Engine Mechanical problem
1998 Audi A4 4 cyl All Wheel Drive Automatic
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Recently I had the motor rebuilt, everything is new including the oil pump but not the crankshaft (it was polished). Since putting the rebuilt motor in I have been experiencing very low oil pressure readings when warm. (at cold idle i get 60-70 psi and at warm idle i get 6-8 psi) At this point I have changed the bearings to check them for wear and also for the possibility that they may have been the wrong bearings (main caps only). This did not correct the issue. There are no external leaks. I have changed the oil filter, experimented with various oil viscosities, but have not found a correction. Would it be possible that the oil pump may be the issue, I primed it when I installed it but didn't get to start it until a week later, could it have lost its prime? And could the relief valve and pump drive gears in the oil pump be the issue? Thanks

Was the engine rebuilt becauce of high mileage or was there a mechanical failure?
If the mains are good was the camshaft replaced?
Excessive cam bearing clearence will really drop oil pressure.
Let me know

Thanks for your quick response. The motor was rebuilt due to high milage but also because when I had purchased it, the vehicle was hard to start. The intake cam had to be replaced(badly damaged). It also looked like the old owner had a timing belt break and then unloaded it. It was knocking when I finally pulled the motor. I replaced the cam bearings and completely rebuilt the head as weel as the motor. Cam clearance could be an issue. The thing that is confusing me is the idea that the oil pressure is good when it is cold but when you reach operating temperature it becomes low. Would bearing clearance be effected by temperature? Like I first thought, maybe an issue with the relief valve? The main bearings have been done twice, once with the new motor and once again yesterday, I did this incase the oil pump may have caused damage. The other thing is that when I first started the car after the engine replacement it seemed to take an abnormally long time for the lifters to pump up. I am going to guess at maybe 4-5 minutes. This is why I thought that it may have been the bearings. Please let me know if you need any more information, I am DEEPLY in debt with this car and it's problems, but I cannot sell it like it is, besides I did like the car before these issues. Thanks again for your response and help.

The relief valve is to dump eccessive oil pressure back to pan. The 60-70 you get on start that is a little high but probably where the relief is set. It is possible the valve could be staying open and dumping the pressure to pan.The lifters taking a long time to pump up where they primed before start up?ie. placed in oil and the plunger operated. Is there any lifter noise now? Also have you tried another gauge?
Put another gauge in at the oil sender and see what you get.
Another way is to pressurize the oil system with an pressure tank of oil take the pan off and valve cover and observe any excessive leaking ie. pump, bearings,camshaft
Most machine shops have this oil pressure tank you might ask around maybe they let you borrow.
Let me know