Hi everyone, I am new to the forum. My son and I rebuilt the motor of his 1998 SLP and on startup the oil pressure was low and about 5-10 minutes of idling the oil pressure went to almost zero so I shut it off.

First a little background on the car. My son purchased the car last year from a private seller even though I strongly objected to it. Within 2 months exactly what I said would happen happened, the engine died. We took it apart and the bearing where worn, the block was twisted and had 4 small cracks in the block. So my son got an iron block 5.3 and had it bored to a 5.7, got a remanufactured crank, rods and cam. He had all the parts checked and polished at a local machine shop.

My son and I assembled it carefully taking no shortcuts and used quality parts such as Arp bolts for reassembly. We just started it last week and it started fine, no smoke or weird noises but the oil pressure seemed low to me at about 15-20 psi. We are using breaking oil recommended by the machine shop. In 5-10 minutes the dropped to almost nothing so I shut it down. I started it a few more times after verifying that the readings on the car gauge matched hand held gauge from a friend of mine. The engine losses oil pressure before it has a chance to get hot.

Any suggestions, is the engine not primed completely? Is it because of the breaking oil or a physical problem in the engine?

I had a shop look at it and they are stumped, I don't know if they really don't want to get involved with it. My son is joining the Navy soon and I wanted to get this fixed before he goes, any constructive ideas or suggestions is greatly appreciated.

 

Verifying the oil pressure with a hand held gauge, I presume it's a mechanical one?
Being a remanufactured crank, it's very likely the journals have been ground to a smaller size and you would need to use under-size bearings. Do you know what specs the journals were machined to, did you use the correct size bearings, and did you use plastigage during assembly to verify a good bearing fit?
What type of oil pump did you use?
I'm leaning more towards a bearing size mismatch issue though. Bearing size too large = low oil pressure.

 

The oil pressure gauge was a mechanical one and it matched the cars gauge almost exactly. We used a stock oil pump. We used the bearings that the machine shop suggested we use, I am not sure everything that the machine shop had done, I let my son handle a lot of the back and forth wight he shop. If the bearings are in deed the problem can they be switched out from the bottom of the car without removing the engine?

 

Yes you can, but removing the oil pan might be the challenge. Also, with the crank still in place, it's going to be hard for you to check the main crank journal sizes accurately with a mic.
One more thought, are you sure you got the main bearings in correctly? The uppers (block side) have oil holes, the lowers (cap side) don't. Here's a helpful video: LS1 Engine Video 7 crankshaft installation - (Carburetorssuck.com) - YouTube

 

Thanks, I am sure the oil holes in the bearing line up. I'll be getting the car home this week sometime and will check out a few more things. My time is very limited and we have been dealing with this car for a year and a half, I may just sell it. Its to bad because its in almost new condition besides the engine issue.

 

Did you plastic gage the bearing when you assembled the motor? Sometimes machine shops make mistakes and sometimes parts on in the wrong boxes. I always plastic gage bearing and check the ring end gaps because I have been burnt with both not being what they should be. I am not a big fan a plastic gage, I prefer to calculate my clearances but it is great for catching math mistakes and wrong part. There has been 2 or 3 times I have had 8 matching bearing boxes and one bearing that barely put a dent in the plastic gage.

Just as easily it can be an issue with the pump. When I start building race motors the old guy I was working with used to pull a part and check every pump before he installed it on the motor no matter who manufactured it. I was standing there once when he found a stuck pressure relief valve. Since then I have found one with had no clearance. Both the stuck open valve and the no clearance issue would have cost us a motor.

 

No, he didn't use plastigage. . . .

 

That sucks, that LS1 pan is going to be a pain even then you can not be 100% sure of oil pressure. This is why a lot of times I recommend used with warranty low miles motors. At the dealership all we only ever see are the projects that fail and man I have seen a lot of them. In the worst cases there is nothing to be saved.

I have not done a LS1 yet but on the old school SBC I used to prime and verify oil pressure right on the stand.

 

I'm still putting my money on it being the wrong size bearings. Especially if say the main journals were only polished and still at stock specs, and the rod journals were ground down needing undersized bearings, or vice-versa. That's where an "oops I accidentally received all the same size bearings" mistake can happen.

 

The other common mistake I have seen a lot is not cleaning the crankshafts. In many machine shops this is considered part of the assembley because no builder would think of install a crank without cleaning it but most new guys do not realize they need the clean out the crank ports. As soon as the motor starts it just dumps at the grinding grit trapped in the the crank oil hole right into the bearings. That will wipe out the bearing is a few minutes.