3800Pro Forums banner
1 - 20 of 27 Posts

·
Registered
Joined
·
57 Posts
Discussion Starter · #1 ·
Hello so i have a 99 buick regal GS and for some odd reason iam getting horrible gas mileage just this week ive had to fill up 3 times. Icommute to school everyday which is 50 miles round trip at 70-75 mph. If iwas to fill up on a monday id have to fill up again wednesday or thursday . Ihave no trouble codes at all . Can somebody please give me an idea of what can be goin on? I have done a voltage test on my MAF sensor ican test again if need be . Iam going to replace the sparkplugs and fuel filter tomarow .
 

·
Registered
Joined
·
83 Posts
I am assuming that the car is running OK otherwise--not shaking terribly, like a cylinder misfiring.

Sudden poor gas mileage is usually a bad MAF or O2 sensor.
Bad MAF sensors often fail to set a code for whatever reason.
The OE sensor was made by Hitachi and is far more reliable than aftermarket sensors, in my experience.
HITACHI MAF0047 you can get it online far cheaper than ACDelco brand (which is the same sensor with an ACDelco label ).

You really need a scan tool to see what is going on.

The MAF sensor should read around 5-5.7 grams per second at warm idle at sea level.
(Different altitudes cause readings between 3 and 6 grams per second)
Hertz readings should be between 1200-3000--again altitude dependent.
The primary O2 sensor should be rapidly fluctuating between lean and rich.
(Also known as the upstream oxygen sensor.)

(The downstream O2 sensor should read rather steady, not rapidly changing. It does not exert the fuel control that upstream sensors do and is used to monitor catalyst efficiency.)
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #3 ·
I am assuming that the car is running OK otherwise--not shaking terribly, like a cylinder misfiring.

Sudden poor gas mileage is usually a bad MAF or O2 sensor.
Bad MAF sensors often fail to set a code for whatever reason.
The OE sensor was made by Hitachi and is far more reliable than aftermarket sensors, in my experience.
HITACHI MAF0047 you can get it online far cheaper than ACDelco brand (which is the same sensor with an ACDelco label ).

You really need a scan tool to see what is going on.

The MAF sensor should read around 5-5.7 grams per second at warm idle at sea level.
(Different altitudes cause readings between 3 and 6 grams per second)
Hertz readings should be between 1200-3000--again altitude dependent.
The primary O2 sensor should be rapidly fluctuating between lean and rich.
(Also known as the upstream oxygen sensor.)

(The downstream O2 sensor should read rather steady, not rapidly changing. It does not exert the fuel control that upstream sensors do and is used to monitor catalyst efficiency.)
.
Okay so i do have a scan tool that gives me readings of the system i have already replaced the upstream O2 sensor about a month a ago and the MAF sensor ican test again with those numbers in mind that you have provided . As for cylinder misfires none that ican notice the car runs pretty good its just that my gas mileage is horrible . So if yud like ican perform some more test .
 

·
Registered
Joined
·
83 Posts
.
Okay so i do have a scan tool that gives me readings of the system i have already replaced the upstream O2 sensor about a month a ago and the MAF sensor ican test again with those numbers in mind that you have provided . As for cylinder misfires none that ican notice the car runs pretty good its just that my gas mileage is horrible . So if yud like ican perform some more test .
So the horrible gas mileage began after you replaced the O2 sensor?

What brand O2 sensor did you install?

Was there a problem that caused you to replace the original O2 sensor?

Please answer these same questions for the MAF sensor.

When you drive the car is the engine temperature as high as it was before?
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #5 ·
.
Okay so i do have a scan tool that gives me readings of the system i have already replaced the upstream O2 sensor about a month a ago and the MAF sensor ican test again with those numbers in mind that you have provided . As for cylinder misfires none that ican notice the car runs pretty good its just that my gas mileage is horrible . So if yud like ican perform some more test .
So the horrible gas mileage began after you replaced the O2 sensor?

What brand O2 sensor did you install?

Was there a problem that caused you to replace the original O2 sensor?

Please answer these same questions for the MAF sensor.

When you drive the car is the engine temperature as high as it was before?



So when i had for bought the car ireplaced the 02 sensor because right away iscanned the car and had a code for a 02 sensor with te bad gas mileage so ireplaced that so the gas mileage had improved a bit . Ithink the brand is Bosch and as for the MAF i have never replaced it iwent to the junk yard and pulled a few MAFS just in case mines was bad i am warming the car up as we speak ijust replaced the spark plugs so once its warm iwill test the maf with my scan tool and as for temperature it is good it doesnt go past the Middle mark of the Gauge
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #6 ·
I am assuming that the car is running OK otherwise--not shaking terribly, like a cylinder misfiring.

Sudden poor gas mileage is usually a bad MAF or O2 sensor.
Bad MAF sensors often fail to set a code for whatever reason.
The OE sensor was made by Hitachi and is far more reliable than aftermarket sensors, in my experience.
HITACHI MAF0047 you can get it online far cheaper than ACDelco brand (which is the same sensor with an ACDelco label ).

You really need a scan tool to see what is going on.

The MAF sensor should read around 5-5.7 grams per second at warm idle at sea level.
(Different altitudes cause readings between 3 and 6 grams per second)
Hertz readings should be between 1200-3000--again altitude dependent.
The primary O2 sensor should be rapidly fluctuating between lean and rich.
(Also known as the upstream oxygen sensor.)

(The downstream O2 sensor should read rather steady, not rapidly changing. It does not exert the fuel control that upstream sensors do and is used to monitor catalyst efficiency.)
So ive let car sit for about 10-15 mins on and my MAF readings so far are around 4.55-4.63 grams/sec iwill go on a drive ina few to see how it is then at idle after a drive .
 

·
Registered
Joined
·
83 Posts
Since you are about to use the scan tool, check the LTFT (long term fuel trim)and STFT (short term fuel trim) at idle and then check them while holding the RPMs at 2000 rpm for 15 seconds.
Post the results.

Now that I see you have posted about the MAF---what about the O2 sensor readings?
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #8 ·
Since you are about to use the scan tool, check the LTFT (long term fuel trim)and STFT (short term fuel trim) at idle and then check them while holding the RPMs at 2000 rpm for 15 seconds.
Post the results.

Now that I see you have posted about the MAF---what about the O2 sensor readings?
okay so at idle STFT its Either at 0.0 to 1.6 bouncing back and forth and my LTFT is at a constant 8.6
So now at 2000rpm
STFT iseen 0.8 , -0.8 and 2.6
LTFT 7.0
And my O2 sensor ready at idle is bouncing around in the .100s so itd bounce any where from 0.100-0.760
 

·
Registered
Joined
·
83 Posts
Your fuel trims indicate that your car's computer thinks it needs some more fuel so it is adding fuel.
The numbers raise my suspicions as they are "borderline".

The reason I recommended checking at idle and at 2000 RPM was to eliminate the possibility of a vacuum leak causing higher fuel trim numbers.

A bad MAF sensor would be under or over-reporting the amount of air flow as falsely lower/higher grams per second--which does not seem to be happening.

There have been numerous reports of Bosch oxygen sensors causing problems in GM 3.8L engines, so I recommend replacing the Bosch O2 sensor with an ACDelco OE (not Professional) or Denso oxygen sensor.

I've never read complaints about either of these brand oxygen sensors on any forum.
Amazon and Rockauto have them for less than $40 and under $30.
 

·
Registered
Joined
·
83 Posts
I should add that when you buy a used car it is never a bad idea to replace:
Spark plugs
(ACDelco 41-101 is the best plug to use for longevity and reliability)
Air filter
Fuel Filter
PCV

I'm not pushing ACDelco parts as a "GM fan" or "ACDelco fan".
The ACDelco OE parts have simply been proven very reliable in both my professional and personal experiences.
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #11 ·
I should add that when you buy a used car it is never a bad idea to replace:
Spark plugs
(ACDelco 41-101 is the best plug to use for longevity and reliability)
Air filter
Fuel Filter
PCV


I'm not pushing ACDelco parts as a "GM fan" or "ACDelco fan".
The ACDelco OE parts have simply been proven very reliable in both my professional and personal experiences.

About 3 months ago ihad replaced everything you just mentioned ihave a routine matience and document everything that ido to the car but where ihad messed up on is iridium plugs are to no be touched wirh a gapped tool and i recall when doin my sparkplugs as a habit i get a gapp tool and check them so once ipulled the plugs off today i had noticed the electrode on some plugs were smashed without me knowing.. now the car has new plugs and its running amazing alot more acceleration and smoother feel then before .
 

·
Registered
Joined
·
83 Posts
Very glad for you.
Thanks for posting back about it.
A good rule to follow is: whenever something is replaced/repaired and a new problem begins, the very first things to check are the things you just did.

I would still keep monitoring the gas consumption.
I find it strange that the narrower gaps did not result in overall negative fuel trims.
If mileage does not substantially improve, a new Denso or ACDelco O2 sensor would be the next step to take.
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #13 ·
Very glad for you.
Thanks for posting back about it.
A good rule to follow is: whenever something is replaced/repaired and a new problem begins, the very first things to check are the things you just did.

I would still keep monitoring the gas consumption.
I find it strange that the narrower gaps did not result in overall negative fuel trims.
If mileage does not substantially improve, a new Denso or ACDelco O2 sensor would be the next step to take.
sounds good thanks for all your help i really appreciate it
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #15 ·
Glad to assist.
BTW, very wise of you to have extra OE MAFs on hand.
I've been known to drive with a freshly cleaned spare in the trunk on long trips.
Hey man so ive been driving to school all this week and my gas milage is still horrible although the car feels amazing now good acceleration and everything . But the gas is just killing me im basicly only getting 250 miles on a full tank thats insane considering ihave a 17gal tank and at those miles ishould be getting more Mpg in my opinion .. iam going to test one of the other MAFS igot on hand too see what my results are .. any suggestions? Thanks
 

·
Registered
Joined
·
83 Posts
Personally, I would change the oil since you recently had misfires and that unburned fuel contaminates the oil and makes the engine work harder.

Clean up a MAF--which I hope is from a 3.8L engine (the AFS number should match the original) and install an ACDelco OE oxygen sensor.

You may not want to spend another 35 or so on an OE sensor but those Bosch O2 sensors have been very problematic in our 3800 engines.This is not just the opinion of one person and I can provide many links to reports on Bosch sensors and 3800 engines being a poor match.
Here is just one http://www.3800pro.com/forum/exhaust/32909-o2-sensor-brands.html
An OE ACDelco or Denso oxygen sensor is cheaper than a single tank of gas.
Look on Rockauto and Amazon for prices.

If your wheels are out of alignment, front end or suspension has badly worn parts, tires are worn and/or underinflated, brakes are sticking or transmission is low on fluid, these will each rob you of fuel economy.
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #17 ·
Personally, I would change the oil since you recently had misfires and that unburned fuel contaminates the oil and makes the engine work harder.

Clean up a MAF--which I hope is from a 3.8L engine (the AFS number should match the original) and install an ACDelco OE oxygen sensor.

You may not want to spend another 35 or so on an OE sensor but those Bosch O2 sensors have been very problematic in our 3800 engines.This is not just the opinion of one person and I can provide many links to reports on Bosch sensors and 3800 engines being a poor match.
Here is just one http://www.3800pro.com/forum/exhaust/32909-o2-sensor-brands.html
An OE ACDelco or Denso oxygen sensor is cheaper than a single tank of gas.
Look on Rockauto and Amazon for prices.

If your wheels are out of alignment, front end or suspension has badly worn parts, tires are worn and/or underinflated, brakes are sticking or transmission is low on fluid, these will each rob you of fuel economy.
iknow i have not mentioned some things but everything you have listed i have done already just recently but ilike the suggestions so thank you very much . I am going to go to the parts store to get a new 02 sensor hopefully they have AC delco i have done a lil search and seen bosch sensors are crap . Iwill get back to you on results on the new sensor in a few days thanks
 

·
Registered
Joined
·
57 Posts
Discussion Starter · #19 ·
You can get the ACDelco sensors at Amazon and Rockauto (assuming you're in the US)--which are probably less shipped than what a local store is able to charge.
Hello iam back to let you kno how my cars MPG has been and I personally think its still guzzling gas .. istill drive to school all week 25miles to school and 25miles from school a day . What really catches my eye is that i only get 25miles per half a quart so in a day to and from school(50miles) it takes a whole quarter which idont think is right
 

·
Registered
Joined
·
83 Posts
What really catches my eye is that i only get 25miles per half a quart so in a day to and from school(50miles) it takes a whole quarter which idont think is right

Do you mean that it is using 1/8 to 1/4 of a tank of gas to go 50 miles?
Did you ever install and ACDelco or Denso O2 sensor or are you still sticking with the Bosch?
What are the fuel trim readings at idle--both STFT and LTFT?
Raise the RPM to 2500-3K for 30 seconds and read the fuel trims again.
What are they then?
Are there any problems with how the the car runs? (Rough idle, hesitation, lack of power, etc.)
 
1 - 20 of 27 Posts
Top