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1999 STS, 84 Seville, 93 FWB, 47 Nash, 2017 Corvette, 99 Z28, 85 Trans Am
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15 Posts
Discussion Starter · #1 ·
Got my 99 Seville STS yesterday, what a car. Nothing in the owners manual nor the shop repair manuals that say not to use synthetic motor oil, doesn't say you can use it. So what's the consensus with using synthetic oil in these Northstar engines.
 

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Premium Member
99 Seville STS
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473 Posts
Synthetic motor oil is ok to use but on a pre-2000 as your 99 it should be 10-30W or 10-40W as this engine has the flat top kind of lifter, (so for ex 0-30W and 05-30W is not suitable).
This viscosity is however hard to find as full Synthetic, at least in Europe, so I run half Synthetic 10-40W in my 99
 

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Administrator
2002 F55 STS, 2014 Explorer XLT, F-150s
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75,811 Posts
Synthetic is just fine. Your engine is spec'd for 10W-30 of whatever oil brand floats your boat.

DO NOT get caught up in yet another opinionated, unsupported oil war. Read the owner's manual, study the stickies in any forum of interest, keep the car clean and drive it.

Stickies are timeless info, marked with a push pin. Start with the Seville and Northstar forums.

1999 STS says it all - Seville Touring Sedan, CVRSS active suspension, 300 hp 3.71 performance drivetrain.

If the water pump drive belt tensioner pulley has not been replaced, do it yesterday - when it goes things get very expensive in a hurry.
 

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1999 STS, 84 Seville, 93 FWB, 47 Nash, 2017 Corvette, 99 Z28, 85 Trans Am
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Discussion Starter · #4 ·
The pulley and belt were ordered this morning at around 4:30 am CT. I have read older posts about that pulley and when I woke up I ordered it. Tomorrow will be 4 days since I took possession of the car, bought the factory repair manuals, and am learning quick about how particular this engine is with the water pump on the back of the motor running off a cam shaft wheel.
Next thing to do is flush out the old coolant because the surge tank is extremely dirty. what would be the best way to do this? The repair manual has nothing specific.
 

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Administrator
2002 F55 STS, 2014 Explorer XLT, F-150s
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If you're just getting your feet wet with the Northstar System I would recommend a complete coolant exchange by a good local Mom & Pop shop. About $120 - $150, they'll use a pressure/vacuum machine to get it all out and all back in. The Northstar cooling system IS snitsy about correct coolant levels and maintenance procedures. DO NOT get into pissing contests over GM DEX-COOL coolant. It was reformulated long, long ago and is an excellent long-life coolant - but exchange it religiously every 4 years. That said, you can use any coolant on earth as long as you maintain correct level and exchange it at the recommended intervals.

Correct coolant level in the surge tank is about half full, ambient cold. See the FULL COLD legend and flat arrow molded into the tank top and side.

The purge line must flow a steady stream back to the surge tank at idle. It removes coolant airlocks and insures "solid" coolant to the water pump.

Motor vehicle Organism Mode of transport Font Line
Motor vehicle Font Auto part Automotive design Drawing
Motor vehicle Font Auto part Circle Electric motor
 

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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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21,486 Posts
Synthetic is just fine. Your engine is spec'd for 10W-30 of whatever oil brand floats your boat.

DO NOT get caught up in yet another opinionated, unsupported oil war. Read the owner's manual, study the stickies in any forum of interest, keep the car clean and drive it.

Stickies are timeless info, marked with a push pin. Start with the Seville and Northstar forums.

1999 STS says it all - Seville Touring Sedan, CVRSS active suspension, 300 hp 3.71 performance drivetrain.

If the water pump drive belt tensioner pulley has not been replaced, do it yesterday - when it goes things get very expensive in a hurry.
==============================
DO NOT get caught up in yet another opinionated, unsupported oil war.

but - BUT - BUT -
Forehead Microphone Tie Coat News



it's been MONTHS since we've had a good OIL WAR!
 

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Registered
1999 STS, 84 Seville, 93 FWB, 47 Nash, 2017 Corvette, 99 Z28, 85 Trans Am
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15 Posts
Discussion Starter · #7 ·
Way to go Brandon
 

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1999 STS, 84 Seville, 93 FWB, 47 Nash, 2017 Corvette, 99 Z28, 85 Trans Am
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Discussion Starter · #8 ·
Thx submariner for info. Yes I'm getting my feet wet on the Northstar. I read in the OEM repair manual , Vol 1, pg 6-281 to add pellets, PN: 3634261. I believe these are the same pellets GM says to use in the 4.1 V8s for the 80-85 bustle back Sevilles. Those that know about those problematic engines says the pellets are required and a must use. Is this the same for the Northstar?
By the way I am most amazed with the engineering in this car and its drive train. I'll find a shop to flush out the system for me.
 

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Super Moderator
2003 Deville Base
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6,638 Posts
DO NOT USE THE PELLETS. GM discontinued that advice years ago. And just in case you already did, I hope you didn't put them in the surge tank. Regular maintenance of the cooling system is likely more important that your brand of oil choice.
 

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Administrator
2002 F55 STS, 2014 Explorer XLT, F-150s
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DO NOT use the coolant supplement tablets or powder. Discontinued by Cadillac/GM TSB (Technical Service Bulletin) in 2006 or so. The service manuals and radiator sight shield stickers were never updated so the only way a casual owner would know would be info from a conscientious (???) dealership.

FWIW, the supplement was used to prevent assembly line nuisance weeps in order to prevent warranty comebacks. The stuff is actually Bar's Leaks Tablets #HDC or Bar's Leaks powder #G12BP.

Once again - read the stickies in Engines; Northstar. You'll find all sorts of good info that's not even in the GM/Cadillac/Helm service manuals.

Study this - written primarily for the pre-2000 Northstar by a CF member who was a GM Northstar System powertrain engineer. Some was superseded with the radical 2000 redesign (like the coolant tablets) but it's still applicable.

 

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Premium Member
99 Seville STS
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473 Posts
You may want to re-route the purge line that is heating up the Throttle body to get a better flow through the purge line, I did after Sub had hinted me about that, its dead simple, you just run a fresh hose directly from the Coolant Reservoir to the thermostat housing after removing the current hoses to the Throttle body.
I also took the Coolant Reservoir off and gave it a real good clean up as it was very clogged up on the inside from them pellets mentioned in the previous post, I think they had put in more of them than they were supposed to, perhaps due to HG anxiety....

Overall the purge line is crucial for the NS as you probably know already
 

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1999 STS, 84 Seville, 93 FWB, 47 Nash, 2017 Corvette, 99 Z28, 85 Trans Am
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Discussion Starter · #12 ·
So that's what that brown shit is in the surge tank. I'm going to get my 84 Seville flushed out too.
 
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