Knocking sound at startup

[rksand]

I have a '94 Suburban, 5.7 with 275,000 miles and still runs great. I overhauled transmission at 250,000 miles and have had a knock at startup since the trans.overhaul. The knock lasts about 1 or 2 seconds then it is gone. It burns a quart of oil every 800 miles. It has done this since I got it at 94,000 miles 3 years ago. Does this sound like bearings or possibly flywheel? The flywheel was replaced when the transmission was. Oil pressure is around 30 when warm and cruising at 70. It has always been this low. I run 20-50 oil with Lucas additive. Pressure is at 50 when cold and just above redline at hot idle.

What should I do? I would like to get maximun miles out of this engine.
I am reimbursed mileage so I don't mind spending a few hundred dollars if it would get me another 50,000 miles.

[Thunderbolt]

Originally Posted by rksand

I have a '94 Suburban, 5.7 with 275,000 miles and still runs great. I overhauled transmission at 250,000 miles and have had a knock at startup since the trans.overhaul. The knock lasts about 1 or 2 seconds then it is gone. It burns a quart of oil every 800 miles. It has done this since I got it at 94,000 miles 3 years ago. Does this sound like bearings or possibly flywheel? The flywheel was replaced when the transmission was. Oil pressure is around 30 when warm and cruising at 70. It has always been this low. I run 20-50 oil with Lucas additive. Pressure is at 50 when cold and just above redline at hot idle.

What should I do? I would like to get maximun miles out of this engine.
I am reimbursed mileage so I don't mind spending a few hundred dollars if it would get me another 50,000 miles.

Have you changed oil filter brands ?? Some oil filters don't have anti drainback valve so the oil drains out after it is parked causing it to knock until it fills the filter back up. The other and more likely cause is a rod making noise. I have a buddy that has one that has been doing it and it started over 1 year ago. My 98 has had a wrist pin noise for the last 80,000 miles and in the last 30,000 has staterd doing exactly what yours is doing. It goes away after it runs a second and makes no noise when driving so I have left it alone. If yours only does it for that inital start it is a sign of wear, But I would still keep driving it until it makes noise all the time. It won't leave you stranded without prior warning and may go thosands of miles yet. Maybe not 50,000 , But it is possible. The knock will start to last longer and get louder andthat will tell you it is time. I would take it easy on it and don't do hard starts or tow heavy loads that strain the engine and you should be fine. The other alternatives are a new long block or if you have the knowlege and are on a super tight budget you could pull the motor and puta re-ground crank, rod and main bearings and oil pump in. You can get a crank and bearings for about $150.00 and the oil pump for about $30.00. You would still have to buy anti-freeze and oil and filter plus rent the engine hoist. If it were mine I would drive it until the knock starts lasting longer, gets louder or doesn't go away at all and then get a different truck.

[rksand]

Thanks for the response. I'm not sure on the oil filter. I usually have it changed at Jiffy Lube. What brand has the anti drain back valve?

[Thunderbolt]

Originally Posted by rksand

Thanks for the response. I'm not sure on the oil filter. I usually have it changed at Jiffy Lube. What brand has the anti drain back valve?

Filters such as the Napa Gold, Wix and A/C all have a good anti drain back vave. Other filters do also, But some don't work as well such as Fram. I personally use the A/C Delco because that is what came on it and they are actually are rated good.

[stubbornmike]

thunderbolt gave some good info, how ever having 275000 + miles isnt 20-50 to thick of oil. During cold start up the oil is very THICK and giving more of a dry start than a 10-40. I know you want thick oil so u dont burn it. I think id rather protect my engine rather than worring about losing oil.
although are close to a rebuild anyways...
good luck pal!

[Thunderbolt]

Originally Posted by stubbornmike

thunderbolt gave some good info, how ever having 275000 + miles isnt 20-50 to thick of oil. During cold start up the oil is very THICK and giving more of a dry start than a 10-40. I know you want thick oil so u dont burn it. I think id rather protect my engine rather than worring about losing oil.
although are close to a rebuild anyways...
good luck pal!

I missed the 20-50 part. I definatley wouldn't run 20-50 in it. You are going to do more harm than good with that thick of oil. If it is your rod bearings you will actually ruin them faster with the 20-50 because on intitial start up it doesn't flow well. I wouldn't run anything thicker than 10-40 and if it uses that much oil then it is time to rbuild or replace the entire engine.

[gregory914]

I have a 97 Burb with 110k miles on it and it has always had a knock for the first mile or two. It was purchased new and I had the dealer do the first couple of oil changes then I have used only Mobil One 5w30 as reccomended in the manual.
The dealer's shop foreman told me the 5.7 motors had "piston skirt slap" which meant the piston would cock itself in the cylinder until the oil worked it's way all the way through the engine. And GM would not fix the problem unless you blew the motor. My noise sounds like a tappet or rocker arm sound and has not gotten any worse in 110k miles and the shop foreman attributes that to the 5w30 Mobil One. He said most motors that had that noise wouldn't last more than 50k miles before the rispin or rod would let go.