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Overheating Mystery


bovet

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I have a 99 STS 4.6 Northstar, 104,000 miles, one fine ride with a real overheating mystery. I was on the interstate and all of a sudden temp rose to 245, I shut engine off coasted to side and waited 30 minutes, started right up , normal temp and drove 5 miles to rest area at normal temp until the ramp when it happened again, shut engine off and coasted in. A tow home,hoses,thermostat,water pump,radiator and antifreeze later, verifying fans working proper it was determined that at hiway speeds the temperature of the engine would shoot up and if you shut the engine off, coasted to the side, removed the small hose off the coolant reservoir and blew through the hose the engine temp would return to normal and you could continue driving. Through experimentation it was determined that you could start from a cool engine temp get on the street speed up quickly to traffic speeds ( 70mph ) and the temp would rapidly rise to hot, stop leave engine running, remove hose, blow through it and temp returns to normal. You could crank car and leave idling for hours and engine temp would remain normal, only when the speeds or I assume pressure increases due to rpms increasing on the engine would this happen. Took thermostat out and replaced again and ran a line through the reservoir hose all the way to the thermostat (no blockages). Also determined that the engine coolant seemed to boil but radiator wasn't as hot, but after blowing on the hose boiling would cease in engine. Short of drilling a hole through the fire wall and running hose through to "blow as ya go " any ideas. The obvious seems to be a blockage, but where and how? And...NO OIL IN THE COOLANT AND/OR NO COOLANT IN THE OIL, No moisture/ steam out the tail pipes. Not the typical Northstar head gasket overheating problem.

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First question.

Are the top and bottom plastic close off panels in place?

Second question.

Have you removed and cleaned the "hollow bolt" fitting where the purge line connects to the crossover assembly?

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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Do yourself a favor and get a block test kit from Napa or rent one from Autozone and test the air in the surge tank for the presence of exhaust gases before you start replacing parts. Your symptoms ARE rather typical of the early stages of a head gasket failure. Rule that out first.

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Do yourself a favor and get a block test kit from Napa or rent one from Autozone and test the air in the surge tank for the presence of exhaust gases before you start replacing parts. Your symptoms ARE rather typical of the early stages of a head gasket failure. Rule that out first.

Makes a lot of sense to rule that either in or out!

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First question.

Are the top and bottom plastic close off panels in place?

Second question.

Have you removed and cleaned the "hollow bolt" fitting where the purge line connects to the crossover assembly?

#1 plastic panels are in place on both the top and bottom.

#2 This is news to me, was not aware of this ' HOLLOW BOLT ", AM NOT SURE WHERE IT IS? I will try to locate it and do as you suggested.

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#2 This is news to me, was not aware of this ' HOLLOW BOLT ", AM NOT SURE WHERE IT IS? I will try to locate it and do as you suggested.

If you trace the plumbing backward from the small hose at the top of your coolant reservoir you will come to a piece of rubber hose clamped to a hollow bolt located directly aft of the upper hose fitting on the coolant crossover. The hollow bolt also secures one end of the engine lift fitting.

Be aware that this plumbing can be difficult to follow because the throttle body is included in the circuit on your year engine.

One day I plan to take pictures of the bolt, plumbing and normal idle RPM purge line flow, but that won't help you today.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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  • 5 years later...

Our family has 3 vehicles with Northstar 4.6L engines build on the same line as Cadillac DTSs. Two are 2006s and one is a 2007. They have from 100,000 miles to 140,000 miles on them. They heat up quickly from a cold start, taking about 5 minutes for the temperature to indicate exactly mid scale and they usually faithfully hold that reading. However, all 3 have demonstrated a mysterious overheat episode.
The 2007 misbehaved about 14 months ago. After a 2 hour 50mph highway trip, I must confess I didn't notice the temperature gauge when I left the car, nor did I notice the cooling fans running. I came back in about 10 minutes to get something and the electric cooling fans were going full blast and the temperature gauge was almost in the red. An hour later the fans were off. I slowly released the coolant cap with no fuss and noted the coolant level was normal. I added about a quart of 50/50 coolant anyway with the car idling and the temperature quickly indicated mid range.
The car didn't give me any more trouble until today, 14 months later. After 10 minutes driving, the overheat light came on and the gauge was in the red. I pulled off the road and let it idle. The dash told me it was overheating, then told me it was turning off the AC and then it was going into idle mode. So I shut it off and the fans kept running. After only 5 minutes I was able to slowly release the coolant cap with only a slight hiss and a perhaps a hint of a gurgle. The coolant level was normal but I added about a quart anyway. When I started the car, the temperature quickly reached normal and stayed there for my 3 hours of driving today.
My sons reported a similar one off experience with the other two vehicles. We all did the same thing - release the pressure, add a bit of coolant and things immediately went back to normal. We have replaced all the cooling hoses and thermostats and used a vacuum tool to get rid of trapped air. The only thing I can think of is a stuck thermostat or an air lock. Any ideas? I just hope it isn't head gaskets.

We had another bad scare this summer when all three engines suddenly had their bottom halves covered in oil. Oh no, not oil pan seals! But how could all three possibly do it at once? I got underneath and with the engine idling used an air gun to clear the oil away from the joints and see where it came back. I started at the oil filter, lucky me. That's where the oil leak was. It had been the first time we used white NAPA oil filters and had tightened them the full turn as labelled on the filter. When we replaced them with Frams, we noticed the gasket was a lot stiffer and took a lot more torque for its full turn. No more oil leaks on all 3 cars. I would hate to think of people getting $4,000 gasket jobs because of lousy oil filters.

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