oh no !!! spun bearing

[Radar]

That is really strange. Why would an engine be okay when cold and then develop a noise after it warms up. I would think that a really bad bearing would be noisy even when cold. I know that the oil thins as it warms but a bearing loose enough to knock when warm would not be silent with the cold oil. I think it is something other than bearings. Your cat theory may work but I think something else is involved.

Is the noise a dull thud, a clicking, a rap, or something else? Is it quiet under load? Is it noisey when coasting with the throttle closed? A good mechanic can tell various things about the bearings based on the type of sounds and when they happen.

The other thing that is wierd is your falling oil pressure. On my 89 L98, the oil pressure is about 53 psig when cold. When hot, it falls to about 28 psig at idle and 45 psig above 1100 rpm.

Just for grins, pull the distributor and check the bushing that drives the oil pump. Perhaps that plastic part has failed and your oil pump loses it's drive when that part warms. A long shot but worth a check.

Good luck on finding the problem and let's hope it is something easier to fix.

 

[89blkvet]

first off, i would like to thank EVERYONE for the overwhelming support, this has got to be the greatest forum out there, as for car, very good questions. under load, the noise completely goes away, and the oil presure is well up there. once it gets warm and I am stopped at idle, the pressure drops TOO low and the noise starts????? when i start the car in the morning at idel, no noise and high oil psi. as far as metal in the oil, as far as I can tell, no. the noise is hard to explain, kinda sound like a rap, but very uniform and goes away with throttle

 

[geekinavette]

I had a very similar problem with mine. "tap-tap-tap" sound at idle with very low oil pressure (about 9 PSI) but ok at higher RPM. I couldn't pinpoint the noise under the valve covers so I found a long enough steel rod, stuck one end right on the block and my ear to the other end and sure enough it was bottom end. After tearing the engine down I found that the crank journals had been ground .010" undersized but whoever put it together did not use oversized bearings so as to achieve the proper clearance.

If you do have a spun bearing the only way to repair it properly (to me anyway) is to have the block align-bored and the crank journals ground (if possible).

Bill

 

[oldace84]

Your thinking the worst.......take your time and figure it out.....you'll get there...
good luck
tony

 

[Dave C. '04 Z06]

Low oil pressure at idle certainly sounds like a spun bearing to me. Those were the same symptoms mine had (which I ignored until the bottom half of the bearing wedged under the top half and then there was some REAL clanging going on as the piston repeatedly smacked into the head...damaged a good set of AFR heads that way....mainly because a piece of the piston decided to bounce around in there independently of the rest of the piston... )

But anyway, you could possible do it from underneath, but trust me... you don't want to. Just rent an engine hoist and pull it out. The engine will probably be completely disassembled anyway.

If you spun a bearing, it's probably a rod bearing, so you'll have to take out the crank and have it polished (or cut if it's been going on a while) and the rod was probably overheated so to take that out (or to put it back in really) you'll have to remove the head. Even if it wasn't overheated, you'll still need to have it re-sized. So it's coming out.
If you do that, you might as well replace all the rings and bearings.
If you do that, you should hone the block (or have it honed. You probably won't need a re-bore unless your engine wasn't maintained properly).

So you see, it just balloons.... so just plan on a rebuild.

But before you do, check to make sure it isn't something like a broken lifter, rocker arm, or valve spring (although these usually won't cause a drop in oil pressure, except maybe the lifter). Its easy enough to check because the heads will be coming off anyway.

Good luck
-Dave C.