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Discussion Starter #1
I just bought this gorgeous White one owner 70,000 mile 1999 Cadillac Deville. (Northstar engine)

The car is pretty much flawless. However there is a problem.




The car remains cool when it idles. But loves to overheat as speeds of 60 mph or higher.

When I run the heater around town (in Phoenix) Yikes! The car seems to run at regular temperature 195-208. The first things I did was remove the thermostat. The drain the green coolant the son in law of the previous owners put in 3 days prior. Then flush the system with clean water from the hose and put in a coolant flush and ran it for 20 mins or so.

Then drained it again. It still overheats. I then took advise from a friend that said the 3/8's coolant hose might be blocked that runs from the reservoir. I put the hose on it both ways and flushed it out. The car still overheats. After getting the water in the system totally clean. Then driving it for a bit. The water seems to have fine sediment in it after close examination.




I felt around the radiator and it seems some parts of the radiator are hotter than others.

I really like this car very much and would love to be able to use it and not have to worry, at the same time I am quite limited in the funs I can spend on it.




Your great advise is very much appreciated.
 

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98 DeVille, 97 DeVille d'Elegance
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Put the thermostat back in and check the hollow bolt above the WP housing. It sticks straight out towards drivers fender. run a bit of metal(I use a drill bit) and ream the bolt out real good.
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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6,735 Posts
Running an engine without a thermostat, expecting it to run cooler is an urban legend from the 40's. Install a 195 F new DELCO one back in it. Check the hollow bolt for any restriction. Report back. And.......... you say overheats. Describe overheats better. Does it boil over when shut off, does it go beyond 225 degrees?
 

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Discussion Starter #6
Hello all,

Thank you for the amazing prompt responses. The car overheated before I removed the thermostat. But I will be sure to put on in Monday. In the mean time I am headed outside to go and look at this hollow bolt everyone is referring to. I will update as soon as u am through.

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Also on the temperature question. The car will hit 250 if I am not very carefull. It shuts the ac off. That has only happend twice.
 

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White Diamond '03 DHS (with DTS floor shift)
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86,579 Posts
To check the hollow bolt, simply pull the purge line off the surge tank and stick it in the open filler neck (engine cold). Start it up and you want to see coolant flow from the line. If not, then the hollow bolt is plugged. If you do, you probably have a bad HG. The block test will tell.
 

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Discussion Starter #8
I just went to O'reilly's and got a new radiator cap. The car is now not throwing coolant out of the reservoir. However after another drive on the freeway at about 65 the temp went from 205 to 230 in about 1/2 mile or so. Around town doesn't seem to bug it too much. The heater takes the temp down a fair amount. The block test this week will tell. Oh geez...
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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67,924 Posts
What was the result of the purge line test ?? If that line/hollow bolt/nipple is clogged it WILL create all sorts of cooling problems.

Proper coolant level is half full, checked cold only. Never check or adjust coolant level with the engine warm or hot.

Observe the FULL COLD legend and flat arrow molded into the top and side of the surge tank.
 

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White Diamond '03 DHS (with DTS floor shift)
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Whether you drive it like an old lady or Mario Andretti is irrelevant.

The purge line runs from the top of the surge tank to the crossover manifold. I'll post pictures of both ends. As I and others have said earlier, if the purge line flows coolant, do a block test.

Purge Line At Tank.jpg
Purge Line at Hollow Bolt.jpg
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,059 Posts
YUP - like Ranger said -

with the engine running - CHECK for a constant - gentle - coolant flow from the PURGE LINE -
if YES -

do a BLOCK TEST -
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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67,924 Posts
Water crossover and purge system hollow bolt/nipple.

Water crossover assembled.gif


bass, Did 1999 still have a coolant heated throttlebody? If so, bypass it with a single length of reinforced rubber fuel line - directly from the hollow bolt to the surge tank upper side.

Coolant heated throttlebody.png
 
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