Inboard Engine Blowing Steam and Coolant Out of Exhaust: Causes?

Coolant and thick white steam exiting your exhaust means the closed cooling antifreeze circuit has breached into the combustion chambers or the raw water stream. On a 1988 petrol V8, the most likely culprits are a blown head gasket, a cracked cylinder head or a rusted exhaust manifold.

Exhaust Manifolds and Risers

Your 1988 Cruisers Sprint 3370 runs twin 5.7L or 7.4L V8s from Mercruiser, Crusader or Volvo Penta. You must first determine if you have a full or half closed cooling system. In a half system, only the engine block sees antifreeze while raw water cools the manifolds. These half system manifolds cannot leak coolant into the exhaust. A full system fills the exhaust manifolds with pressurised antifreeze.

Cast iron manifolds last five to seven years in saltwater and ten to fifteen years in freshwater. Coolant dumps directly into the exhaust passage when the internal walls rust through. Replacement cast iron manifolds cost £300 to £500 per side. Scrape the mating surfaces perfectly clean before fitting new ones and tighten the bolts to between 25 and 30 lb ft.

Blown Head Gasket

A failed head gasket allows pressurised engine coolant to leak directly into the cylinders. The heat of combustion instantly turns this liquid into thick white steam. The engine then fires this steam out of the exhaust alongside unburnt antifreeze. You will notice a sweet smell to the exhaust smoke. This scent indicates vaporised ethylene glycol.

• Check the spark plugs: Remove all the plugs on the starboard engine using a standard 5/8 inch socket. A plug from a cylinder burning coolant looks exceptionally clean compared to the soot on the others. The coolant effectively steam cleans the ceramic tip.
• Check the oil: Pull the dipstick. Milky brown oil means coolant has breached the oil galleries. You must complete an immediate oil and filter change to prevent the main bearings from corroding.

Cracked Cylinder Block

Freezing temperatures expand weak antifreeze mixtures and crack cast iron engine blocks or cylinder heads. A crack in the exhaust port of a cylinder head pumps coolant straight into the exhaust manifold.

Look closely at the exterior of the block just beneath the exhaust manifolds. Freeze cracks often push outwards here and create a visible weeping line of rust. Internal cracks require a cylinder leak down test or a magnetic particle inspection by a machine shop. A replacement bare cylinder head for a GM block V8 costs £400 to £600.