PCM delivering wrong fuel mixture?

[Matthew Jeschke]

This has been extremely difficult to track down. My truck is starting to get horrible mileage. I am building it and changing quite a few things along the way. I had issues with the evap and thought that might be it too. I revered my tuning changes to a point where I knew I had better mileage. I also fixed up the evap.

Logged a bunch of data with HP Tuners MVPI after reverting my changes. Truck got good mileage. Figured I fixed it so I drove around. Filled up and back to bad mileage. I think what's actually happening is a electrical issue with the oxygen sensors. If I unplug the OBDII tuning adapter my wideband reports a mixture of 14.0:1 if I plug back in the OBDII adapter the wideband reports 14.7:1.

I don't even have a computer hooked up, I'm just plugging the HP tuners adapter into the OBDII Port.

Long story short, do these P59 controllers go bad? I went through grounds and everything seems fine, from what I can tell. Oxygen sensors are also new within last 10k miles or so.

 

[Marky Dissod]

If I unplug the OBDII tuning adapter my wideband reports a mixture of 14.0:1.
If I plug back in the OBDII adapter the wideband reports 14.7:1.

Is it a coincidence that the stoichiometric AFR for pure petrol is 14.67:1, but 14.0:1 for E10?

Lambda = 1.00
100% pure unleaded gas = 14.67:1 tough but not quite impossible to find once in a while
90% gas / 10% ethanol = 14.06:1 typical pump gasoline, today
15% gas / E85 ethanol = 9.84:1 actually this can be anywhere between 51 to 83% depending on the time of year, hence the flex fuel sensor
0% gas / E100 ethanol = 8.99:1 drink it up!

 

[Mudsport96]

2001 right? What operating system is it running? 1344? 2088?
There are a few things. Like above post, you may think you are putting in standard 10% but be getting something else. I don't use hptuners so not sure why just the dongle being plugged in would change your afr.
Could be an electrical issue shorting something with the connector plugged in I suppose. I would have to look at an electrical schematic to be sure, but I don't think there is a common power or ground to the O2 sensors.
Does it drive okay otherwise?
@Tonyrodz Does this sound like your old issue?


Edit. Your sig says 01 but you asked about a p59... should be a red/blue pcm p01 for a 2001... is this for a different vehicle than the 2001 tahoe in the sig?

 

[Tonyrodz]

My fuel %'s were all over the place. Turned out my injectors were maxed out, and I think it had something to do with the old tune/tuner. Got some new bigger injectors and a new tune and its been running great ever since.
Mine is non FF, and NJ doesn't have E85.

 

[strutaeng]

This has been extremely difficult to track down. My truck is starting to get horrible mileage. I am building it and changing quite a few things along the way. I had issues with the evap and thought that might be it too. I revered my tuning changes to a point where I knew I had better mileage. I also fixed up the evap.

Logged a bunch of data with HP Tuners MVPI after reverting my changes. Truck got good mileage. Figured I fixed it so I drove around. Filled up and back to bad mileage. I think what's actually happening is a electrical issue with the oxygen sensors. If I unplug the OBDII tuning adapter my wideband reports a mixture of 14.0:1 if I plug back in the OBDII adapter the wideband reports 14.7:1.

I don't even have a computer hooked up, I'm just plugging the HP tuners adapter into the OBDII Port.

Long story short, do these P59 controllers go bad? I went through grounds and everything seems fine, from what I can tell. Oxygen sensors are also new within last 10k miles or so.

Is this a flex fuel vehicle?

I don't think the P59 computers go bad. If anything, I believe they are considered an improvement over the earlier 0411, et. al. which are pretty robust and reliable.

Can you list your modifications? On another thread you mentioned you did a 6.0 swap, and I can't recall what else? You did the ECU reflash yourself? Any other mods on HP Tuners?

 

[Matthew Jeschke]

I tuned it to 14.7:1, pump gas, for non-PE 12.0:1 for PE.

@Marky Dissod I think 12.6:1 is best for PE but honestly, I seldom if ever get into PE so it hasn't bothered me much. I just basically turned PE on (isn't on in Truck tune, I think it's rather baked into VE table) and got it close enough that I don't have to worry about knock / pre-detonation.

1273059 HP Tuners Speed Density Real Time Tuning OS

@Mudsport96 wow good memory! My truck is a 2001 Z71 Tahoe 3.73 gears and cammed 6.0L / LQ9 swap. The cam is a custom grind by Roger Vinci, he made me a sort of hybrid RV cam for great low in torque. Idea being I could climb better and improve part throttle performance / efficiency for highway driving. Really sad that I had a couple tanks of horrendous gas mileage. I really need to update the way I log data so my tuning is based around gas mileage / best pumping efficiency per fuel. I believe both lead to improved horsepower.

@Tonyrodz I had same experience! I used the stock 6.0L injectors, but bought new ones off eBay. I didn't realize they were cheap for a reason! I didn't even realize they were cheap. They leaked, lucky I didn't blow a cylinder. I swapped them out for the flex fuel injectors which work awesome. They have a bit of overhead left in them. My mountain driving / city stayed about the same 14 to 15. However, my highway driving got up over 17MPG, 19MPG on one tank (maybe a fluke?). Pretty cool... The old injectors I got 15MPG everywhere.

@strutaeng great memory, yup a 6.0 swap. Oh lord modifications... This is on my todo list, I need to make a video or something to list them all. Basically there's nothing stock in the engine or transmission.

Rebuilt 4L60E with boosted pump, clutches, sunshell, Lower stall Circle D Converter, etc. etc. etc.

Engine - Balanced LQ9 (w/ stock 317 heads and full float pistons), custom valvetrain, full roller shaft mount roller rockers, custom cam, dual valve springs (lower seat pressure than high performance ones), custom cam etc. Just added a banks cold air intake a couple weeks ago.

Tons of other mods but kept factory configuration for most part.

Further testing of PCM:

I maybe chasing my tail here. Watched wideband for 150 miles and it was correct. I'm unsure why, maybe it's just a fluke but there's chassis ground and signal ground on the OBDII connector. Those don't seem to be a bit high. Last I measured signal ground it was 3.2ohms (maybe normal as signal tells me it's an isolated ground / not chassis ground). Chassis ground was 1.5 ohms.

PCM richens the fuel mixture at start-up. Then slowly goes to 1 lambda as engine warms up. I wonder if the PCM just goes to 1 lambda when the OBDII connector is plugged in? A glitch in OS?

 

[Mudsport96]

I tuned it to 14.7:1, pump gas, for non-PE 12.0:1 for PE.

@Marky Dissod I think 12.6:1 is best for PE but honestly, I seldom if ever get into PE so it hasn't bothered me much. I just basically turned PE on (isn't on in Truck tune, I think it's rather baked into VE table) and got it close enough that I don't have to worry about knock / pre-detonation.

1273059 HP Tuners Speed Density Real Time Tuning OS

@Mudsport96 wow good memory! My truck is a 2001 Z71 Tahoe 3.73 gears and cammed 6.0L / LQ9 swap

Hptuners has its own operating system so I don't know that number. But I do know that you should try putting your PE back to stock fueling, but bring the time delay down to Zero and rpm delay down to just above 2500. Leave the tps activation at 90%. After you sort out the fueling issue you have, then use PE to fill in the gaps. You need to do data collection first. PE will throw that off. But you have injector problems I think...see below

. They leaked, lucky I didn't blow a cylinder. I swapped them out for the flex fuel injectors which work awesome.

okay here is a question about your setup. Are you currently running the 2001 intake with the fuel pressure regulator on the fuel rail? If yes, did you get the flex fuel regulator when you swapped to the flex injectors? They run at a different pressure to get their flow rating. So if you did not, they are flowing more than what the computer is expecting them to, therefore all of the fueling calculations will be off.



Higher psi is an old trick we used to use on obd1 (eec 4) ford's that werent as tuneable in the 90s. Turn up the fuel pressure to get more fueling when you get into the throttle. So now your ecm isn't getting injector pulses right because it thinks your injectors flow less than they are.

Just added a banks cold air intake a couple weeks ago.

Further testing of PCM:

I maybe chasing my tail here. Watched wideband for 150 miles and it was correct. I'm unsure why, maybe it's just a fluke but there's chassis ground and signal ground on the OBDII connector. Those don't seem to be a bit high. Last I measured signal ground it was 3.2ohms (maybe normal as signal tells me it's an isolated ground / not chassis ground). Chassis ground was 1.5 ohms.

PCM richens the fuel mixture at start-up. Then slowly goes to 1 lambda as engine warms up. I wonder if the PCM just goes to 1 lambda when the OBDII connector is plugged in? A glitch in OS?

 

[Matthew Jeschke]

@Mudsport96 wow, thanks for all your input. I've done extensive amount of idle and fuel tuning. The fuel is dialed in and for all intensive purposes the PE as well. The injectors are also tuned. It has a new pump but stock LM7 regulator (not the higher pressure L59 flex regulator) however, that was factored into the tune. The biggest thing with having the other regulator is the max injector flow rate isn't quite as high but I had quite a bit of headroom anyways. I don't recall exactly what I did for injectors w/o looking but a number of parameters had to be changed in the PCM. I'd have to look at my tuning log but have had that dialed in for quite a while, maybe a year now.

You reminded me of my 1994 Z28 I built up. I cammed it and put an adjustable fuel pressure regulator on it for exactly what you mentioned, to get more flow. Now I think there's more adjustability in the injector settings for the newer OBDII PCM. The even newer return less stuff has a LOT more adjustability yet. Thank God I didn't have to tune that!

I keep a mileage log for every tank of gas I put in. I had three tanks with horrible mileage. Worst since I built the truck up. I had done several things...

1. Advanced timing 2 degrees around cruise, and a bit farther along acceleration line in spark table.
2. Broken (then repaired) evap tank to canister line.

I thought maybe issue was my evap so I repaired it but mileage still sucked so I revered all my timing to prior baseline. That helped. The next tank I got reasonable mileage. I had also installed a banks cold air intake and TruCool Trans cooler... I don't think, or hope not at least, that they had a negative impact on mileage.

The last tank of gas during my experiment I did a short trip in mountains and got 12.5MPG again (typically it's 14 to 15MPG). I did use 4wd a LOT which I virtually never use, so maybe it was that? However, I plugged in my wideband and saw It was running around 14.0:1 AFR. It should be 14.7:1. So I plugged in the HP tuners cable to start logging data and figure out what's going on... as soon as I plugged in the cable the PCM went to 14.7:1. Started me down this rabbit hole / post.

Last tank was *eh* mileage. I drove 130 miles on highway, 20 miles on mountains. Mountains were no faster than 25MPH, decent roads dirt roads climbing over a pass. Then the highway was 55 to 65 mph. I got 16.4MPG. I think maybe I used a bit more gas climbing. I'd kind of expected 17ish but seems closer to what I'd expect.

I'd just like to know what I saw when the PCM was reading that rich AFR. Then get passed that and start tuning engine for efficiency. Pretty well have fuel dialed in, idle needs a bit of work, PE could be a touch leaner, but spark isn't anywhere nearly as close to what it should be... Despite that truck runs good. Just I think lots of power left on table. I'm running LM7 spark config with my LQ9 crankcase, both are quite conservative in terms of timing from looks of it. That said, I've never tuned spark... Just studied it a bit and looked at a handful of tunes.

 

[Mudsport96]

@Matthew Jeschke the flex regulator is actually lower pressure according to oriellys site.
like I said in other posts I haven't used hptuners, but in tunerpro I have done a fair amount. I know 5.3s have more aggressive timing than 6.0s, and 4.8s are more aggressive yet. And there is a timing difference between manual and automatic 4.8s too. Timing is a lot of work. Getting fuel worked out first is a must, then a timing loop can be run.
Sounds like you are figuring it out though

 

[Marky Dissod]

OK, first of all:
what i was trying to say at 130am was that it may be better to simply monitor O2 voltages than to look at the 'reported AFR'.

Second:

I tuned it to 14.7:1, pump gas, for non-PE & 12.0:1 for PE.
@Marky Dissod I think 12.6:1 is best for PE but honestly, I seldom if ever get into PE so it hasn't bothered me much.
I just basically turned PE on (isn't on in Truck tune, I think it's rather baked into VE table) and got it close enough that I don't have to worry about knock / pre-detonation.

The best PE fuel ratioS at any given RpM need to be discovered through trial and error.
12.6:1 may be great for cylinder cooling, but not for best power - and I do think the best PE fuel ratio fluctuates a little.

If I remember correctly, PE is on in the truck tune, except it waits for like a minute or some ridiculous delay.