OK, the car is running hot. I'm driving a 96 Eldorado with 157,000 miles on it. It's in the 90's here and the analog gauge is showing a range between first and second marks past the center line with the A/C on or off. If I get bogged down in slow freeway traffic it may pass the second line and move toward the red line ( but it never gets there). If I come to a stop on a city street or on the freeway the RPMs go down to about 750-800 and the oil pressure light starts to flicker intermittently. Yes the heat then rises to the warmer end of the range I've described. When my speed is resumed or the RPMs exceed 900 the oil light goes off. As the speed gets up to cruising range the temp goes back down to the lower end of the range described. I work in the mountains and when I go to one destination, requiring a drive of 12 miles at 45-60mph up moderate grades (5-10%), I let off the gas just as I pull into the parking lot so the RPMs drop down from around 2400 to the idle of 800 and the oil pressure light comes on strong and the warning message comes up saying to "Stop engine - Low oil pressure" and the audible alarm goes off. All those bells and whistles cease about 50 yards into the parking lot and after the RPMs are back up to at least 1200.

I've had the thermostat replaced, changed the oil and filter ahead of schedule, changed the air filter, checked the cooling system for leaks (none), checked the belts and replaced one (that drives the alternator - it quit a few weeks ago), and let the mechanic run the car on idle in his shop for over 20 minutes with the A/C on full. He said that the temperature was constant at 205 degrees but that when he turned on the heater it dropped 20 degrees.

The mechanic concludes that I need a new radiator at $275.00 plus "a difficult installation" charge. He thinks that the oil pressure light comes on because I am exceeding the maximum temperature range of the oil so that it thins out too much and triggers that sensor. (I use Valvoline 10/30.)[sm=dontgetit.gif] Does any of this make sense - especially the part about the mechanic's diagnosis?

10w30 in a high mileage engine is a little thin IMHO.
Try 20w50 in the summer. Yes oil pressure goes down as temps rise.
Cold climates take a thinner oil to get it flowing quickly on start up.
Without a doubt fix the overheat problem.
I can't tell you if it's the radiator or not.
If you don't trust him, get a second opinion.

As oil gets warmer, it "expands" and therefore gets thinner, so the oil pressure can drop. Carmakers usually recommend relatively thin oil like 5w-30 in order to give an easy engine start in colder climates or in the winter. Thinner oils also give a better mileage. But if your engine heats up like that you should get much thicker oil. Just ask somebody at Walmart about which oil is thickest, although you might have to do an oil change to a lighter grade later on before the onset of winter.

The temperate gauge on my 99 Eldorado seems to hold steady somewhere between the middle and the line to its right. But the driver info center permanently says "check coolant". I just had some coolant refilled at the Walmart. Apparantly, there is a litte rubber hose that simply leaks out extra coolant without recycling it.

So definitely also make sure there is enough coolant in there. I really don't trust the sensors or the driver info center anymore so I prefer to see for myself if there is enough oil and coolant in the car.