2001 Cadillac SLS Northstar Motor

They are known for leaking head gaskets, and the head gasket is not the problem. The block is aluminum, and the head bolt threads as well. What happens is the pressure applied to the threads, as well as expansion and contraction of the metals, eventually causes the threads to fail. The repair method is installing steel thread inserts called a 'timesert'. In my 12 years with Cadillac, I have never seen a repeat failure after this repair has been performed, the only problem is the cost. You are limited to putting in the same engine that you removed, and a possibility that the replacement engine has aluminum threads, putting you at risk for a potential failure of the same nature in the future. In my opinion, money would be better spent on the repair, assuming that someone qualified does the job with the right tools and procedures. My dealer does anywhere from 4-6 of these a month, and has no problems.

Your talking about a motor, not a car, correct? I don't know where you would find a motor where the thread inserts are known to be installed. There is no way of telling without tearing into it. That info is usually not disclosed, and the seller may have limited, or no, knowledge of the previous history of the motor.

At this point I am a little confused. You are talking about a leaking intake, this is not the concern that requires thread inserts. Often times a leaking intake is the result of a split plenum boot, the rubber that is between the throttle body, and the intake manifold. Head bolt repair is usually required as a result of head gasket failure indicated by coolant loss with no external leak, or overheating with no obvious cause, and most often confirmed by a failed block test indicating hydrocarbons in the cooling system. I can't say that everyone opts for the repair, but being in the repair business, I do see quite a few being repaired. I would not recommend removing the bolts and reinstalling them. They are torque to yield bolts, meaning they stretch when torqued to maintain clamping force. That is the head bolts, the intake bolts can be removed and reinstalled, but red threadlocker(or any for that matter) is not necessary.

If you chose to have a pre-purchase inspection, I would recommend a block test. If a boot is leaking, it usually causes check engine light/lean codes, and may have higher than normal idle. Visually, there is no way of telling if the inserts have been installed. Visually, with some specifics, you may be able to tell if repairs have been performed. Retainers broken, new cotter pins in ball joints, etc, just some of the tell-tale signs that someone has been there. You would not know how far the repairs went. You may have options in buying from private party, where they know the repair history, and may have receipts to prove it. From a dealer, sometimes the car is a trade-in, and most times it was bought at auction, so you may or may not have repair history. If you are looking toward a Cad, they never had a 3.8. A 3.6 came out in 2004 CTS, and SRX, wasn't until 2005 that the STS used it.