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99 STS, White Diamond
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Discussion Starter #1
99 STS, 143k. Head gaskets (with threaded inserts), water pump and radiator replaced when I purchased thirteen months ago at 139k miles. Temp never past center until about a month ago when weak spot from a previously misrouted upper rad hose developed a tiny leak. Repaired of course after removing and verifying thermostat for proper function.

Went touring Sunday on curvy, hilly country roads. Rarely above 60 mph. Sunroof open, radio on and having nice conversation with passenger.

On way home (after about 2/3 tank of gas used) was enjoying some "spirited" passing and driving--floored it maybe six times in sixty miles. Well after my last pass, engine began to overheat very rapidly. Was coasting to side of road as protection mode became active. Stored snapshot shows 268F @ 794 rpm @ 16 mph and 2% calculated load. No DTCs other than record of the overheat event.

Went for long walk and returned to engine cooled below 12 o'clock on gauge. On very short drive for a better parking place engine rapidly began to overheat again. Just as I was about to shut it down, I saw the temp needle back down one notch and within a minute, it was back at center where it stayed for the remaining 80 miles or so home.

"Service Engine Soon" illuminated as well as "change engine oil". Service light extinguished after two more short drive cycles with temp never above the middle. Changed oil the next morning, reset reminder and immediately after temp gauge crept above the middle again, wavered around the mark after for about a mile then went back to the middle. Not above the middle since.

Have had a passenger watch my scan tool during some decent drives and coolant temp seems perfectly normal, varying between about 210 and 215 degrees in town driving with temp not going up at all (usually down a touch) under heavy throttle. Intake air temp rock steady at 89-90F.

What's up? "Sticky" thermostat? Cooling fan(s) that occasionally drop out without leaving any code? Or is the wife right? I'd just finished the spring detailing (to hear wife the eight hours or so spent were more like 40) then took her touring, shopping and to dinner. Is the thing Christine?
 

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White Diamond '03 DHS (with DTS floor shift)
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No, your spirited driving had nothing to do with it. Hard to say what did though. Where the Timeserts done by someone who knew what they where doing? Thermostat is a possibility. How about the water pump belt tensioner and purge line?
 

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99 STS, White Diamond
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792 Posts
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No, your spirited driving had nothing to do with it. Hard to say what did though. Where the Timeserts done by someone who knew what they where doing? Thermostat is a possibility. How about the water pump belt tensioner and purge line?
Good shop well known for working on "good" cars, so I presume they knew how to install the inserts properly. Tensioner is working well; no noises and purge line (presume you mean to pressure reservoir) is properly routed with no signs of leakage.

Would improper timeserts result in a sudden, complete cooling system failure that doesn't repeat?

Checked thermostat in pan of water on stove and it was fully open at the proper temp having started opening about 5 degrees before. Guess I'll put one in the trunk in case...

Finicky cooling system in this car. Upon purchase it took quite some time for me to find that the occasional burst of coolant immediately after shutdown was due to a bad cap and rough filler neck. With the tiny hole in the upper hose it lost less than a cupful after shutdown, but engine immediately overheated with next uses without leaking or, it seems, circulating. The tiniest leak seems to lead immediately to runaway coolant temp.
 

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White Diamond '03 DHS (with DTS floor shift)
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87,112 Posts
purge line (presume you mean to pressure reservoir) is properly routed with no signs of leakage.
Yes, it runs from the water pump to the surge tank. Check it at idle (cold) and see if it has coolant flow.

Would improper timeserts result in a sudden, complete cooling system failure that doesn't repeat?
No, it would result in another head gasket failure. Hard to say what it is being intermittent like that.
 
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