1999 GMC Suburban P0305 Fault: Engine Dies, Rough Idle, Zero Compression in Cylinder 5
My wife was driving our 1999 GMC Suburban on the highway when the check engine light came on and the truck suddenly shut off. Upon arriving at the scene, both the check engine and gauge lights were illuminated, with the temperature gauge reading abnormally high—currently stuck to the right side of the dashboard. After allowing the engine to cool, I added approximately 1.5 gallons of coolant, and the vehicle restarted. While driving back home on the highway, a distinct clacking noise was heard around 45–50 mph. To avoid further issues, I drove slowly and noticed a severe rough idle upon arrival at home. A diagnostic scan revealed fault code P0305. A compression test on cylinder #5 showed nearly zero pressure. Despite the engine starting when I tried to pull into the garage, it continues to exhibit rough idling and chugging under acceleration. I'm seeking advice on possible causes and solutions—especially given that this is a 170k-mile vehicle. Any insights or troubleshooting steps would be greatly appreciated. Thanks, Tim
In the worst-case scenario, the engine may have ingested coolant into cylinder #5, potentially causing a bent connecting rod. If you still value this truck, consider replacing it with a used, low-mileage unit. Before installation, ensure intake gaskets and fuel injectors (with revised pressure regulator assembly) are replaced. While the engine is out, inspect the radiator, replace belts and hoses, and check the thermostat. Also don't forget to replace distributor cap, spark plugs, wires, and rotor. Install new exhaust manifold studs and gaskets for a full rebuild—though this may not be worth it depending on your budget or expectations. Sorry to hear about the damage!