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Gas mileage dropping?

K

Karrade

New member
I have a 2001 Grand Prix GT and I've noticed over the past few weeks that if I drive at 60 MPH and either just keep it there with my foot or cruise control my gas mileage is 29-30 and the RPMs are steady at 2000 however once I hit 65 or over I lose 5-6 MPG and it drops down to 24-ish and when I first bought it the gas gauge didn't drop nearly that fast when doing 65-70. Are there any common gas mileage problems with the Grand Prix's?

I know that there is a minor coolant leak in the throttle body/upper intake and it comes out a little bit when you really punch it as well as the top right seam on the radiator is hissing which will be getting fixed soon but the car has never overheated, or gotten any hotter according to the temp gauge unless it sits running in hot weather on the freeway in traffic. The transmission has been replaced with a TripleEdgePerformance one when the original one went bad and seafoam has been used as well.
 
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Hard to say here. Cover all your bases first though.

Clean air filter(?), clean the MAF sensor, do any tune up stuff required. If you can get a scan tool you can watch different parameters while the engine is running to see if anything is going wrong. High positive fuel trims would mean it's using extra fuel, so like a vacuum leak.

Check tire pressure, I'd suggest 35psi myself.

Even a failing o2 sensor could cause a drop in mileage.
 
I definitely plan to get a clean air filter soon, that's the easiest thing to start with out than a fuel injector cleaner. Thank you for the suggestions! Is there a good way to find a vacuum leak if there is one?
 
You can have the engine running and spray carb cleaner around all the vacuum fittings. If you find one that makes the engine change idle then that's a vac leak. Other way is a smoke machine.

Since you have a known coolant leak near the TB/UIM I'd suspect at least a bad TB to UIM gasket. At worst a warped UIM. If that is the stock UIM then I would consider replacing it since if it decides to crack internally then it will dump coolant into the motor and either hydro-lock it or cause the lower end to eat a bearing due to it ingesting coolant. Either way not good on the motor.

When I started having this similar issue I went all out and just replaced the LIM gaskets, new UIM, injector O-rings and all the other bits. Also blocked off the coolant to the TB. Seems my UIM was causing my vac leak.
 
Sounds like you've had the car for a while.. please give us more info.

When did you get it?
Last tune up with plugs, wires?
Ever clean the throttle body? Perfect time to fix that coolant leak at the TB btw
Any exhaust leaks?
Any check engine light?

Personally, I like to fix the known issues and then go from there. Otherwise you are saying..these might be the problem and I haven't addressed the obvious yet.
 
If I can keep it close to 100 km/h (60mph) on the freeway, I can get and extra 50+ km (31 miles) out of a tank. Sure this hasn't happened before?
 


Apologies for the super late reply. After getting the UIM replaced with new gaskets for that and the lower one as well as having the LIM refinished to remove any rust/buildup it's definitely being more consistent and removed some vacuum leaks. Next up I need to clean the throttle body and replace a few of the sensors if nothing but for the fact the car is over 17 yrs old.
 
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