Depends a lot on what year. The early RWD cars had the most trouble, because the engine was not designed for them. If it's an '82 or '83 full size RWD, I would be cautious but not run from it. Check the valve covers. If they're chrome or black with aluminum fins, it's the original engine (not a deal killer, but not the best news). If they're a plain black cover, it's been replaced with an updated engine (very good). If it's an '85 to '87 FWD car, there is much less to worry about.
On the car you are looking at, check for any sign of oil/coolant contamination: i.e. a milkshake color to the oil on the dipstick, or black oily slicks in the radiator or overflow tank. If the coolant is the color of a cup of coffee, that's very good - it means the cooling system sealer has been used.
Also check trouble codes on the onboard diagnostics (see the sticky post at the top of this section for instructions).
The 4100 got a bad reputation for several design flaws, most of which were resolved in the FWD cars of 1985-87. The primary problem areas are head and intake gaskets, weak camshafts, an overloaded distributor gear, and weak timing chain sprockets. But the truth is the catastrophic failure rate of 4100s under warranty was not that high. In fact, the 4.5 and 4.9 engines, which were updated versions of the 4100, remain some of the most reliable engines Cadillac ever produced. Plenty of people (myself among them) have driven 4100 powered cars for years with no serious issues. Mine gets 25 mpg on the highway and purrs like a kitten.
The three keys to success with a 4100 are:
1) Get a service history if one is available.
2) CHANGE THE COOLANT EVERY YEAR AND USE THE GM SEALER SUPPLEMENT TABS.
3) If you're mechanically inclined, purchase a Factory Service Manual for your year and model car. It can save you a fortune in labor.
These are complex but not terribly complicated cars, well within the skills of the average shade-tree mechanic. By comparison, consider that large numbers of pre-2000 Northstar engines have similar head gasket problems, but they are so complicated hardly anyone but Cadillac dealers can even work on them. Head gaskets on a N* mean dropping the entire engine cradle out the bottom - thousands of dollars. On a 4100 - you can have the job done for less than a grand, or do it yourself for less than $400.
Just go into it with your eyes open. Any old car is going to have things go wrong, and keeping a luxury car on the road is going to cost more than a Corolla. But then, a Corolla doesn't ride like - or have 1/100th the class of - a Cadillac.
