P0304 on Cadillac: How to Diagnose & Fix

The 6.2L supercharged LT4 V8 in CT5-V Blackwing models has experienced premature engine failures including bottom-end bearing failures, lifter collapse, and metal shavings found in oil. Failures have been reported as early as 5,000-15,000 miles. Car and Driver documented a catastrophic engine failure on their long-term test CT5-V Blackwing. GM initially claimed it was an isolated incident but forum reports suggest the problem is more widespread than acknowledged.

The 2.0L LTG turbocharged four in the third-generation CTS is direct-injected, so no fuel ever washes across the back of the intake valves. Oil vapor drawn in through the PCV system bakes onto the valve stems and seats and builds a hard carbon crust that chokes airflow. The result appears gradually as a rough or lumpy cold idle, a stumble under light throttle, misfire codes and a measurable loss of power and fuel economy - typically becoming noticeable somewhere past 60,000-80,000 miles, sooner on cars driven in short trips. Owners on Cadillac Owners Forum also report oil pooling in the intake plenum at the PCV connection, which accelerates the deposit rate. This is a direct-injection design characteristic, not a defect covered by any recall, and it is specific to the DI engines - it should not be conflated with the V6 timing chain story.

The 3.6L V6 (LFX naturally-aspirated and LF3 twin-turbo) uses direct fuel injection, so no fuel washes over the backs of the intake valves. Combined with a PCV system that routes oil vapor into the intake tract, carbon coking accumulates on the intake valves over time. By roughly 60,000-100,000 miles this can cause rough idle, misfires, hesitation, and reduced fuel economy/power. This is separate from timing-chain-stretch and high-pressure fuel-pump issues; it is a fueling/airflow deposit problem inherent to GDI engines.

The 1993-2002 Eldorado was one of the first vehicles to receive the Northstar 4.6L V8 and has the same head bolt/head gasket failure. Early Northstar engines (1993-1995) are particularly prone as the design was less refined. The Eldorado ETC (Touring Coupe) uses the high-output 300hp Northstar. Same failure mechanism and repair as all Northstar-powered Cadillacs.

The Cadillac Northstar 4.6L V8 is infamous for head gasket failure caused by the head bolts pulling out of the aluminum engine block. The original design used steel bolts threaded directly into the aluminum block without inserts, and the threads strip over time from heat cycling. This causes coolant to enter the cylinders and exhaust gases to enter the cooling system. The issue typically occurs between 100,000-150,000 miles but can happen earlier. This is one of the most well-known engine failures in automotive history and affects every Northstar-powered Cadillac (DeVille, DTS, Seville, Eldorado). The repair requires engine removal and Time-Sert or Norm's head stud installation.

The 2006-2011 DTS (DeVille's successor) continues to use the Northstar 4.6L V8 with the same head bolt thread-stripping issue. GM made minor improvements to the later Northstar engines (L37 and LD8 variants) but never fully resolved the fundamental design flaw of steel bolts in aluminum threads. The DTS Northstar is slightly improved over the DeVille era but still fails at 100,000-150,000+ miles. Same symptoms, same repair as the DeVille.