Chevrolet LQ1 (3.4 Twin Dual Cam)

GM's 3.4-litre Twin Dual Cam V6 — four camshafts and twenty-four valves — in the Lumina Z34, Monte Carlo Z34, Grand Prix GTP and Cutlass Supreme.

It is not the 3.4 pushrod. The LA1 / 3400 shares its displacement and nothing else: two valves per cylinder, one camshaft, a chain, and a fraction of the parts count. A listing that says only 3.4 V6 covers both, and ordering for the wrong one is the commonest mistake made on these cars.

Its complexity is its problem. Four camshafts in a transverse V6 leave almost no room at either end of the engine, so routine work here costs several times what the same job costs on the pushrod 3.4 — and that is before anything has failed. The timing system in particular is a large job.

The head gaskets are the recurring failure, and by now most surviving engines have either had them or are on borrowed time; coolant loss with no external leak is the symptom to act on. It also shares the GM-wide intake manifold gasket. The engine is powerful for its era and was expensive to keep, which is why so few survive relative to how many were built.