: What else is possibly left???



yankee1021
07-25-10, 10:02 PM
Ok,
It's a 98 DeVille D'Elegance with a Northstar 4.6L with app 150,000 miles. This car has run fine for quite awhile. About 2 months ago it over-heated while climbing a pretty stout hill on a very hot (90+) summer day.
Since then, I've tried everything I know of to try to get this thing to NOT over-heat. Here are the symptoms: Will run fine for several miles, temp will vascilate between 196 to 212 without any problem. Suddenly the temp will start to climb and will continue upward (often pretty hastily) until either it shuts itself down at around 260 or I shut it off. I've replaced the thermostat twice, replaced the water pump. Flushed the whole system out twice, refilled with 50/50 mix of coolant, have checked the purge valve, have added BarsLeak Pellets. Fans work. A/C works. Heater works. Temp sensor seems to work since when it says the coolant temp is HOT it obviously is. Have also replace the purge tank pressurized cap. The hoses all appear fine, no signs of collapse. No leaks visible. Have considered the idea of a air pocket and have run wide open with the heater on full blast.
I have a ton of experience working on engines and never in my life have encountered a system such as this that will work fine (sometimes for days in a row) then suddenly and without warning overheat.
Any brilliant ideas?

Submariner409
07-26-10, 12:34 PM
The '98 and '99 Northstar is statistically the highest risk of head gasket failure (poll thread at the top of Northstar performance), leading to combustion gas in the coolant, overpressurizing the system and blowing off coolant through the cap pressure relief. The displaced coolant causes overheating. The gasket failure is caused by head bolt thread failure in the block, causing loss of clamping pressure and subsequent gasket rupture. It can happen gradually or catastrophically - yours seems to be middle of the road.

Find a Lisle 75500 cylinder block leak test kit, follow the instructions, and test the airspace over the coolant in the reservoir for the presence of exhaust byproducts. (Normal coolant level, cold, is halfway up in the reservoir, no more.)

No amount of snake oil, tablets, thermostats, etc. will repair this: it takes engine removal and machine work to repair the block cylinder head bolt threads.

Many, many, many posts and threads on all this up in Deville and in Northstar Performance.

FWIW, This thread belongs up in Northstar Performance, not "Technical Tips".

Ranger
07-27-10, 12:37 PM
Yup. Head gasket. Do the test Sub suggested to confirm it.

zonie77
07-27-10, 05:22 PM
Ok,
It's a 98 DeVille D'Elegance with a Northstar 4.6L with app 150,000 miles. This car has run fine for quite awhile. About 2 months ago it over-heated while climbing a pretty stout hill on a very hot (90+) summer day.
Since then, I've tried everything I know of to try to get this thing to NOT over-heat. Here are the symptoms: Will run fine for several miles, temp will vascilate between 196 to 212 without any problem. Suddenly the temp will start to climb and will continue upward (often pretty hastily) until either it shuts itself down at around 260 or I shut it off. I've replaced the thermostat twice, replaced the water pump. Flushed the whole system out twice, refilled with 50/50 mix of coolant, have checked the purge valve, have added BarsLeak Pellets. Fans work. A/C works. Heater works. Temp sensor seems to work since when it says the coolant temp is HOT it obviously is. Have also replace the purge tank pressurized cap. The hoses all appear fine, no signs of collapse. No leaks visible. Have considered the idea of a air pocket and have run wide open with the heater on full blast.
I have a ton of experience working on engines and never in my life have encountered a system such as this that will work fine (sometimes for days in a row) then suddenly and without warning overheat.
Any brilliant ideas?

Not brilliant, just the result of experience...Head Gaskets.

tateos
07-27-10, 08:26 PM
Yup - HG. Sub and Ranger are probably telling you the prudent thing to do - do the test, but we've all experienced and/or seen and/or repaired enough of these to know that if it walks like a duck, and quacks like a duck... it's the HG.

zonie77
07-28-10, 01:01 AM
Did you replace the pressure cap? Less than $10 and worth a try.