View Full Version : Major oil leak in '98 Seville STS Northstar engine

11-25-10, 04:19 PM
This is my first post and i could really use some advice here.

I bought a 1998 Seville STS a year ago (with 170,000 mi.) and 3 months later it began leaving oil spots on my garage floor. I took it to the local dealer who ended up having to drop the whole cradle that holds the motor/trans and replace the gasket/seal between the upper and lower crankcase. Well, after 2 more attempts by the dealer to fix the oil leak, including 2 different rear seals and various gaskets, it leaks like a sieve now. I leak about 2 qts. in a week with 120 miles of city driving. When I am idling with the engine warm, I can see a steady stream of oil coming off of the drain pan. After shut-down, it will still seep for awhile and leak about a one foot diameter wet spot on the cardboard. The dealer also detected a mild coolant leak "somewhere in the engine" but it would never overheat. In desperation I used Barr's Leak Head Gasket sealer which stopped my coolant loss. The dealer said the rear seal kept going bad because of of the coolant leak into the crankcase. I'm not buying it, but I have given up on them having the technical competence to help me.

Here's the deal: Once upon a time (1985) I worked as a front end mechanic after completing auto mechanics training. Is it worth my while to pull the motor myself and go to all the trouble to fix this leak the right way? I need to get another vehicle anyway, so I need fix this, sell it, or junk it. I really enjoy driving it and thought this might be a nice project for my 15 year old son who is also interested in cars.

11-25-10, 05:42 PM
IF you have other transportation and can devote some time - at least a couple of part-time weeks - to the job, a halfcase reseal on a Northstar is a steep learning curve experience. Much is posted in here and in Discussions: Technical Tips on the job. Basically, the oil pan and the halfcase are integral parts of the engine structure - the halfcase is also the lower main bearing caps (4 bolt) and a girdle rolled into one. The seal was originally a formed silicone "gasket" (think curvy O-ring) but has been superseded by GM RTV sealant beads. Surgical cleanliness and attention to detail are the necessities for this job. If you decide to tackle it, also use the stud kit from www.northstarperformance.com (http://www.northstarperformance.com) - head bolt hole failure in the block is common in the pre-2000 engines, and you would hate to do the lower end job and have a gasket blow in a month............You will positively need a real GM Service Manual for this work - Chilton or Haynes are far too generic. You'll need to duplicate the dealer removal procedure: the whole drivetrain comes out the bottom of the car. The cylinder head and cam chain drive system can be daunting - but it's all logical and covered in detail in the Service Manual.

11-25-10, 09:51 PM
WOW! Sounds like some real incompetence in that service department to me.

11-26-10, 09:07 PM
To: submariner409

Wow, you sound you've "been there, done that." Yes, it sounds daunting, especially since my experience with pulling engines was with rear wheel drive vehicles, the traditional way, from the top. I'm concerned how I would get enough ground clearance to drop the whole drivetrain since I don't have access to a hoist. I would like to research this quite a bit before deciding if I'm up for the challenge. The cleanliness part is no problem since I am somewhat of a perfectionist with my vehicle projects in the past. Where would I read up more on the GM RTV sealant beads so I can understand this aspect completely? I will see what I can glean from the other discussions here as well. I'm having difficulty wrapping my brain around the whole half-case architecture. It sounds like the short block is in two parts, an upper and a lower, that bolt together. Does the GM service manual come as a CD or is the paper 3 volume set the only source? I really appreciate your help!

11-26-10, 09:38 PM
Half case. Picture the block split in half at the crankshaft center line. The lower half becomes a one piece main bearing cap and lower crank case.

Here's a diagram that may help you understand it.

11-26-10, 11:03 PM
To Ranger:

Thanks for the diagram link. That helps a lot. It is an interesting design. If you have any other cautions for me, don't hold back.

11-27-10, 03:35 PM
Before you do ANY work - pour a UV dye in the oil system, have it up on very secure ramps or stands (or a hoist) and with a black light, determine the source. You should have the vehicle running for about 5-10 minutes to circulate the dye. The Aurora I just did some warranty work on had a leak from a source I would NEVER have expected (aside from the cracked oil pan and lack of thread sealant on the flywheel bolts). UV dye will pinpoint the source.

I'd just hate for you to do all of the work again and find a weird source like I did - a void in the bottom of a threaded hole, side of the block. Porous block casting.

11-27-10, 03:36 PM
While you're in there, check your P/S pressure hose. They're quite common to leak on these '98+ cars. If you end up removing the engine it's much easier to replace that hose/line at that point rather than later.

11-28-10, 06:16 PM
At the last oil (filter) change at the dealer, I had them put in some more UV dye to see if the massive increase in oil leakage was from a new source. The service manager attempted to show me, but the source was too high up to see well and there was UV dyed oil all over the place, so I can't see that it was very helpful. She thought it was coming from the same area as before, for whatever that's worth. Like I said, I've given up on them helping me fix this problem. Good call on the P/S pressure hose. It started leaking about 5 months ago and in desperation I used some P/S fluid leak stopper which actually worked, though if I pull the drive train, I'll be sure to replace the hose. If anyone has recommendations on how to drop the power train in my driveway, I'd love to hear them. I appreciate you guys chiming in to help.

11-30-10, 05:52 PM