99 grand prix gtp poor fuel economy

[wizardsinsight]

99 grand prix gtp runs good. Poor fuel economy. Had to replace orifice tube in a/c. When put everything back together it runs good but has poor fuel economy. about half of normal. Found leak at TBI/Supercharger. Surface was warped. Car got hot last month due to bad radiator. Repaired surface and leak is gone. Still poor fuel economy. Have done all normal steps tune up, fuel pump, fuel pressure regulator. All of these solved specific symptoms but still poor economy. The DIC information is sporadic and can't be relied on but the fuel gauge is accurate. No codes or lights. Any ideas would be greatly appreciated.

 

[melsg5]

Using a vacuum gauge see what your idle speed reading is. Then raise the engine RPM to 1,500 to 2,000 and see if the reading goes down. If it goes down good chance you have a restrictive exhaust caused by a clogged cat. You can also check for other vacuum leaks using a spray can of carb cleaner and note any change in engine idle.

 

[brandont96]

unfortunatly your car is considered old. older cars tend to get poor fuel economy after a while

theres not much u can do to fix it

mynes a 2000 grand prix se and only gets 15 to 16 mpg when it use to get 20 to 23


heres a list of what you should check/change

air filter
oxygen sensors
spark plugs/wires
fuel injectors
change your oil try synthetic or use what the owners manual recommends.
change your fuel filter

your looking around 150 to 200$ to replace all listed above.

those are somthings u should deffinitely check into and if you got the money to replace them all go for it because it wont do any harm it will just do the car some good.

 

[GrandPrixGtp]

Might I add DO NOT use synthetic oil if you haven't been using it since day one! It will clean up any sealed leaks and I guarantee you will have many leaks after one synthetic oil change! I only know this because it happened to me!

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[brandont96]

i had a grand am that i changed to synthetic with no issues.