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Proper operating temp 1998 Deville

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Hello Gentlemen, (and ladies) My first post here, and frankly I'm not very Caddy savvy. I'm posting out of curiosity, and trying to learn something. I have driven GM products all my life, and am actually much more into the early Diesels 6.2/6.5TD in trucks. Just so you know I'm not a total idiot, regardless of this question......

My cousin drives a 1998 Deville with a Y code Northstar V8. I believe theres about 60K on the clock. Ambient temps here in south FL are running around the usual 90*+ for this time of year. He tells me he routinely sees temp readings of 220* and higher. This morning he told me it was reading 238*!!!!I put the exclamation marks there because for a guy that drives a GM Diesel, thats the kiss of death for the head gaskets, and likely the heads themselves. He recently(within a month) had a cracked coolant bottle replaced, and the thermostat changed, system flushed. This came on the heels of a very minor front end collision, prior to which there were no problems with coolant loss, or overheating. Unfortunatly, he doesnt know what temps the vehicle ran at before, never paid any attention until the low coolant indicator. My quick inspection showed no exhaust pressure in the bottle, and the oil looks clean of any water. Using my IR thermometer this morning, the temp on the engine side of the stat housing was at 207*, after parked for about 40 minutes, after the gauge reading of 238* After looking up the factory spec Tstat, and seeing it is rated at 180*, I'm even more concerned. I realize the Northstar is a gas engine, in a car, and a very different animal than a turbo Diesel in a truck. I just dont want to see my cousin wreck an engine on some bad advice, nor do I want to continue to give him bad advice, if my warnings arent warranted. Should that engine be running that hot?

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The first fan kicks on at around 223 to cool the engine, and the second comes on around 229 I believe.

Here's part of a post from WarrenJ:

Your DIC won't even start barkin' at you until the water temperature hits 252 degrees. Then it will go into "camel mode" and suggest you shut down.

The NorthStar is certified to travel 100 miles in 100 degree heat at 55 mph with NO COOLANT in the engine. That should give you some confidence.



Another post on here says not to worry until you hit 240 and it keeps climbing. It's important that the coolant is a 50/50 mix, and change the green coolant every 2 years an the dexcool every 5.

WARNING: I'm a total car newbie, don't be surprised if I ask a stupid question! Just trying to learn.


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Does his coolant temp seem higher since the surge tank was replaced? If so there is a bleed line that might be clogged, see below.

There is information missing from all of the information you provided.

What is he doing when he 'routinely' sees temps of 220 degrees?, traffic?, local driving? stop and go?, driving highway speeds? Is he running with the AC on or the AC off, it makes a big difference, with the AC on, the cooling fans are commanded on all the time, with the AC off, the cooling fans come on at 228 and turn off at 217. With the AC off he will see wider temp swings.

IF he saw 238 in bumper to bumper traffic, in 90+ degree heat, that can happen, its at the extreme high end but that can happen.

"Normal" temps are dependent upon the ambient temperature, and I do not envy you living in Florida, but that said, even though there is a 180 stat in there, his temp driving 65 will typically range from 196 to 205. In traffic his temp will rise to 220 to 226+, in DEEP stop and go traffic with high ambient temps 95/100 it can get up to 230 to 235 with NO adverse impact, if his cooling system is in good shape, those temps should NOT bother him in the least.

He is driving a 98, I am driving a 96, so I can tell you that, if he has NOT renovated his cooling system his temps will run high. Do you smell coolant? Does it hold the caps rated pressure without leaking? Does the pressure leak down? Has it been drained and refilled. Things to look at/check:

Pressure test the cooling system, it should hold pressure

The radiator end tanks are plastic and can develop stress cracks and it can leak under pressure, you won't see coolant leaking once the coolant hits the atmosphere it vaporizes, look at the end tanks for white stains where they attach to the core.

If the cap has not been changed, change it

If the stat has not been changed change it

If you smell coolant, find it, there are 4 crossover seals that can leak coolant and pressure

Check the water pump belt for cracking and slipping (shiny, glazed), if the water pump belt tensioner is binding it will not provide adequate tension and the belt will slip

There is an AIR purge line that attaches to the coolant tank, pull it off while the engine is cool and while it idles and see if coolant flows, if coolant is not flowing come back and we will explain. There is a 3/8" hose attached to the top of the tank, with a clamp, it is attached to a pipe that runs across the engine. Coolant should flow when the engine runs, it lets air out of the system, if it is clogged, air can not get out.

Make sure the cooling fans are operating as I noted above, make sure they are not just turning weakly, they could be free wheeling, grab them and see if their bearings are worn

Make sure the coolant concentration is at least 50/50, in Florida you can go up to 70/30 and the additional coolant concentration will raise the boiling point to 276 degrees

Make sure nothing is blocking air flow through the radiator and that all air deflectors are in place

Use the GM cooling supplement tablets

Last year I totally rebuilt my cooling system and was in 60 miles of bumper to bumper traffic on Route 95 in 100 degree heat from Baltimore to Philadelphia and my temps ran 228 to 235 and I didn't sweat a drop! Let us know what you find, I hope this helps.

Cooling system schematic, notice highlighted RED location, that is the purge line at the tank


Here is the "bolt with a hole in it", this is the other end of the purge line located near the throttle body, this hose connects to a bolt with a hold it in, if no coolant is coming out of the purge line at the tank, remove tis bolt and look for an obstruction. DO NOT blow air through the line from the tank toward this bolt, you will only temporarily dislodge the obstruction. Remove this bolt, remove obstruction, and air will then be purged from the cooling system. As I said, if this 'bolt with a hole in it' is obstructed air will not be allowed to escape from the cooling system


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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One thing I would like to add is that the Northstar was designed to run for 50 miles with NO coolant (265 deg) and not damage the engine, so your cousins temps, while a little high are far from the "kiss of death". IF the head gaskets are gone, they went first and where not caused by those temps. "Normal" is about 205-215.

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Thanks for the excellent info. I just wanted to confirm what he was told, and had relayed to me, as being the correct info. It appears that it is. I've found that guys on forums, that drive 'em and are enthusiastic about it, generally know more about them than the GM tech that works on them. I value your info highly.

The coolant tank was replaced by a radiator tech, that I trust, and referred my cousin too. He did pressure test the system, replaced the Tstat, and gave it a clean bill of health. My personal ignorance of the Northstar, and safe op temps is what caused my concern. Thanks again for clearing it up for me.

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Thanks let us know if we can help

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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  • 3 years later...

98 CAR LOST 2 PLUS GAL OF COOLANT: I found the low coolant sensor not working repaired and did the following testing.

My concern is the head gasket if it is leaking but no other issues with car.


Pressure tested to 17 lbs hot waited 10-min looked for white smoke non apparent.

No misfires on any cylinders.

Looked for leaks with pressure tester did not find any.

Coolant changed every 30K miles with Dexcool.

Found cap not holding pressure very slow leak down replaced cap.

After 100 miles coolant seems to be at same level.

I plan on flushing system with distilled water adding 2-gal of coolant to flush out the 6-taBs of bars stop leak.

You cannot use coolant dye with Dexcool so plan on using plan water to test for leaks.

So what is next did I miss anything?

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yeah have you checked the radiator cap? 16 psi I believe. I know you said the tank was changed but what about the cap?

I am a big fan of the 6.5 Diesel. It and the 6.2 are pretty bulletproof. Most common thing I have done on them is the injecctor pump and fuel drivers. I love them though. Havent had one blow yet even with 300000miles on them.

As for the northstar To give you a heads up I had my 96 Eldo with blown head gaskets blow about 30 miles from home and I made it all the way back home with the engine in one piece and still running. It didnt like it so much but it made it. long story short the car had too many problems to list. It was shot when I bought it


Nice, clean, luxury= fine automobile

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