Aftermarket Intake trouble shooting MPG

Bit of a long story, I went from 15MPG down to 12.5MPG. Did a few changes to tune, fixed evap issues, etc. Reverting tune seemed to have a limited effect on MPG. Last thing on my list seems to be the intake. I called the company. They were very responsive and tried to answer my questions but maybe a bit too in-depth for tech support. They said they didn't have many calls about MPG concerns... and if they did cleaning the MAF sensor always seemed to help. I really don't think that's my problem but cleaned it anyways.

I'm going to save my notes here as I troubleshoot the MPG further.

Air Intake: I think the aftermarket CAI may have created a scenario where cylinder fill / vacuum is less efficient at part throttle. I imagine there needs to be a gradient of pressure, from intake to piston such that air can for lack of a better vernacular slide into the cylinder efficiently. I'd THINK each intake would be optimized for a different gradient / RPM (air demand). When I called they were not sure what scenario they optimized the intake for, in hindsight I'm assuming high RPM WOT. My build has been focused around part throttle optimization and NOT just WOT.

Engine Advance: I'm assuming MORE engine advance would improve gas mileage. I advanced my timing, basically had a LM7 spark map, changed it to LQ9 map (I have a 6.0 but used parts from 5.3 including controller). Then reverted LQ9 spark settings back to my former baseline of LM7. Seemed to have no impact on improving gas mileage.

VE Calibration: I have a speed density tune, MAP only (not using MAF for time being). This has been what I used before and after the intake. Added up to 7% fuel but in most places is really around 4%. O2 switching at idle is a bit lazy and fuel trims are less consistent. I think I can improve by further tuning fulcrums / voltage settings. Unsure if this will help with MPG but was always that way before so probably not issue. I assume trims would have kept the engine running around 14.7 AFR regardless of VE table fine tuning which I'm sure is what I observed in closed loop.

EVAP: After months of trouble shooting I traced my EVAP issues to wiring fault. DEFINATELY was affecting gas mileage as Purge was stuck open at 100% all the time. Truck runs WAY better with it fixed than blocked off. However, I don't think it was really the source of my issue as I can still get tanks at 12.5MPG which I NEVER got prior to all this mayhem. A healthy evap system definitely helped part throttle response!

Transmission Thermostat: installed a oil thermostat as trucool 40k cooler came with a REALLY crappy one. Shame on Dana who makes those things. Works like a CHAMP. I think this may have helped. It's full open temperature is 180F. Hard saying if helped gas mileage. In theory it would but maybe 1/100 mpg? who knows...

Anybody else wants to chip in feel free... I just SWAPPED out the CAI to factory so I can collect some data on that and see if it fixes MPG issue.

Almost sounds like you have a partial alcohol tune. No misfires detected on live scans during MPG runs? I would expect you to see 18 unless big tires/lifted.
Long ago I had a couple econoboxes that ATE MAP sensors. Every junkyard run I would get all they could spare for $5 each. Funny thing- every time mileage slipped, swap map. Never had one quit. Just that some were better than others after 5 mile relearn. Those engines known for burning valves if mpg slipped below 40. I still carry a map in my Suburban.

Notable issues I've seen from aftermarket intakes, in no particular order:

- They remove the resonators present in the OE intake tube
- Stock intake is already cold air. Pulls air from the wheel well. Depending on how it's routed and where it's located, it could be sucking hot engine air
- If oil wetted, check the MAF. Oiled filters tend to catch fewer particles AND shed oil mist - leads to dirtying the things up
- Some of the cone filters can introduce turbulence in the airflow stream over the MAF, especially if the omit the smoothing screen. Can check this by putting a steady load on the engine and looking at live data off the sensor. The stock intake is fairly stable. Turbulence will see it bouncing around more than ~5% of median

I would put the stock air box setup back in, and drive. Do an A-B-A test to make sure it is the cold air setup causing the issue. And not some other coincidental problem that happened around the same time.

Also as a side note.
The 4.8, 5.3, 6.0 and 8.1 all use the same air filter.... So a quality filter that flows enough for an 8.1 making 335hp and 445tq will definitely flow enough for a 6.0 (if stock) making 330hp and 360tq. The mid pipe I think cam be improved on like an airaid intermediate pipe. I have one on the Tahoe, it was free, and I like the sound without the silencer.

You probably already checked the injectors. I was thinking about pulsing each injector and recording system pressure drop for each one. If that test did not reveal any differences between injectors, I probably remove them and measure the amount of gas each one puts out. There is probably better and easier methods of gathering that data.

I think I already asked this on your other post (and I can't remember), but do you have an aftermarket camshaft? Are the fuel trims less than 10%?

What kind of driving is this? A mix of city/highway, I assume?

I would actually start looking at other things. Things like dragging brake calipers and maybe hubs. Jack the truck up and start spinning the tires. You could probably remove the driveshaft and spin the yoke and see if there's excessive drag. Not sure if you could do the same for the front differential? Isn't there an actuator on the front differential that engages it when switching to 4x4? You are in 2HI on the transfer case, right? Not on the Auto mode?

I may have FINALLY found my issue, but honestly, this might be the 10th time I found the issue haha. Need to collect more data yet...

exp500 said:

Almost sounds like you have a partial alcohol tune. No misfires detected on live scans during MPG runs? I would expect you to see 18 unless big tires/lifted.
Long ago I had a couple econoboxes that ATE MAP sensors. Every junkyard run I would get all they could spare for $5 each. Funny thing- every time mileage slipped, swap map. Never had one quit. Just that some were better than others after 5 mile relearn. Those engines known for burning valves if mpg slipped below 40. I still carry a map in my Suburban.

Click to expand...

I had to turn off misfire detection. I'm running an aftermarket cam. Otherwise, as far as the engine running smooth... It runs SILKY smooth. Power is good, etc. I could do the exhaust temperature check / spray bottle. Misfires had crossed my mind, however, with it running so well, I suspect it's something else.

I had also wondered about an "alcohol" tune. I'm running HP Tuners RTOS and I cannot confirm yet but suspect, it's not quite as good at metering fuel as the factory OS. I occasionally see the tune going rich, to 13.7, and getting stuck for no reason. It only clears up / reverts to 14.9 (I chose 14.9 instead of 14.7 in my tune) when I plug the programmer in. It did this even when I had everything calibrated to 14.7. I think it maybe an oddity with the HPT RTOS but cannot confirm yet. I know my PCM grounds are good, presume wiring to o2 sensors is good, and my wideband is reading correctly.

What is an econobox?

SnowDrifter said:

Notable issues I've seen from aftermarket intakes, in no particular order:

1) They remove the resonators present in the OE intake tube
2) Stock intake is already cold air. Pulls air from the wheel well. Depending on how it's routed and where it's located, it could be sucking hot engine air
3) If oil wetted, check the MAF. Oiled filters tend to catch fewer particles AND shed oil mist - leads to dirtying the things up
4) Some of the cone filters can introduce turbulence in the airflow stream over the MAF, especially if the omit the smoothing screen. Can check this by putting a steady load on the engine and looking at live data off the sensor. The stock intake is fairly stable. Turbulence will see it bouncing around more than ~5% of median

Click to expand...

1 - yeah sounds pretty sweet. Its so loud I can scare people on sidewalk lol
2 - Yeah, 100%. This Banks CAI is awesome construction though. However, it lacks the gasket to link it to the fender like factory, which made me wonder if it's really going to out perform stock as it can suck hot engine air there.
3 - Called Banks guy was helpful. He mentioned same thing. I cleaned the MAF but swapped in the factory intake immediately there after.
4 - WOW super cool. I will check this! I still have the smoothing screen on the MAF.

One thing worth noting though, I'm running speed density tune. Not using MAF at moment, just temperature reading for IAT from the sensor.

Mudsport96 said:

I would put the stock air box setup back in, and drive. Do an A-B-A test to make sure it is the cold air setup causing the issue. And not some other coincidental problem that happened around the same time.

Also as a side note.
The 4.8, 5.3, 6.0 and 8.1 all use the same air filter.... So a quality filter that flows enough for an 8.1 making 335hp and 445tq will definitely flow enough for a 6.0 (if stock) making 330hp and 360tq. The mid pipe I think cam be improved on like an airaid intermediate pipe. I have one on the Tahoe, it was free, and I like the sound without the silencer.

Click to expand...

On it I've swapped back in the factory intake. Trying to get original baseline numbers from prior to when MPG issues arose. Interestingly, my WORST highway mileage was realized after the swap. However, I'm keeping the stock intake on and reverting out other changes in my build / tune till configuration looks much closer to when I got my baseline MPG I'm trying to get back to.

Great side note! I am considering keeping the stock box. The damn filter is HUGE on the Banks. I wonder if it's not leading to decreased cylinder vacuum at low RPM (where I'm optimizing motor for). I'm not entirely sure how to quantify / measure this in my data logging... Some mathematic transforms and MAF vrs MAP readings in regards to TPS and RPM I think. Also gripe per above comment, the Banks CAI has a fender port but no seal! He is animate about not sucking under hood air but then doesn't provide a seal or provision to mount a seal for fender port *sigh*.

Donal said:

You probably already checked the injectors. I was thinking about pulsing each injector and recording system pressure drop for each one. If that test did not reveal any differences between injectors, I probably remove them and measure the amount of gas each one puts out. There is probably better and easier methods of gathering that data. I recongize that I am slow. I was the only veteran in the tenth grade at Plains High School. I finally graduated in 1962.

Click to expand...

Is another great tip. I wish I'd done this before I installed my injectors. Unfortunately, the flow bench was super price and I didn't know a shop that had one. However, I've since seen people make them from factory rail, and pump. Then it just injects into tubes w/ measurements on them. All aside though, fueling / AFR / Wideband is quite good. I could probably improve VE table a bit yet but truck runs like a top. However, you're really close to what I tested last.

I had turned off LTFT in my tune. My VE table was way off and I suspected it was LTFT... However, it was my table. I never turned back on LTFT and so I did that and logged 160 miles yesterday. The mileage was MUCH improved. 15MPG for 60 miles of mountain driving about 20 city and remainder highway. Still going to log more data though to make sure that's what it is... Then my VE must be off a bit as MPG should be fairly close w/ and w/o LTFT in my opinion.

strutaeng said:

I think I already asked this on your other post (and I can't remember), but do you have an aftermarket camshaft? Are the fuel trims less than 10%?

What kind of driving is this? A mix of city/highway, I assume?

I would actually start looking at other things. Things like dragging brake calipers and maybe hubs. Jack the truck up and start spinning the tires. You could probably remove the driveshaft and spin the yoke and see if there's excessive drag. Not sure if you could do the same for the front differential? Isn't there an actuator on the front differential that engages it when switching to 4x4? You are in 2HI on the transfer case, right? Not on the Auto mode?

Click to expand...

You are thinking like me! Tuning learning curve is HUGE. My trims are within 5%... However, I had turned off LTFT somewhere in this process and didn't turn them back on. It was tricky to get truck to run right, as soon as I turned them off it ran fine. However, I suspect my fueling was off so much that LTFT was creating a lean condition. I turned them back on yesterday and it ran fine. Drove 160 miles.

After this MPG issues arose I did a BUNCH more VE tuning. I adjusted stoichiometric to 14.9 from 14.7... I changed O2 fulcrums to 410mV across the board... and tuned the VE table a bunch more along with PE. My original efforts were off by a considerable amount. Truck runs WAY better. O2 sensors aren't quite as responsive at idle, so I may fiddle with fulcrums there a bit and a touch more VE tuning. This is part of issues that sent me down this rabbit hole originally.

I suspect (1) disabling LTFT hurt my MPG (2) HPT RTOS maybe doing some odd things. Then for further improvement I may switch around configuration of airbox. I really feel that filter element is WAY too big for what I'm trying to do. It doesn't provide enough restriction to create a strong vacuum. That said, it may have actually improved my gas mileage hard to say just yet.

RE post 2 Econobox. In this case a pair of Suzuki commuter cars, generic nearrly and throwaways.

exp500 said:

RE post 2 Econobox. In this case a pair of Suzuki commuter cars, generic nearrly and throwaways.

Click to expand...

Ah super cool! I am so annoyed that they come up with all these technological gadgets to boost MPG. When we had it several decades ago with small commuter cars. Additionally, Germans are AWESOME at MPG (they also over engineer other things to insane levels but another topic).

I was FACINATED by the YouTube video "40MPG V8! Lawn Mower Carb..."
I theorize he got more efficiency by shifting peak vacuum to lower RPM than from the carb alone. He made the intake REALLY small. I'd thing that'd dramatically increase piston scavenging and improve fill at lower RPM. He probably completely cut his top end off... but who cares. Lamborghini used to make a variable geometry intake manifold so they could maximize Torque curve. I don't know why manufacturers chose a completely different route over the years. Surprised Koenigsegg hasn't tried something like that yet... I'd thought you could take a cross section of a root style compressor and adjustable geometry by degreeing the compressor one way or the other so you could maximize cylinder scavenging on intake stroke. But maybe I'm just crazy.

Matthew Jeschke said:

I may have FINALLY found my issue, but honestly, this might be the 10th time I found the issue haha. Need to collect more data yet...

I had to turn off misfire detection. I'm running an aftermarket cam. Otherwise, as far as the engine running smooth... It runs SILKY smooth. Power is good, etc. I could do the exhaust temperature check / spray bottle. Misfires had crossed my mind, however, with it running so well, I suspect it's something else.

I had also wondered about an "alcohol" tune. I'm running HP Tuners RTOS and I cannot confirm yet but suspect, it's not quite as good at metering fuel as the factory OS. I occasionally see the tune going rich, to 13.7, and getting stuck for no reason. It only clears up / reverts to 14.9 (I chose 14.9 instead of 14.7 in my tune) when I plug the programmer in. It did this even when I had everything calibrated to 14.7. I think it maybe an oddity with the HPT RTOS but cannot confirm yet. I know my PCM grounds are good, presume wiring to o2 sensors is good, and my wideband is reading correctly.

What is an econobox?


1 - yeah sounds pretty sweet. Its so loud I can scare people on sidewalk lol
2 - Yeah, 100%. This Banks CAI is awesome construction though. However, it lacks the gasket to link it to the fender like factory, which made me wonder if it's really going to out perform stock as it can suck hot engine air there.
3 - Called Banks guy was helpful. He mentioned same thing. I cleaned the MAF but swapped in the factory intake immediately there after.
4 - WOW super cool. I will check this! I still have the smoothing screen on the MAF.

One thing worth noting though, I'm running speed density tune. Not using MAF at moment, just temperature reading for IAT from the sensor.

On it I've swapped back in the factory intake. Trying to get original baseline numbers from prior to when MPG issues arose. Interestingly, my WORST highway mileage was realized after the swap. However, I'm keeping the stock intake on and reverting out other changes in my build / tune till configuration looks much closer to when I got my baseline MPG I'm trying to get back to.

Great side note! I am considering keeping the stock box. The damn filter is HUGE on the Banks. I wonder if it's not leading to decreased cylinder vacuum at low RPM (where I'm optimizing motor for). I'm not entirely sure how to quantify / measure this in my data logging... Some mathematic transforms and MAF vrs MAP readings in regards to TPS and RPM I think. Also gripe per above comment, the Banks CAI has a fender port but no seal! He is animate about not sucking under hood air but then doesn't provide a seal or provision to mount a seal for fender port *sigh*.


Is another great tip. I wish I'd done this before I installed my injectors. Unfortunately, the flow bench was super price and I didn't know a shop that had one. However, I've since seen people make them from factory rail, and pump. Then it just injects into tubes w/ measurements on them. All aside though, fueling / AFR / Wideband is quite good. I could probably improve VE table a bit yet but truck runs like a top. However, you're really close to what I tested last.

I had turned off LTFT in my tune. My VE table was way off and I suspected it was LTFT... However, it was my table. I never turned back on LTFT and so I did that and logged 160 miles yesterday. The mileage was MUCH improved. 15MPG for 60 miles of mountain driving about 20 city and remainder highway. Still going to log more data though to make sure that's what it is... Then my VE must be off a bit as MPG should be fairly close w/ and w/o LTFT in my opinion.


You are thinking like me! Tuning learning curve is HUGE. My trims are within 5%... However, I had turned off LTFT somewhere in this process and didn't turn them back on. It was tricky to get truck to run right, as soon as I turned them off it ran fine. However, I suspect my fueling was off so much that LTFT was creating a lean condition. I turned them back on yesterday and it ran fine. Drove 160 miles.

After this MPG issues arose I did a BUNCH more VE tuning. I adjusted stoichiometric to 14.9 from 14.7... I changed O2 fulcrums to 410mV across the board... and tuned the VE table a bunch more along with PE. My original efforts were off by a considerable amount. Truck runs WAY better. O2 sensors aren't quite as responsive at idle, so I may fiddle with fulcrums there a bit and a touch more VE tuning. This is part of issues that sent me down this rabbit hole originally.

I suspect (1) disabling LTFT hurt my MPG (2) HPT RTOS maybe doing some odd things. Then for further improvement I may switch around configuration of airbox. I really feel that filter element is WAY too big for what I'm trying to do. It doesn't provide enough restriction to create a strong vacuum. That said, it may have actually improved my gas mileage hard to say just yet.

Click to expand...

Damn that is some impressive low level control you have on that

I guess the easiest thing to differentiate hardware from software at this point would be to load up the stock tune for a tank and see where things land