Asked by Aug 04, 2013 at 05:39 AM about the 1998 Mercury Grand Marquis 4 Dr GS Sedan

Question type: Maintenance & Repair

About a month ago my 1998 Mercury Grand Marquis started running rough after I pumped regular gas at a Chevron Station. As the car has 67,000 miles I took it to Sears Auto Center to have the spark plugs replaced.It continue doing the same.

Then I decided to take to the Ford Dealer and ran a diagnostic test. They said the problem was with a coil and 2 spark plugs. I gave the okay to replaced them. When I picked it up and started driving it I noticed that it was running the same way as it was before I took it for the diagnostic test.

I returned to the Ford Dealer the next day and their foreman kept it for 2 days. At the end of the second day I decided to picked it up and the foreman wanted to take a ride with me. When I started driving it the car was running smooth, so I took it home and it was running smooth for the 5 mile trip.

The following day I went out for a ride and I soon as started it was acting again.

The next day I rode the car and it was running smooth.

I checked in the internet and found that the problem could be with the Regular Gas, it may contained WATER, and the suggestion was to pump PREMIUM GAS so it will absorb the water, if any.

I have done the regular OIL and FILTER replaced every 3,000 miles and never had had any problem with the car.

REPLACEMENTS: alternator, serpentine belt, battery (twice), brake job, etc.

I am retired and drive the car about 4,500 miles a year.

As the car is running rough now and then, when is running rough is sounds like it is COUGHING.

Do you have any suggestion?

Thank you.

20 Answers

Tom Demyan

You car is supposed to run on '87 octane. Maybe you got some bad gas, but how often do you drive? Adding premium fuel won't help you. Maybe some concentrated fuel injector cleaner would. But the running rough should have cleared up by now if it was water in the gas. I see you took the normal course of action to correct the issue. Replacing parts and a tuneup. Is the check engine light on? Without seeing the car, I can't help much further, but would recommend taking your car to a local mechanic and have another set of eyes to look at it. Since the problem is intermittent, those issues are harder to fix. Have the throttle body cleaned, look for vacuum leaks, clean the EGR of carbon, check the PCV valve.

4 out of 4 people think this is helpful.

When Sears did the plugs, did they replace the plug wires, cap and rotor too? They should have. As dandyoun mentioned if it mainly acts up in damp conditions that definitely points to spark plug wires, cap and rotor. Your Mercury is designed to run on regular grade fuel. Since you drive it only about 4,500 miles per year I get the impression it sits a lot. Try keeping the gas tank as close to full as possible. This will prevent condensation from forming inside the gas tank. Run a bottle or two of "Dry gas" through the fuel system. This will absorb any water in the fuel system. Premium fuel will not. HTH. -Jim


NEVER DONE: (Ford Dealer ran a Diagnostic Test) The throttle body cleaned, look for vacuum leaks, clean the EGR of carbon, check the PCV valve NEVER REPLACED: Plug wires, dist cap and rotor button DONE: concentrated fuel injector cleaner

1 out of 1 people think this is helpful.

Yup, definitely change those plug wires, cap and rotor. - Jim

1 out of 1 people think this is helpful.
Tracy Hooks

Hey Wait! Isn't this car equipped with the 4.6L engine? THERE IS NO DISTRIBUTOR, CAP OR ROTOR! I'd go with the "BAD GAS" theory. Octane only relates to resistance to predetonation (Ping). I would suggest you let the diagnostician at the dealer take the car home and drive it for several days so he can get a feel for the car under all conditions. With difficult a diagnosis, you will have to be patient and let them figure it out.

3 out of 3 people think this is helpful.

Good catch Tracy. I looked up a dist for it before I posted earlier but after your post I double checked my search and I had entered the wrong information. Thanks for catching that.


As I live in South Florida we either have RAINY DAYS or SUNNY DAYS! Our time is now 2:00 PM. It has been a SUNNY DAY ! SO GUESS WHAT ! ! ! I went out this morning and the car was running SUPER SMOOTH . I drove around 10 miles and the car was not "COUGHING". Any thoughts?

4 out of 4 people think this is helpful.

I would tend to lean toward coil/ plug issue. Did they only change two of them? If so why? Normally when it's time to change one its a good time to do all.

1 out of 1 people think this is helpful.

With each plug having its own coil one of them could be jumping spark out somewhere. This would be more noticeable on a damp/rainy day. Causing it to cough. If it was mine I would do me up a mist bottle with water in it and lightly mist the plug wire / coil areas with car idling. If one is going to jump spark you will here it make a cracking/snapping noise and see blue spark jump out of it to ground.

2 out of 2 people think this is helpful.

Oops, well then yes, replace the other coil-overs. By the recent description of today's events (smooth then chugging) I'd say one or more coil(s) is getting hot and breaking down. HTH. -Jim

4 out of 4 people think this is helpful.
Blanche Blevins

Add a can of gas treatment. It's about $1.60 at WalMart. All gasoline has some water in it but if it collects more that 12% water the car will run rough and can eventually damage the engine. Learned this from my manager when I worked at a convenience store. She checked the water content in our gas every morning, but some don't check it or at least not on a regular basis.


The 4.6L engine in the Grand Marquis is different in that it has separate spark coils attached through rubber-booted springs to the top of each spark plug. I caught Sears in Florida trying to steal two new tires from my car that they even installed on a tech's truck when I called them on it. They also do tune-ups without replacing plugs after insisting on it when dropping the car off. So I do not go to Sears... (Makes you wonder why Ford would need to replace two new plugs.) Anyway humidity can cause problems with older spark connections, and the high voltage coming from the spark coil to the plug can cause intermittent shorts as the rubber boot loses it's insulation near the plug end. When I replace plugs on my 2003 Grand Marquis, I also replace the rubber boot above it going to the spark coil. I use a SMALL amount of dialectic compound on each end of the boot and on the contacts of the plug going into the spark coil to help control moisture. My car just hit 110,000 mi, so if I discover one coil going bad, I will probably replace them all. This is an outstanding car, and I hope I can keep it forever. My Dad always took his straight to Ford, but I am too cheap to do that, and no one is going to care as much about keeping it running perfectly as I will.

1 out of 1 people think this is helpful.
Tracy Hooks

No individual coils on this vintage Ratpuppy. 2 coils with 4 posts each. I wouldn't recommend replacing all of the coils on your car - We have Police Interceptors with 150,000 with no ignition problems at all. We've found a couple that missed their tune-up intervals and still had the OEM plugs! We seldom ever need to replace a COP coil.

1 out of 1 people think this is helpful.

Catalytic converter plugged up?

6 out of 6 people think this is helpful.

Had this problem with my 2008. The air cleaner is a little tricky on this car. If it is not perfectly seated, the idle is very rough and the car stalls. Cost = free.

2 out of 2 people think this is helpful.

If I understood correctly you said it runs rough intermittently. When you have a vacuum leak it does not come and go, When you have coils or spark plugs bad they usually do not come and go and if the coils or spark plugs were bad it would miss when accelerating. Secondary ignition always breaks down when the engine is under load. The most likely cause if it is intermittent is a sticking EGR Valve. The EGR valve should never be open when the engine is at an idle. If it is sticking open or just part way open it will cause a rough idle. The EGR Valve allows exhaust gas to flow into the intake manifold for emission purposes. It is possible to have a PCV Valve that is bad but the computer will have a lean code because of the size of this leak. I did not see anywhere in the responses the need to check the computer for codes. This should be done before any work is done and in many cases will lead you to the problem. Do not add any of the dry gas additives be cause they are mostly alcohol. If you are using ethanol gasoline you already have plenty of alcohol at 10%.

4 out of 4 people think this is helpful.

Replace the feul filter


If your Ford 4.6 misses on rainy days but runs OK in the dry, try spraying your coils with this clear protectant coating from NAPA.


Have the self puckering sphincter valve adjusted and re-tension the muffler bearings. Then if all else fails have the windshield regulator re dimmed because the way the light refracts through the three tone frequency generator is setting up an infinite loop of mechanical malarkey. I am sure that the AM / FM 8 Track player has lost a 1/4 track off the greasy side and shiny side up Goofy buddy hamer down on the knob polishing seat cover.

Cory Wayne

I have a 1991 grand marquis will a 2004 alternator off of a grand marquis work for the 1991

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