my parents had one and it started having problems at around 100K. i replaced the cam, had the heads resurfaced and a valve job done. after that and a few other parts, it ran fine. i worked in an auto shop after high school and saw a bunch of these come through. i found this cool write up with a bunch of the common problems.... it is not listed, but i thought i remember hearing that there was a defect that allowed coolant/water to drip onto the first lobe of the camshaft causing it to flatten.
"Previous to 1982, Cadillac's fuel injection systems had been electronic - good systems when they worked, but prone to vacuum leaks, and hard to diagnose because of the variable nature of electronic systems. The HT4100 was a product of the post-fuel crunch era and was intended to give excellent fuel economy in a lightweight V-8 package.
The HT4100 engine was destined to become one of Cadillac's most dismal failures. Numerous problems plagued the design, from porous block castings, which allowed oil and coolant to mix over time; to coolant leaks caused by warping of intake manifolds and block to head mountings caused by overheating or even normal operation of the engine. Weak crankshafts, camshafts and distributor drives could shatter and cause instant failure, or cause a slow death as they wore down and reduced the power output of the already anemic 120 hp engine to almost nothing. Failed head gaskets and intake manifold gaskets were common to even low-mileage cars.
The 1982-83 model years were the worst. The engine was not extensively tested before production, and problems began to appear after 50K-60K miles of ownership in many cases. By 1984, Cadillac had contracted with Mercury Marine, an engine company experienced in manufacture of aluminum-block engines, to work out some of the problems in the design.
This led to the 1984-87 engines being less prone to problems, but still troublesome nonetheless. A general recall of all HT4100 engines produced in 1985 was carried out, at which time stop-leak pellets were crushed and added to the radiators of all HT4100-equipped cars to seal block porosity and gasket leaks. It is important to note that while many HT4100-equipped cars ended up with engine replacements or in junkyards very early on, there are also many HT4100 engines still on the road with 170K-200K miles and counting. Longevity and reliability seen to be a hit-or-miss proposition with the drivetrain. Proper maintenance can serve to extend the engine as far as possible.
Repair and general maintenance of the HT4100 are also quite different compared to previous Cadillac products. Due to the aluminum construction and complex design, certain procedures and torque specifications must be followed religiously. Repairs that were simple on olded Cadillac engines are quite different than on the HT4100, and very little information is available outside factory shop manuals and supplements. "