I have a 99 GP GT which calls for platinum plugs. well i m agetting horrible gas milege at 29,000 miles. like 20 mpg on the highway. Well I put new air filter plug wires fuel filter and when i got to the spark plugs i replaced them with the correct plat ones. Well My 90 Lumina with 140k gets about 30 mpg on the highway. the plugs in that car look like they were just put in. Beautiful after the 100K tuneup. But the plat ones in GP are dark color like they are not hot enough and i was htinking of putting regualr plugs in tehre. what can I do?

 

Sounds more like fuel trim is off , are you still in warrenty?
If so complain , if not complain to GM directly . If you complain enough you will see that you can get alomost everything for free.

 

Not always the case.


Do not put regular plugs in the car. For some reason unknown to me the platinum plugs are the only ones that will allow the car to run correctly and its not a joke. I pulled regular AC's out of the same type of car you have (running horrible after a recent tune up) and replaced them with cheap Bosh platiunm plugs and it ran 100% better. $6 is outrageous for an PT Delco plugs use Bosh, $1.99 is a lot better deal and works just as well.

 

Have you changed the O2 sensor? That can cause a rich condition that will turn the plugs black and get bad mileage.

 

Theres 4 O2 sensors. And at 60 bucks a piece that gets pricy. Naw its out of warrenty cause the 3 years ran out before the milage, but while it was in there i complained and they had they couldnt find anything wrong. Only thing they did then was gave it an injector cleaning. I do knotice that it takes like 10 years for the car to heat up and on the highway in the cold right now it barely gets above 175* so I may swap out the thermo. maybe its not heating up fast enough.

 

that could cause the car to fowl the plugs , ussually they have a time out code for the car not comeing up to operating temp , or should i say not going into operating temp . Bot i also bhav only worked on cars with a supperior ecm . I know that on a trionic 7 sys from saab it will through up a manufatures code . Try a new stat . and go from there . Also i know you said the car is out of warrenty but you stated that you had it in for a similiar complaint . If there is one thing i learned about dealerships its that if you camplian you most likly get a goodwill . If the dealer wont help you , you bypass them and go to gm . keep on it and you get just about anything , trust me on this.