99 Grand Marquis loses power after it warms up

Asked by Jan 03, 2016 at 07:56 PM about the 1999 Mercury Grand Marquis 4 Dr LS Sedan

Question type: Maintenance & Repair

I just bought a 99 grand marquis. The seller
told me it would die sometimes when idling
after it warms up. Ive replaced the tps, the
idle air control sensor and the spark plugs.
After finishing everything i drove it up to the
gas station and it didnt have any power. It
struggled just getting to 20 mph. The engine
light is on but i cant get it up to autozone to
have them scan it. I ordered all new coils off
of amazon and im waiting for them to show
up. They were $50 for the set when advance
and autozone wanted $50 each. Im hoping
this fixes it but if not i dont know what else
to do. Any thoughts or suggestions would
be greatly appreciated. Thanks

21 Answers


Would be nice to have the codes... Does it seem like it is holding back? Any change in the sound of the exhaust? Thinking maybe a catalytic converter.

2 of 2 people found this helpful.

That was 1 of my thoughts too. I drove the car home when i bought it, keeping my foot on the gas at stop lights, and it drove fine other than that. It does sound like theres an exhaust leak though


Still could be a lot of things. If the cat is plugged, when it gets hot it will be glowing orange. Another thing could be the Air Idle Control Sensor. The sensor and the port it goes into gets all gummed up and needs to be cleaned. This is a major source of vacuum, and need to make sure the sensor seals when re-installing. Also there may be vacuum leaks. You can take a can of carburetor cleaner while it is idling, and spray along the intake gaskets, throttle body and anywhere there may be vacuum. It would be nice to have the codes. This points a person into the right direction of a fix. As you can see it can be numerous things.

2 of 2 people found this helpful.

I miss the good ol carburator days. When all you needed was gas, spark, and air.


The concept has not changed, but the way we make it work certainly has...

2 of 2 people found this helpful.

It does idle fine after everything ive already done. But after it warms up if i push the gas it sounds like it could be pulling too much air, or something could be clogged. Like the cat converter. The car only has 124,000 miles and i paid $1150 for it. As of now i still feel like it was a good deal but if i have to drop $500 or $600 more into im not so sure anymore.


It is sounding like a catalytic converter, when plugged it will make that noise. Get the car hot at night, trying to rev it up once warm, leave the car running or have someone with their foot on the gas, and look underneath at the cat. it will be glowing orange if it is plugged up.

2 of 2 people found this helpful.

Will do. I appreciate it


RF, I think you are correct about the cat con. Dunno why but we've seen more than unusual here.

1 of 1 people found this helpful.

Any idea what itll cost me? Rough neighborhood?


Ford is about 2k, aftermarket should be about $500 for both. Let your fingers do the walking and call some of the local muffler shops. If you can find one use a locally owned shop that's been around a while, although old school guys are grumpy they usually do good work.

1 of 1 people found this helpful.

$500 for both? So theres 2 of them on it?

sounds like your catalyst converter's pretty sure their part of the y pipe. I think it's about $500

Well, I had it towed to a mechanic today. He said the converters are bad but they shouldnt be causing this issue. He scanned the check engine light and it showed cylinders 4 and 5 are not firing. He said its the coils which are supposed to be delivered to my house tomorrow


So you have a miss on top of it. Keep us informed, always like to hear about what was done...

2 of 2 people found this helpful.

Will do

SUCCESS!!! Just spent about 45 mins in 20 degrees changing the coils in the mechanics parking lot. Im not gonna pay someone $150 to do something I can do. When i fired it up i could see the engine shaking from the misfire but after 2 or 3 mins it smoothed out and purred like a kitten, other than the catalytic converters making their noise. I appreciate everyones input and suggestions. Now I know where to go if I have any other issues


Good to hear...thanks for letting us know...

2 of 2 people found this helpful.

Glad that the converters weren't causing it to not go and you have your car back running again.

2 of 2 people found this helpful.

Got another question guys. Could the muffler alone cause these symptoms that made everyone point to the cadalytic converters


No, only if someone shoved up something in the exhaust to block the flow.

1 of 1 people found this helpful.

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