LML fueling

[ikeG]

Have a 15 LML in here. Tuned, SOTF switch, non compliant. It runs super lazy and sluggish, checked all the normal stuff. I noticed it maxes out at a desired FRP of 72xx psi. Ive tried all positions of the SOTF switch, cleaned MAF, checked air filter, etc. What would limit the desired rail pressure, except for the tune itself? Ive not ran into this one before.

 

[Ron Nielson]

MAF, MAT, ECT, etc all the usual suspects. How's the boost? If you demand full fuel pressure with your scanner, will it max out the actual fuel pressure?

 

[2004LB7]

Fuel temperature would be my first thought. Isn't that how they do it on the LML switchable tuning? Maybe somethings up with the SOTF switch. What does the scanner say for fuel temperature?

 

[ikeG]

I'll dig into that stuff tomorrow. Thanks guys

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[JoshH]

What is the commanded fuel rate?

 

[ikeG]

What is the commanded fuel rate?

Wot is 120mm3. Desired frp maxes out at 7251.

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[ikeG]

Maf flow looks correct, 32ish g/s at idle and goes up to 418 ish wot.
Map is same as baro at idle, then up to 82 in at wot.
Boost is always correct during the run, mirroring map minus baro.
Desired Boost is 14.4 at idle and up to 94.9 psi at wot.

I don't have access to lml tables to see what else would limit frp. It's mystery tuning. Idk

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[JoshH]

Is this a problem that just recently started, or has it been like this since it was tuned?

 

[ikeG]

Is this a problem that just recently started, or has it been like this since it was tuned?

It's a trade in at the dealership I do work for, so I'm not sure at this point. In fact, sluggish/low power wasn't even on their list of things to look at.

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[2004LB7]

It's a trade in at the dealership I do work for, so I'm not sure at this point. In fact, sluggish/low power wasn't even on their list of things to look at.

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I would not be surprised if the customer had this issue for a while. Maybe tried tuning to fix it with no luck. Had multiple shops try and fail. Then got frustrated and decided to dump it for another new one. And wouldn't say anything to you guys about it as it may reduce the trade in value. So now you're stuck with the problem

Might be time to flash it back to stock and put the emissions system back on it. At least that might give you the codes needed to properly diagnose the issue. Even if you don't put the emissions systems back on it might give you enough codes to figure out which ones are related. Problem is, without being able to put the tune back in you kinda get stuck. Unless you can just do an ECM swap for a non tuned one just to see what it says

 

[ikeG]

I'm going to see if the dealership can get ahold of the previous owner. I cannot see anything that would limit rail.
Question, does the sotf switch make fuel temp stay constant? I'm assuming so. Like tune 1 is -40*, then tune 2 is 32*, and so on?

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[2004LB7]

I'm going to see if the dealership can get ahold of the previous owner. I cannot see anything that would limit rail.
Question, does the sotf switch make fuel temp stay constant? I'm assuming so. Like tune 1 is -40*, then tune 2 is 32*, and so on?

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That's how I understand it to work too

Then in the tune, the compensation tables are used to limit the power. So the tune is made hot, or whatever max the customer wants. Then the SOTF tricks the truck into thinking the fuel temperature is getting hotter and so pulls back fuel pressure and whatever other tables are connected to that fuel temp in order to reduce overall HP

 

[JoshH]

Yeah, SOTF tuning in the LML is usually a fuel temp based fuel limiter. Some guys do it differently, but most use that method. The switch is still a resistor like the old DSP switches, but rather than changing the voltage on a PIN the references a different set of tables, it changes the voltage on the fuel temp wire and sets a fuel limit that is written into the tune file for the different temperatures. It almost has to be a tune issue causing the low rail pressure if everything else looks correct. I took a peek at a stock file to make sure I wasn't forgetting something because I didn't really recall any tables that modified the desired rail pressure being available for the LML. There are 2 modifier tables with multipliers that I saw. One is a fuel temp based which is zeroed out. The other is VSS based which has a max adjustment of around 2500 PSI. The thing I found interesting is there is a minimum fuel pressure table that at its highest setting matches what you are seeing for your maximum rail pressure.

 

[ikeG]

It was the tuning. The exhaust was pretty old, based on the rust. So it's pretty crazy to wonder how long the previous owner might have driven this way. If you had tow anything, it wouldn't have been able to get out of its own way.

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