Only getting 5 MPG, excessive fuel consumption

Tiny
TOTALMC
  • MEMBER
  • 1999 PONTIAC FIREBIRD
  • 3.8L
  • 6 CYL
  • 2WD
  • MANUAL
  • 180,000 MILES
Car listed above 3800, just replaced heads on motor and car seems to be running a little rough. Blamed it on 4-month-old bad gas, filled the tank fresh and now I only get about 90 miles out of a tank of gas (about 5 MPG). No fuel odor , so I don't think it's a leak

Car has a new catalytic converter and O2 sensors, new tune up parts, and injectors were cleaned and tested, along with a set of brand new heads that were just installed.

Any ideas on why it's using 3 times as much fuel as it should?

Thanks
Do you
have the same problem?
Yes
No
Friday, November 6th, 2020 AT 2:48 PM

19 Replies

Tiny
KASEKENNY1
  • EXPERT
That much fuel usage is most likely due to faulty injectors that are just dumping too much fuel. We should do two things even though you tested the injectors. First check the fuel pressure because high pressure can cause this as well.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Next is to monitor the live o2 sensor data with a scan tool. You should have an OBD II vehicle which means most scan tools (not code readers) should be able to do this. We can either monitor the short term fuel trims or o2 sensor voltage so that we can see that it is in fact running rich. If the o2 sensors are not showing a rich condition then that could be the issue.

Is there any smoke coming from the exhaust?

https://www.2carpros.com/articles/engine-black-smoke

Black smoke shows a rich condition. Let's start with this and go from there.
Was this
answer
helpful?
Yes
No
+1
Friday, November 6th, 2020 AT 3:05 PM
Tiny
TOTALMC
  • MEMBER
Thanks, I'll check the fuel pressure again (it was fine before I installed the new heads), but I do not have a live scan tool (been meaning to get one, just don't really have enough use for it to warrant the cost).

If I do go pick one up, I'm not sure I follow the second half of your answer - " if the O2 sensors are not showing a rich condition, then that could be the issue " - what is " that " in that sentence?
Was this
answer
helpful?
Yes
No
Friday, November 6th, 2020 AT 3:16 PM
Tiny
KASEKENNY1
  • EXPERT
If you are getting real poor fuel mileage, we would expect the o2 sensors to show the exhaust is very rich. If the o2 sensors are not showing the exhaust as rich then they are not operating and giving the PCM correct information so it just keeps dumping more fuel. We can get to that after the fuel pressure checks out.
Was this
answer
helpful?
Yes
No
Friday, November 6th, 2020 AT 4:50 PM
Tiny
TOTALMC
  • MEMBER
Okay, so by " that " you meant the O2 sensors? Wasn't sure. So if the O2 sensor doesn't show a rich condition should I pull a plug to confirm if it's running rich? If it's not the O2 sensor - which are brand new ac delco's, could a vacuum leak be causing it to dump so much fuel, and shouldn't it be a large easy to leak if it's dumping fuel at three times the rate?

Thanks again
Was this
answer
helpful?
Yes
No
Friday, November 6th, 2020 AT 5:01 PM
Tiny
KASEKENNY1
  • EXPERT
Good thought. I didn't think of a vacuum leak. We would have gotten there but that is pretty easy to check.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If the o2 sensors are new, let's assume they are ok. If you want to go ahead and pull a plug you can however, if you are getting that poor of fuel mileage, we can assume it is at this point.
Was this
answer
helpful?
Yes
No
+1
Friday, November 6th, 2020 AT 5:05 PM
Tiny
TOTALMC
  • MEMBER
Okay, so when measuring stft. What kind of numbers am I looking for? (As in what would be a normal number). And the same for the O2 sensor voltage and what kind of fluctuations am I looking for? Thanks in advance.
Was this
answer
helpful?
Yes
No
Thursday, November 12th, 2020 AT 2:20 PM
Tiny
KASEKENNY1
  • EXPERT
Okay, here are the basics:

O2 voltage and stft is measuring the same basic thing. The o2 sensor is measuring the amount of oxygen in the exhaust which we can determine air/fuel ratio based on that. It does not measure fuel. We just know that there is fuel and oxygen in the exhaust so if there is more o2 then there is less fuel. When there is less o2 then we know that is because there is more fuel.

So here is where people get confused when looking at the +/- number of the fuel trims. Keep in mind this is a measurement of o2 in the exhaust and nothing else. So if the number is negative, that means there is less o2 in the exhaust then desired. You want the number switch around 0. It will never just be zero because the exhaust is always changing and the PCM is constantly adjusting it. So that is why we want it going negative a couple points, and then it adjusts and goes positive. Then negative and it keeps switching back and forth from negative to positive.

So if a negative number is less o2 that would be considered a rich exhaust. This means a positive number has a larger amount of o2 in it then desired. This would be considered a lean exhaust.

Take a look at the attachment that illustrates this.

As for the voltage, that is just a different way of looking at the same thing. The lower the voltage, the more lean the mixture. The higher the voltage the more rich the mixture.

Let me know what questions you have. Thanks
Was this
answer
helpful?
Yes
No
+1
Thursday, November 12th, 2020 AT 6:07 PM
Tiny
TOTALMC
  • MEMBER
I'm not sure, but I think the numbers looked good while reading the live data of the fuel trim (I have some readings written down I'll send them later to get your opinion).

I pulled one plug only, and it smelled like fuel.

Tested the resistance on the coils, on two of them the secondary resistance was just a little over 6, but the third was showing a resistance over 9 ( I read they should be between 5 and 8, and also they should have similar resistance ), so could that be a problem? Also when checking primary resistance, it would show about.5 when the leads were first touched, but would drop to zero almost instantly which I would think would tell me the coils are faulty, but the car runs strong under load, just a little rough at idle (kind of sputters, all the way through the tailpipe). If the coils were faulty the car wouldn't really run at all, right?

Also, fuel pressure is at 40 PSI - no smell of fuel while the car is running (or off), and exhaust smells okay, doesn't seem overly rich. But it's only getting half the MPG it should.

Thanks in advance for any help you can give.
Was this
answer
helpful?
Yes
No
Saturday, November 14th, 2020 AT 7:07 PM
Tiny
KASEKENNY1
  • EXPERT
As for the coils, your secondary side should be around 5-8 K ohms. I am sure that is what your meter was reading but I just want to make sure that is clear in case others read this in the future. So that means 5,000-8,000 ohms. The primary side should be less then 1 ohm. So they appear to be working. If you meter is dropping to 0 and not showing an open circuit then that is probably just the meter as it gives you an average of what you are trying to read so that it is not bouncing all around so it is not uncommon for it to tell you 0 on something like this. Also, the fact that the engine runs well tells us they are operating.

However, this appears to be conflicting information. The reason is, if you are getting that poor of fuel mileage, I would expect you to smell fuel in the exhaust and in the engine compartment. Plus the o2 sensors should be showing very negative fuel trims or the voltages should be below.2 volts. What were the numbers when it was running?

Also, I never asked, how did you calculate the fuel economy?
Was this
answer
helpful?
Yes
No
Sunday, November 15th, 2020 AT 2:37 PM
Tiny
TOTALMC
  • MEMBER
Calculated fuel economy by the fact that 3/4 of a tank of gas went 111 miles, when a full tank usually lasts over 300. STFT covers right around zero on both Banks - LTFT B1 is around 14, B2 at 10.9. Guess I forgot the 02 voltage readings ( should have bought the scan tool that would record and graph, may bring mine back and upgrade it this week )

Also it just threw a p0300 multiple misfire code ( new plugs may be fouled, I'll clean them and drive it to see if it comes back ). But could this be a vacuum leak?
Was this
answer
helpful?
Yes
No
Sunday, November 15th, 2020 AT 2:51 PM
Tiny
TOTALMC
  • MEMBER
I guess I can hear the misfires at idle under the hood, and even more through the tailpipe. But it seems to settle under acceleration.

Thanks again.
Was this
answer
helpful?
Yes
No
Sunday, November 15th, 2020 AT 2:59 PM
Tiny
KASEKENNY1
  • EXPERT
Okay. That would make more sense. Let's address the misfire issues and then retest the fuel mileage. Misfires are horrible on economy. So let's not worry about the fuel trims or voltages at this point. Let's change out the plugs and if you can get pictures of them so we can see the condition and we can go from there.
Was this
answer
helpful?
Yes
No
Sunday, November 15th, 2020 AT 4:08 PM
Tiny
TOTALMC
  • MEMBER
Did a smoke test and found a leak somewhere between the plenum and middle injector on a driver's side. Couldn't really see where it was due to fuel rails in the way. Going to try an inspection camera or partial disassembly to pinpoint leak.
Was this
answer
helpful?
Yes
No
Tuesday, December 1st, 2020 AT 9:54 PM
Tiny
KASEKENNY1
  • EXPERT
Sounds great. Let us know what turns up. Thanks for keeping us updated.
Was this
answer
helpful?
Yes
No
Wednesday, December 2nd, 2020 AT 7:47 PM
Tiny
TOTALMC
  • MEMBER
I took some more live data readings. Here's what I found:

STFT B1 / STFT B2
+5 / -5 fluctuates back and forth
LTFT B1
+17, then settles to 5.5
LTFT B2
+15, then settles to -3
O2 B1S1
.8 to.025 fluctuates
O2 B1S2
.75 holds
O2 B2S2
.8 to.125 fluctuates
STFT B1S2
99 steady

All readings - live data set to record at time of trouble code, 100 frames of data.

Any help deciphering these numbers would be very greatly appreciated.

Thanks
Was this
answer
helpful?
Yes
No
Sunday, February 7th, 2021 AT 3:41 PM
Tiny
KASEKENNY1
  • EXPERT
What was the vehicle doing when you took these numbers? Did you find anything with the vacuum leak that you mentioned above?

Basically these numbers are not out of line. The numbers should be bouncing back and forth from positive to negative. This shows they are going rich then lean as the PCM makes adjustments.

The only one that is concerning is the Bank 1 sensor 2 that holds steady at.75 volts. This means the sensor is seeing low oxygen content after the converter. Which would mean it has a rich condition after the converter. This means either the sensor is faulty or the bank is really rich, or the converter has failed.

Swap the down stream o2 sensors and see if the bank 2 sensor 2 then reads the same.75. If it does, change the sensor.
Was this
answer
helpful?
Yes
No
+1
Monday, February 8th, 2021 AT 6:19 PM
Tiny
TOTALMC
  • MEMBER
Scanner was set to record when the trouble code was set. Just took a quick drive around the neighborhood (took it close to 60 MPH). As for the vacuum leak; I found a bolt that was missing and replaced it, but did not do another smoke test (my new $100.00 smoke machine is not very good, only blows smoke for about 5 seconds).
Was this
answer
helpful?
Yes
No
Monday, February 8th, 2021 AT 7:00 PM
Tiny
KASEKENNY1
  • EXPERT
Okay. If you found a bolt missing then we may still have a leak as once there is a leak the gasket (depending on what type) will fail at that spot and it will not seal.

Let's use this guide for the vacuum check and do the method of spraying the engine with a combustible fluid so that we can listen to the RPM. If there is a leak the RPM will raise when it pulls in the liquid.

Or put a vacuum gauge on it and get a video of the gauge when running the engine.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
Was this
answer
helpful?
Yes
No
+1
Tuesday, February 9th, 2021 AT 6:27 PM
Tiny
TOTALMC
  • MEMBER
I do not believe the missing bolt is going to cause a seal issue with the gasket, it was a bracket bolt going into the middle of the intake - pretty sure it does not even reach the head (just left an almost pinky sized hole in the top of the intake).

I'll try your recommendations this weekend and let you know how it turns out. Thanks again.
Was this
answer
helpful?
Yes
No
Wednesday, February 10th, 2021 AT 9:15 PM

Please login or register to post a reply.

Sponsored links