1997 GMC Other

Have a '97 Pontiac Bonneville SSEi, 142,000 miles. At speeds above 50 mph, the engine temp. Indicates 215- 218 deg, seems the car use to hold it's design 195-200 deg, at nearly any speed, at any ambient temp. I've pulled the radiator, had it rodded out, & exterior cleaned, new thermostat, cap, cleaned the A/C coils in front of Radiator. Should the Dash gauge be suspect, in this car? Will a partially plugged Catalytic Converter contribute to this? The EGR valve failed, a few days ago, replaced it, but saw no change, (except, it cleared the computer code, & most importantly, it idled & started). Any way to test the Dash Gauge? Dont see any leaks, on 3 yr. Old water pump. Am suspect of a tiny leak, perhaps on the Intake, as has been the case, on many of these cars. But coolant loss is perhaps less than 6 oz. Per month. Any suggestions appreciated.
May 13, 2006.

Most over heating problems in GM products with that many miles is caused by a clogged radiator. The factory coolant is good for about 50K miles and it looses it's additives and slowly builds up residue in the radiator. It is almost impossible to have the radiator cleaned because of the plastic top. Rodding out is at best a 50% solution because the construction doesn't allow for a 100% fix.

The dash gauge is about 75% accurate. I recommebnd getting an infra-red digital heat gauge. They are about $65.00 and look like a small pistol. You can point and read temperatures.

A catalytic converter can cause an engine to run hotter. Most garages have an exhaust pressure tester and emissions analyzer to determine if you need to replace it.

If nothing else works my recommendations are to replace the radiator. It costs about $250.00. It is a problem that occurs slowly and you don't see it happen. When you replace the radiator, replace the hose and thermostat.

May 13, 2006.
The problem may be related to the intake manifold gaskets (Upper and Lower). GM just redesigned the lower intake manifold gasket to be metal framed. Use that one if you swap them out. They are famous for leaking and may let water/coolant enter your intake directly. This can cause all kinds of problems. Don't sit on it too long.
Good luck!

Aug 19, 2006.