Crap fuel mileage
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
Crap fuel mileage We have a 2000 4x4 1500 Suburban with the 5.3 and 3.73 gears. It is totally stock right down to the tire size. We bought it with a blown head gasket. I had a set of 706 heads that had 5k on them and not Casetech so I used them instead of the Castech originals. They were also 706 heads. It runs beautiful. No miss fires on the scanner, no miss listening to it. I can't find a vacuum leak. Warm idle IAC counts are around 30 at 550 rpm making me think that there isn't a vacuum leak. I replaced the thermostat and temp sender when I did the heads and it runs right at 195-200. I also did new NGK standard replacement plugs. I cleaned the throttle body and MAF (used CRC MAF cleaner) when I put it together. No codes are popping up. I had one for the purge solenoid but I replaced it and still crap mileage. My long term fuel trims show around +20. I'm not a scanner master but as I recall if it is lean then it will command more fuel (high trims). I even swapped in a known good set of upstream O2 sensors just because I had them. No change. Does anyone have any other ideas? I also have a spare MAF that I may try.
This is a post I started recently with some really good links on finding/diagnosing lean codes; http://www.hotrodders.com/forum/lean...re-497322.html
Your LTFT being that high is telling you that the computer believes for some reason that the engine is lean and it is adding the extra fuel. Figuring out WHY the computer thinks it is lean is the challenge.
Your LTFT being that high is telling you that the computer believes for some reason that the engine is lean and it is adding the extra fuel. Figuring out WHY the computer thinks it is lean is the challenge.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
This is a post I started recently with some really good links on finding/diagnosing lean codes; http://www.hotrodders.com/forum/lean...re-497322.html
Your LTFT being that high is telling you that the computer believes for some reason that the engine is lean and it is adding the extra fuel. Figuring out WHY the computer thinks it is lean is the challenge.
Your LTFT being that high is telling you that the computer believes for some reason that the engine is lean and it is adding the extra fuel. Figuring out WHY the computer thinks it is lean is the challenge.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
I have checked it. It was at 58-60 koeo and 55ish running. I snapped the throttle and it popped up to that 58-60. If I recall correctly that is in spec?
I think he means, check to see if the fuel pressure regulator diaphragm is leaking...(if the vacuum line is pulled off the diaphragm, it will usually be wet)
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Staging Lane
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If your LTFT is at 20% + then it's compensating for lean and you should get a lean code. The max compensation before CEL trigger is 10%
When I'm checking for lean codes I bring stft and ltft up on the graph in the scanner, select only those 4, then I start messing with things and watch for them to start coming back down.
Watch them at idle and at cruising. You should see the largest compensation at idle if you have a vacuum leak and larger compensation at cruise if you have a fuel supply issue.
I've done several sets of intake gaskets on the 5.3 for lean codes but that is about it. Check all vacuum lines carefully and your intake tube. Pull off the brake booster hose and hold your finger over the line and watch the fuel trims for any change to see if your booster is leaking (not common on these trucks it's just one of my typical checks).
What is your MAF reading in grams at idle?
What fuel are you using? Do you know what the ethenol content percentage is?
Are you using 5w30? What air pressure is in the tires? I gained several mpg last month by changing my driving style, upping air pressure to 40, and fresh plugs. The majority of the gain was me being easier on it under acceleration.
When I'm checking for lean codes I bring stft and ltft up on the graph in the scanner, select only those 4, then I start messing with things and watch for them to start coming back down.
Watch them at idle and at cruising. You should see the largest compensation at idle if you have a vacuum leak and larger compensation at cruise if you have a fuel supply issue.
I've done several sets of intake gaskets on the 5.3 for lean codes but that is about it. Check all vacuum lines carefully and your intake tube. Pull off the brake booster hose and hold your finger over the line and watch the fuel trims for any change to see if your booster is leaking (not common on these trucks it's just one of my typical checks).
What is your MAF reading in grams at idle?
What fuel are you using? Do you know what the ethenol content percentage is?
Are you using 5w30? What air pressure is in the tires? I gained several mpg last month by changing my driving style, upping air pressure to 40, and fresh plugs. The majority of the gain was me being easier on it under acceleration.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
If your LTFT is at 20% + then it's compensating for lean and you should get a lean code. The max compensation before CEL trigger is 10%
When I'm checking for lean codes I bring stft and ltft up on the graph in the scanner, select only those 4, then I start messing with things and watch for them to start coming back down.
Watch them at idle and at cruising. You should see the largest compensation at idle if you have a vacuum leak and larger compensation at cruise if you have a fuel supply issue.
I've done several sets of intake gaskets on the 5.3 for lean codes but that is about it. Check all vacuum lines carefully and your intake tube. Pull off the brake booster hose and hold your finger over the line and watch the fuel trims for any change to see if your booster is leaking (not common on these trucks it's just one of my typical checks).
What is your MAF reading in grams at idle?
What fuel are you using? Do you know what the ethenol content percentage is?
Are you using 5w30? What air pressure is in the tires? I gained several mpg last month by changing my driving style, upping air pressure to 40, and fresh plugs. The majority of the gain was me being easier on it under acceleration.
When I'm checking for lean codes I bring stft and ltft up on the graph in the scanner, select only those 4, then I start messing with things and watch for them to start coming back down.
Watch them at idle and at cruising. You should see the largest compensation at idle if you have a vacuum leak and larger compensation at cruise if you have a fuel supply issue.
I've done several sets of intake gaskets on the 5.3 for lean codes but that is about it. Check all vacuum lines carefully and your intake tube. Pull off the brake booster hose and hold your finger over the line and watch the fuel trims for any change to see if your booster is leaking (not common on these trucks it's just one of my typical checks).
What is your MAF reading in grams at idle?
What fuel are you using? Do you know what the ethenol content percentage is?
Are you using 5w30? What air pressure is in the tires? I gained several mpg last month by changing my driving style, upping air pressure to 40, and fresh plugs. The majority of the gain was me being easier on it under acceleration.
I'm hoping to have a couple hours with it tomorrow. Thanks for all of the suggestions so far everyone.
Unfortunately my scanner is an older snap on red brick without graphing so I will just have to watch and compare. I was a cheapskate when I did the heads and I reused the original intake gaskets. After this code popped up I redid it right... I got the good felpro ones and put them in there. My MAF is reading a little low at idle, if I remember correctly it should be somewhere around the displacement size and it shows 4.6-7ish. Fuel is 87 octane, e10, no knock showing. I get fuel from different distributors all the time so it's not a brand of fuel but typically I use either Chevron or Mobil. Oil is 5w30 Amsoil. Driving habits are easy to account for in that I also teach defensive driving. I typically pull mid-forties out of my wife's Honda Civic where she can only pull upper thirties out of it.
I'm hoping to have a couple hours with it tomorrow. Thanks for all of the suggestions so far everyone.
I'm hoping to have a couple hours with it tomorrow. Thanks for all of the suggestions so far everyone.
Did you install new intake gaskets? Throttle body gasket?
GM don't seem to be as sensitive as Fords in regards to the PCV but I've had several Fords come in with a lean code that needed a PCV valve, the flow rate of the PCV is pretty important.
Over the years I've noticed when everything is perfect LTFT will be what I call between the 3's. -3 to +3
I just checked mine and haven't in a long time, mine is still great so I can use my truck for an example of what you should expect to see.
On one bank I have 0 LTFT and on the other bank I have +3 LTFT. My maf reading is 5.5 - 5.8 at hot idle between 580-600 rpm with all loads off, a/c lights etc.
I just checked mine and haven't in a long time, mine is still great so I can use my truck for an example of what you should expect to see.
On one bank I have 0 LTFT and on the other bank I have +3 LTFT. My maf reading is 5.5 - 5.8 at hot idle between 580-600 rpm with all loads off, a/c lights etc.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
Sorry to take so long getting back.
KOEO fuel pressure 55
Running 51
Running with vacuum line unplugged 58
Spraying carb clean in vacuum line all 4 O2s over 900mv
Mass vacuum leak all 4 O2s under 75mv
IAC at idle 44-47
MAF gm per sec 4.7-4.8 with 2 different MAF sensors but the original one is much more responsive
The only thing I found is the evap solenoid on the intake is pulsating causing a vacuum leak. To experiment I left the vacuum line plugged in and plugged in a spare solenoid into the wiring. I reset the fuel trims and now I average 13 on the long term fuel trim. Still no codes. Any thoughts? What causes the evap solenoid to operate?
KOEO fuel pressure 55
Running 51
Running with vacuum line unplugged 58
Spraying carb clean in vacuum line all 4 O2s over 900mv
Mass vacuum leak all 4 O2s under 75mv
IAC at idle 44-47
MAF gm per sec 4.7-4.8 with 2 different MAF sensors but the original one is much more responsive
The only thing I found is the evap solenoid on the intake is pulsating causing a vacuum leak. To experiment I left the vacuum line plugged in and plugged in a spare solenoid into the wiring. I reset the fuel trims and now I average 13 on the long term fuel trim. Still no codes. Any thoughts? What causes the evap solenoid to operate?
Sorry to take so long getting back.
KOEO fuel pressure 55
Running 51
Running with vacuum line unplugged 58
Spraying carb clean in vacuum line all 4 O2s over 900mv
Mass vacuum leak all 4 O2s under 75mv
IAC at idle 44-47
MAF gm per sec 4.7-4.8 with 2 different MAF sensors but the original one is much more responsive
The only thing I found is the evap solenoid on the intake is pulsating causing a vacuum leak. To experiment I left the vacuum line plugged in and plugged in a spare solenoid into the wiring. I reset the fuel trims and now I average 13 on the long term fuel trim. Still no codes. Any thoughts? What causes the evap solenoid to operate?
KOEO fuel pressure 55
Running 51
Running with vacuum line unplugged 58
Spraying carb clean in vacuum line all 4 O2s over 900mv
Mass vacuum leak all 4 O2s under 75mv
IAC at idle 44-47
MAF gm per sec 4.7-4.8 with 2 different MAF sensors but the original one is much more responsive
The only thing I found is the evap solenoid on the intake is pulsating causing a vacuum leak. To experiment I left the vacuum line plugged in and plugged in a spare solenoid into the wiring. I reset the fuel trims and now I average 13 on the long term fuel trim. Still no codes. Any thoughts? What causes the evap solenoid to operate?
But mine again was at 580 - 600 RPM what RPM was yours reading 4.7-4.8
The evap solenoid is supposed to pulse.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
550 for the idle. That was hot, in gear with hvac off. With hvac on it was still 550 but 6.0-6.2.
Have you ever done intake gaskets? If not maybe slap a set in there and retest.
Also pinch off the booster line and see if anything happens, could be a small leak there. Check your PCV rubber connector at the PCV, those rot sometimes from the prolonged exposure to oil, I had to change mine recently it fit pretty sloppy and I swapped it when I did my valve cover gaskets.
Also pinch off the booster line and see if anything happens, could be a small leak there. Check your PCV rubber connector at the PCV, those rot sometimes from the prolonged exposure to oil, I had to change mine recently it fit pretty sloppy and I swapped it when I did my valve cover gaskets.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3
Have you ever done intake gaskets? If not maybe slap a set in there and retest.
Also pinch off the booster line and see if anything happens, could be a small leak there. Check your PCV rubber connector at the PCV, those rot sometimes from the prolonged exposure to oil, I had to change mine recently it fit pretty sloppy and I swapped it when I did my valve cover gaskets.
Also pinch off the booster line and see if anything happens, could be a small leak there. Check your PCV rubber connector at the PCV, those rot sometimes from the prolonged exposure to oil, I had to change mine recently it fit pretty sloppy and I swapped it when I did my valve cover gaskets.
I just did the intake gaskets as I suspected that. I plugged the booster line at the intake with no change as well as pinching it. I repaced the line from the intake to the air pump (CA emissionsl) because it was suspect. I replaced the 3/8 line from the throttle body to the passenger valve cover but I haven't replaced the pcv one.I also replaced the pcv when I did the intake gaskets. I'm out of ideas.
Sometimes I have to reset the flex fuel adaptives to get the system to "reboot" and read the correct octane and adjust accordingly.
Thread Starter
Staging Lane
Joined: Sep 2006
Posts: 80
Likes: 3