in which the noise takes place. He said that his truck made that exact same noise on cold starts as well. He said his trucked started doing that @ about 50K miles and now it has 300K and still makes the noise. HE said oil filters with check valves and radiator oil coolers could easily cause the problem. He told me that if I wanted to buy an engine he could gladly sell me one, but he told me that he would hate to see me waste money on an engine when the engine I have has a knock that is little more than an annoyance on cold starts. He said use 20 W30 ( thicker oil) and STP and the noise should go away. HE also recommended marvel mystery oil.
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Then thats what Id do. Go with a heavier oil, add a little additive, and ride it out. Your guy in Ga is describing a pressure problem. Once the engine has run for a few seconds the pressure builds and the knock goes away. Just remember, this may be an annoyance, but it "is" a weak spot in the engine. It may run another 100,000 miles before she blows, just be carefull not to be drag racing between stoplights and it may run a long time. Oil pressure is the life of an engine, even for a few seconds rattle, on a cold start, can be doing damage, may not get you now, but it will eventually. Change the oil, put in the additive, and good luck!
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MY truck has now started doing the knock on a warm start. I want to replace my main/crankshaft both while I have a little extra money and before my engine locks up . How much would you charge someone to do it or how much would you pay someone to do it? $300.00???
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Wow, Im not sure 300 bucks will get the engine pulled. Much depends on the condition of the crank. If it needs turning (probably will), plus the cost of a gasket kit, seals, and labor, you are looking at way more than 300 bucks. Id get estimates from a few mechanics before I take it in.
If you can do the labor yourself, 300 bucks might get it done. Theres not that much to a bottom end, make sure each rod/main cap goes back to the same place, use a litle STP on the new bearings, torque everything down. Oh...just for kicks, Id replace the oil pump, you never know, that could be the weak spot that led to bearing failure...and take the crank to your local parts house have them check its tolerances, and turn if need be. Then they can match up your bearings.
...now back when I was a teenager, I replaced my mains/rod bearings in my old 67 Chevy truck, lying on my back, in the gravel, with the motor still in the truck. (straight six) Of course these days, there not that kind of room underneath one....
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I checked, I'm looking at about $120.00 for parts, that's new main/crank shaft bearings, gasket sets, and oil pump. I met a guy that said he would change out the bearings for $300 labor. I have access to an auto hobby shop here on base. They have truck lifts , cherry pickers and engine stands. SO I may try an do it myself. But I'm not sure if I'm up to doing something that in-depth...
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Dunamis via CarKB.com wrote:

Before you do all that work for all that money, I think you should google "piston slap." Its really common in lat model GM motors. It happens in new trucks and GM won't fix it. They say its nothing to worry about and its not covered under warranty. The point is, if its piston slap, you can do all kinds of work to it, and the day you start it back up, the rattle will still be there. Get a professional opinion first. If its a bearing problem, then you got to work on it.
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I know what you guys are gonna say, "this guys is wishyiwashy!" You are right. I have crate engine in my truck , so I called the manufacturer of the engine to find out my bearing size so I could replace the bearings. He put it to me like this. If your knock goes away in just a second or two, not much to worry about. Now if the knock persists the whole time, you've got an issue. Even then you'd be throwing good money to bad by working on the engine because if there is damage done to the metal, it's probably shot metal shaving throughout the engine that you'll never be able to completely clean out. At this point I'm going to run some 10w40 and STP and see what happens.
Thanks all for your input!!!
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Sometimes when you take precautions like heavier oil and additives, then take it easy on the engine, you can get em to go a good ways before the problem really bites you on the ass. Really, the cheapest way to eek out whatever you can get out of it.
Good Luck!
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