Low Fuel pressure issue ? & HIGH fuel trim

[bg1988]

 

 

 

 

43.51PSI   With engine ok

77.45 PSI   or 534 KPa  Was the peak in auto stop (slowly rose)

 the suddenly drops to  43.51 PSI when the engine starts  

 

 for like 10 seconds  58.01PSI  on a cold start   then went down to 43.51PSI 

 

 

Also MPG decreased    typically 20MPG to 12MPG

High fuel trims (LTFT)  10.9 on average at 1150rpm  @ 35mph...

 

  STFT is  ± 3 from 0

 

I switched back to my normal MAF  it has fixed the cold start stumble..

 

No misfires

 

 Catalytic converter is reaching 500-550C   while driving  Normally this is about 350-390C....

Edited March 8, 2021 by bg1988

[CamGTP]

What year is this truck?

 

The earlier trucks don't have a fuel pressure control module and they should be well above 43psi of fuel pressure at idle and while driving.

[bg1988]

3 hours ago, CamGTP said:

What year is this truck?

 

The earlier trucks don't have a fuel pressure control module and they should be well above 43psi of fuel pressure at idle and while driving.

2008 chevy tahoe 6.0L

  the fuel trim is good tell it  drops to 43psi from 58 psi... that is when it starts dumping fuel into the engine and CAT  520C to 560C 

 

it should be at 350C-380C or so...  this is a known good configuration

 

 

 could it be a restriction  in the air filter??  maybe a few leaves got sucked up>?

 

 

I read online  a Vacuum leak causes "low  or negative long term fuel trims"

 

 

88666 was the last time i replaced the Air filter(according to my fuelly log)    currently has 103700

 

i want to figure it out before I drop a grand or two  throwing random parts at it..

Edited March 9, 2021 by bg1988

[CamGTP]

Are you measuring the cat temp with a temp gun or going by what your scan tool says?

 

The scan tool reading is just a calculation, there are no actual sensors that read how hot the cat is. The o2 sensors themselves do no have temp sensors in them. They are just regular heated narrowband o2 sensors. The cat temp is just a calculation from the computer and not an actual value.

 

 

If this vehicle has a fuel pump control module, which many do in that year range, the fuel pressure should vary with load. With no load the fuel pressure is going to dial way back into the 40's and will jump up when the demand is needed. It should go all the way back up to 55-58psi at full throttle.

 

 

Back to the fuel trim part. If the fuel trim is a positive value that means it's lean and not rich, so it's not dumping fuel and making it run rich. It's adding in fuel because the fuel trim says it's lean and needs more fuel to bring it back to the correct stoichiometric ratio. Either way it's going to use more fuel than before because the fuel trims are making it. So either you have a vacuum leak, dirty MAF sensor, exhaust leak before the o2 sensors or your fuel pump is not working at the correct pressures.

[bg1988]

can a dirty air filter cause high fuel trim  i.e  air restriction it's been awhile since the filter was changed   15k miles ago or so

 

this is a LEV2 emissions system

 

.

  also i think the O2 sensor(s)  might be lazy  as i noticed the  sensor 2 creep up  while it was running also the values between the banks don't match  (sensor #2)  (slowly creep up tell it matched)  the other was 0.785 steady (i'll have to confirm)

 

 

 the values on sensor  one were changing like normal..

Edited March 9, 2021 by bg1988

[CamGTP]

Air filter won't do that because it's before the Mass Airflow sensor. It doesn't know what is before the sensor, it's just reading the airflow after it and telling the computer what to do.

 

Bank 1 sensor 1 and  Bank 2 sensor 1 will move a lot. Both upstream o2 sensors will never match, they will swing lean and rich of stoich (450mv) at all times. It will be a constant up and down switching. Obviously the more exhaust going through the exhaust the more the sensors will move, they generally swing slower at idle.

 

The down stream o2 sensors should be very similar if the cat converters are doing their job. They will bounce around a little while the cats warm up and start working. Then settle into a short range of movement. Maybe something like 550-750mv range. If they start swinging up and down like the upstream sensors, then usually the cat(s) has failed and a check engine light would come on at some point for that.

[swathdiver]

1 hour ago, CamGTP said:

Are you measuring the cat temp with a temp gun or going by what your scan tool says?

 

The scan tool reading is just a calculation, there are no actual sensors that read how hot the cat is. The o2 sensors themselves do no have temp sensors in them. They are just regular heated narrowband o2 sensors. The cat temp is just a calculation from the computer and not an actual value.

 

 

If this vehicle has a fuel pump control module, which many do in that year range, the fuel pressure should vary with load. With no load the fuel pressure is going to dial way back into the 40's and will jump up when the demand is needed. It should go all the way back up to 55-58psi at full throttle.

 

 

Back to the fuel trim part. If the fuel trim is a positive value that means it's lean and not rich, so it's not dumping fuel and making it run rich. It's adding in fuel because the fuel trim says it's lean and needs more fuel to bring it back to the correct stoichiometric ratio. Either way it's going to use more fuel than before because the fuel trims are making it. So either you have a vacuum leak, dirty MAF sensor, exhaust leak before the o2 sensors or your fuel pump is not working at the correct pressures.

My fuel pressure is always 43.5 psi but pump volume % changes according to load or throttle position.

 

Brent, check your intake for a vacuum leak.

[bg1988]

14 hours ago, swathdiver said:

My fuel pressure is always 43.5 psi but pump volume % changes according to load or throttle position.

 

Brent, check your intake for a vacuum leak.

 i checked with water i did not hear any slurping noises.

 

  I did notice this abnormality with the sensor #2 

 

 

under idle condition  and while driving at 1200 rpm

 

both O2  down stream #2 sensors  dropped to voltage 0.165   then went back to 0.75-0.765 ish  it did it multiple times at certain  engine speeds(keeping in mind it has aggressive Deceleration fuel cut off  )  so there is no way to know for sure with out something that can see that data

 

also while it was running both downstream sensors dropped to 0.165 (with the Engine on at 1050 rpm)  M4-Gear   at 8mph..  it   seemed like a Deceleration fuel cut off event (as noted by the instant mpg reading showed 99MPG )but was steady speed.. of 1050 rpm

 

 

I did tighten up the connection at the throttle body ( the plastic  tube )  it did help slightly   in the single digits mostly   6-8 .  with some peaks at 10.

 

  should i try and reset the connection tube?

 

[swathdiver]

10 hours ago, bg1988 said:

 i checked with water i did not hear any slurping noises.

While spraying water or whatever along the intake and heads, you watch the O2 sensors on a graph and or the trims, you're not likely to hear anything.

[bg1988]

On 3/10/2021 at 12:13 AM, swathdiver said:

While spraying water or whatever along the intake and heads, you watch the O2 sensors on a graph and or the trims, you're not likely to hear anything.

i see,

 

 

  I going to try and reset the seal on the  throttle body ( clean it ) and MAP sensor.. 

 

i'll try the map sensor  first then the I also check the bolts for the intake manifold.  they could have been sabotaged by that last place i had taken it to...  they also messed with the PVC  valve 

 

 

 

also do you think it could be the use of 91 octane? causing the higher fuel trims ?

 

 The  timing advancement is in the 38 to 42 range  while driving..... I checked my fuelly MPG it says i averaged 23.9mpg(100% City only)  over  283 miles..  (11.838 gallons fuel used)   this is since march 1st....(Yes everything is super far and  i'm glad i can go 70mph on the surface streets)..I have not had any issues with the engine   misfiring on start up( I have been Holding the key in the start position for an extra second)   this should not hurt the starter motor in anyway... as that motor is designed to move the whole SUV anyways 

Edited March 12, 2021 by bg1988

[bg1988]

I ran her at 2500 rpms    70-72mph    the fuel trims were almost spot on  at 3.5+   even went down to 0 a few times

 

 

maybe it just needed an italian tune up ?

Edited March 17, 2021 by bg1988

[Kcfarmer]

Did you ever figure it out I have the same issue and it is driving me nuts.