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Bad fuel economy

Blackbird87

Blackbird87

New member
Hey guys new to the site and new to the car, so bare with me. Just bought a 99 gp se with the 3.1 117,000 on the clock. I'm getting about 15-17 mpg city driving and driving like a sally. I changed the plugs, injectors (owner said one of them stuck when the car sat so i just changed em), air filter and cleaned the throttle body plate. The plug wires look new so i left them. The car runs and drives great nothing weird going on or ses light so im stumped.
 


Try a Coolant Temp Sensor. These little guys control a lot of stuff and if it's reading that your engine is below operating temp, it may add more fuel. Also, is the actual temp of the engine (when warm) at operating temp?
 
I think its at a normal temp it sits at 180ish. Thats where my firebird is always at. I dont believe the temp sensor is bad cuz when you reststart the car after its been driven awhile it starts just fine. Ive had one go out on a different car and it struggled to start. I did notice my lim gasket is starting to seep so thats getting changed right away, ive already checked the oil for coolent and vice versa
 
You have the 3.1? In the 3.8, it's 195 degrees. If the sensor goes out fully, that's one thing but it may just ohm out at a different level and not drop enough as the engine warms, it may cause the system to go open loop, it wouldn't effect how the car starts, in fact the only thing you may notice is mileage or a difference in gauge temp to actual. if the connector has corroded terminals that will mess up the resistance.
 
The temp sensor is right under the tb to the left?, ill take a look at that when i do my lim gasket. But the gauge moves normally and sits at 190ish now that i took a second look at it. Are there two temp gauges? one for the pcm and one for the gauge
 
What you did was all good. However there are a couple more obvious places to look. I would start with the MAF sensor, just clean it. Just use electric contact cleaner, or actual MAF cleaner. Be very gentle, and whatever you do don’t touch it. You could go blind :th_winking:
The second item is the front O2 sensor, if it has never been changed just do it. It is time, and it is the single most important mpg related sensor.
On a side note, the old clean air filter bit loosely applied to old carbureted vehicles. However, on modern FI vehicles like ours it makes zero difference in mpg. But a new/clean filter does help with acceleration and engine protection.
Just one other thing it is winter, and ALL vehicles get less mpg. In fact at work, just last week I had a discussion on the topic. He has a nearly new Honda CRV and his mpg has dropped by about 20-30% recently. The cold weather has several deleterious effects on mpg.
Change out that front O2 sensor, I think you will be happy with the results. Especially come spring.:th_thumbsup-wink:
 


The maf looked really clean when i checked it so i left in alone. im going to assume that theres only one o2 and not one on each manifold. The weather was something i was thinking about my civic would get 30+ summertime but drop to lows 20's in the winter. Maybe someone can chine in on what theyre getting in the winter.
 
GPs have one O2 sensor just behind the rear exhaust manifold; you can see it plainly looking down from above the engine compartment. And FYI, there is a second O2 sensor under the car behind the catalytic converter. Don’t worry about the second one; its only function is to check the catalytic converter.
As far as what to expect for mpg, considering Chicago traffic and it being winter you should expect about 21-22 mpg average. And of course this is just rough numbers, there are only about 40 factors effecting mpg. And that is assuming your car is working correctly. Just remember the biggest mpg changer for any vehicle is the input device behind the steering wheel.
 
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To 110% true...vacuum leaks will give you worse MPG's along with clogged cat's more than anything will, including a driver mod.
 
Well i have a lil bit of work cut out for me this weekend but ill let you know what happens. The best ive seen is 17mpg and thats with 40 mi of highway. I live in a suburb of chicago so traffic isnt even that bad and as for driving 2.5k is the highest revs the car sees
 
Maybe try giving it a little juice to blow the spider webs out?

I'm betting the cat is clogged...common issue on N/A cars.
 


Replace the Thermostat!

It appears that DTC P0128 is not turned on from the factory.

You may have a Thermostat that is opening partially to early. I see this all the time on V6 cars.
 
CalEditor said:
Replace the Thermostat!

It appears that DTC P0128 is not turned on from the factory.

You may have a Thermostat that is opening partially to early. I see this all the time on V6 cars.
Click to expand...

Its on my list for the weekend haha. What are my options for datalogging this car. I have an obdII cable and laptop. I do know what im doing as ive tuned my firebird im just not familiar with these cars yet
 
little update, i replaced the lim gasket, thermo, temp sensor and o2. mileage is pretty close to the same and now my temp sits right at 210 on the gauge and its a 195 thremo :th_scratchhead:. How do i check for a clogged cat without cutting the pipe right in front of it. One last thing i do notice is the car drives like im pulling a trailer or like it has a drag but it coasts fine.
 


did ya bleed all the air out of the system????

a clogged cat will cause the car to bog down and if it was really bad and rotted away, it would prevent the car from moving itself.
 
I did bleed all the air out but I have a feeling the damn thremostat is bad cuz it will spike up to over 210 before it opens. So I'm gonna replace it again. I'm thinking it could be the cat. What's the easiest way to check it. I was just going to cut the pipe on the inlet of the cat and just weld it back up.

The fuel filter hasn't been changed but I did pick one up and will change it when I get it on the lift and check out that cat.

Yesterday I did go on an all highway trip and I got 22-23 mpg
 
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