The 1996-2001 Mercury Grand Marquis with a 100% plastic intake manifold is known for engine overheating, even with normal use, and a coolant leak from the front of the manifold. This will often trigger the check engine light and low engine coolant warning light .
The engine coolant leak is from a factory defect, which causes cracking on the front coolant passage of the intake manifold. In a very short time, this leak will lead to engine overheating issues, especially while the vehicle is idling.
Between 1999 and 2001, Ford released an updated version of this engine, known as the 'PI' version with a metal coolant passage on the front of the intake manifold, to prevent future issues.
There is no factory authorized repair for the intake manifold, and replacement is required to correct the engine coolant leak and overheating issues.
Transmission problems are common in this model, often as a result of a failed front pump. The pump loses its ability to create hydraulic pressure, which an automatic transmission needs to function. A sign of front pump failure can be a whine type noise.
The throttle position sensor (TPS) may fail. Common symptoms of a defective TPS can be rough idle , hard starting, hesitation on acceleration, poor fuel economy, and stalling.
The heater hose outlet may leak coolant at the back of the intake manifold. Sometimes this is misdiagnosed as a leaking intake manifold gasket because it resembles a leaking manifold.
The Mercury Grand Marquis 4.6L V8 engine is known for displaying the normal symptoms of a coolant leak, including overheating, especially when the vehicle is idling, the strong smell of coolant from the engine, and illumination of the low engine coolant warning light .
This leak is difficult to locate as it is buried underneath the intake manifold , and only begins to leak coolant onto the ground in advanced stages of disrepair. This leak springs from the heater tube, which allows coolant to flow between the water pump and HVAC heater core .
When the connection for this tube begins to leak, the coolant burns on the hot engine, and produces a sweet smell that is unmistakably engine coolant.
The remedy can be complicated, and will necessitate removal of the intake manifold, heater tube, and possibly the water pump. After removal of these items, the connector may be replaced, or a set of o-rings, depending on the year of the vehicle. Most vehicles, model 2002 and later, will have o-rings, and the water pump will not need to be removed.
Wear on the rear axle shafts near the bearings can create excessive play and lead to gear oil leaking past the seals. Gear oil will leak onto the brake backing plate, brakes, and wheels.
One or more oxygen sensors may fail resulting in illumination of the Check Engine Light .
The steering gear may have an internal problem that makes turning to one side difficult; it may need to be replaced. A wheel alignment is needed after the steering gear is replaced.
The alternator may get too hot, causing premature bearing failure.