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Matt, Engineer
Category: Mercury
Satisfied Customers: 20389
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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2000 grand marquis, will run very well most of the time, but

Customer Question

2000 grand marquis, will run very well most of the time, but when i take it on the highway, after approximately 100 miles, runs rough, misfire codes and len code
Submitted: 2 months ago.
Category: Mercury
Expert:  Matt replied 2 months ago.


This sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,

Ideally measure the pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

Customer: replied 2 months ago.
at turnpike speeds car runs smooth, rough only at idle, and that is when i get the obd codes. ill check fuel pressure, but i would think a fuel problem would also appear at high rpm under load?
Expert:  Matt replied 2 months ago.


Ok in that case check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

Expert:  Matt replied 2 months ago.


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