I have a 97 mercury grand marquis with unknown miles. the odometer drive gear was broken and it was stuck on 125k. the car runs great and all seems well but after driving it for 30 min or so it starts to rattle under a load and seems to lose power, not much but a little. I recently put injector cleaner in it with hopes it might be a sticking injector but it hasn't improved. it almost sounds like a diesel engine, but once it sits for a while it's ok, until I drive it that much again. the engine is a 4.6 and runs strong until I hit the 30 to 40 minute run time. I do not hear it when I stop, put it in neutral and rev the engine, it's only under a load. today it seemed to idle rough, sort of a pulse but I thought it might be the ac cycle, I'm not sure. the car is sound and I only drive it around town, rarely exceeding the 30 min time but its a concern and I need some ideas as what to look for.

 

Disconnect the battery.
Take off the NEGATIVE cable first, then the Positive cable.
Leave the cables disconnected for one (1) hour.
Replace the battery cables. Positive first, then Negative.

You're good to go !!

 

thank you 95, I'll give it a try

 

What you did by disconnecting the battery was re-setting the ECM (computer).
Driving the car for an hour should tell you if the problem has returned.
The "rattle" is a puzzlement, since it's really a hard thing to describe.
Ignition problems usually cause the "Check Engine" light to come on, and codes should show up that tell you which cylinder(s) are bad.
However, the "broken" speedometer indicates that some monkey has tampered with the car in an effort to cheat a potential buyer.
Sometimes, the bulb on the "Check Engine" light has been removed.
That can be checked by turning the key "on" without starting the car.
All bulbs on the dash should be lit -- if the "Check Engine" light is not lit, it's a good sign that it's been tampered with.
If the "Check Engine" bulb DOES light, then the system is working, and a Code Reader should find the problem.
If there are NO CODES, it would indicate a fuel delivery problem.
Replace the in-line fuel filter (under the car, on the driver's side) with a genuine Motorcraft filter.
If the car has sat for a few months, the fuel may be contaminated, especially with ethanol fuels -- standing fuel gums up the injectors.
I recommend ISO-HEET to remove any water in the system. There's nothing better for that purpose.
The fact that the problem is worse when the car has been run for 30-40 minutes indicates that there's something wrong with the emissions-control system.
A scanner-code reader should find and identify the problem.
A vacuum gauge is a cheap way to diagnose bad valves, worn rings, and/or vacuum leaks.
When checking this stuff, ALWAYS make sure that ALL accessories are turned "OFF".
If you're not familiar with using a vacuum gauge, just google it.

Lemme know how you make out -- I'm really interested in your problem, and would like to help.
Our engines are SIMILAR, but not identical.
I have Mercury Factory Shop Manuals for the body, engine, electrical, and vacuum systems.
Let's git 'er done !!