Engine rebuilt but low pressure and knocking.

I just rebuilt the engine in my 99 Tahoe and when I went to prime the engine it was only showing 40 psi and no oil was coming out of the pushrods. There was also alot of air bubbles in the air and it the pump was making a gurgaling sound. I swapped out the new oil pump I installed with the rebuild with the old one and the noise went away and oil came out of the pushrods, but barely. I figured it was just because the the drill was not fast enough. I started the engine and the oil pressure was at 20. I throttled the engine to 1500 and the pressure went to 40, but dropped to 10 at idle. The engine started to make a knocking noise when it decelerated. Idle and reving up sounds fine, just decelerating. I think it could be piston slap with the new pistons, but with the oil pressure problems I think it is in the main or rod bearings. On my first rebuild had a spun rod bearing, but you could only hear that when reving. This is the opposite.

Could the pressure just be low because it is breaking in? It used to be almost 60 at startup and no lower that 40 at warmup. It could be a bad lifter but it sounds heavy like a rod or piston. You can feel it through the block.

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Nobody?

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Did you plastigauge the rods and mains?

yea, what was all done to it? was it a professional rebuild with a warrantee or did you do it in your garage?

I did a full rebuild with new heads and pistons. I did it myself. I Plastiguage everything and it all checked out.

The only thing i can think of is camshaft. I sent the block to the machine shop to be cleaned and they removed the cam for me because one of the bolts on the retaining plate was stripped. When they gave it back to me they forgot to install the cam when they replaced the bearings I had to pick it up. When I installed it I noticed there was play in the cam. It would slide between the rear of the block and the retainer about 1/8 inch. I don't remember any play before I sent it off. Also, the timing at seems to be loose. I installed a new set but there is just as much play as the original.

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I just checked again and it holds 40 when it starts up but slowly drops to 18 as it warms up. The knocking sound doesn't start until the pressure drops. I think that rules out piston slap. There is also almost no oil on the rockers so it could be in the valvetrain but seams lound for it. It is a deep sound and it can be felt through the engine.

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NacIK said:

I just checked again and it holds 40 when it starts up but slowly drops to 18 as it warms up. The knocking sound doesn't start until the pressure drops. I think that rules out piston slap. There is also almost no oil on the rockers so it could be in the valvetrain but seams lound for it. It is a deep sound and it can be felt through the engine.

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Click to expand...

I see the same oil pressue drop on my '97 gmc z71 5.7 vortec. Only 160,000 km since new. I installed a separate oil pressure gauge underhood. A 1970 z28 oul pump pressure releif valve spring would bring the pressure up but i think these generations of small block chevy engines are bleeding off pressure as the come up to temp somewhere in the cam bearing area.Sounds like a guestion for tech guru Marlin Davis at Hot Rod Magazine. Nothing gets by him.

Before my rebuild my motor was at 2 psi oil pressure and it didnt knock.
Somethings up and it sounds like the cam bearings. That a very well known reason that the push rods wouldn't get oil if the bearings had clearance issues.

Here is a good read .
http://garage.grumpysperformance.com/index.php?threads/not-getting-oil-to-rockers.4537/

Edit , Did you reuses your lifters? If so did you take them apart and clean them individually ? Mine were so bad they were caked shut and operated like solid lifters. Cleaning mine made the operated like new again.

I took the liters apart and dipped them. I also checked the check ball to make sure it worked. The only thing I didn't do was test them to see if they pumped up. I think it might have to do with the cam. I don't think there is supposed to be play in it so either something is missing or they messed up the bearings. The freeze plug behind the cam was replaced so maybe it is not pushed far enough in.

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I give up. This thing is kicking by but. I have have never had an engine give me this much crap.

The machine shop did tell me they installed the rear cam plug with clearance so that the cam wouldn't wear it down, which might cause timming issues, but I don't think it would cause the oil problem.

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