This last week I tore down a high milage 1.9 engine that had oil pressure problems when the engine was at as a hot idle. the rod bearings looked reuseable but the main bearings rattled in the case they looked ok if you don't bother to look a little closer. The low oil presuure in the 1.9 engine is due to the main bearing fit in the case. Replacing the bearings without restorng the the proper crush is a short term cure. All 1.9 cases need to be align bored when rebult.  
The pressure relief spring does affect the pressure cold but not at hot idle or cruising speed. The con rods are the problem with the high mileage 2.1 engines. The big end of the 2.1 rod gets smaller at a point in line with the beam of the rod or in other words 90 degrees from the parting line of the rod cap. The decreasing clearance of the bearing to crank is a slow process. The result of this distortion is that the rod bearing wears over time and in some cases the engine may never have a low oil pressure at warm idle or the dynamic oil warning buzzer going off.
I also just disassembled Frank Condelli's 2.1 engine with 200k  on it and all 4 rods showed major bearing wear no babbit left just the underling layer of copper my guess would be that the rod to bearing clearance was in the 004 to 006ths range my new spec is 0015th  The main bearing showed no signs of scuffing or obvious wear. Frank as I recall said he had no oil pressure problems to report but he did say he had been using synthetic for years. That might be the only reason Frank still had a core with no rod poking threw to turn in.
Bob Donalds 
http://www.bostonengine.com
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