Hello, I have a 2008 6.6L LMM drivetrain with 140k miles (it is a converted 2007 suburban 2500). When I accelerate hard the fuel rail demand is dropping to a negative number, in the attached example it goes from about 9k demand to -8k demand. I also noticed that even in WOT the demand never exceeded 13k. During the time when the demand drops it will also hiss and hesitate for a few seconds or less. The fuel rail pressure is also significantly higher than the demand (max 28k FRP and 13k DRP on a 10 minute diagnostic drive). The previous owner told me that the CP3 was replaced within the last year. I am also getting an intermittent shark bite pattern on my FRP. the My fuel filters (lift pump and OEM) both have less than 1k miles on them. EFIlive 5 stage (set on stage 3, 50 HP when not towing), S&B CAI, AEM intake elbow, FASS lift pump. I am interested in ANY suggestions or feedback on checks and/or diagnostics.
Negative Desired Fuel Rail Pressure
- Thread starter Dmaxsuburban
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- fuel rail pressure
I'm going to guess that the actual fuel pressure is rising faster or is higher then the desired and the ecm is trying to pull back pressure. Maybe a sticking regulator or a bad pressure sensor. Do the actual and desired follow each other most of the time?
They do track with each other in relative terms, however actual is ALWAYS higher than demanded, sometimes by 1,000 (idle), sometimes a whole lot more. Also, for what it is worth I do not experience the hissing and hesitation unless I am at reasonable temps and WOT, or if I am hot and over 50% throttle.
To make this a bit more confusing, I do occasionally throw a low fuel rail pressure code and go in to limp mode (P0087) while towing uphill in hot ambient weather. I think I have narrowed this to hot fuel temps (as high as 216 over this last weekend) 110 ambient, IAT around 100. No other codes of any kind
To make this a bit more confusing, I do occasionally throw a low fuel rail pressure code and go in to limp mode (P0087) while towing uphill in hot ambient weather. I think I have narrowed this to hot fuel temps (as high as 216 over this last weekend) 110 ambient, IAT around 100. No other codes of any kind
Will do the same drive on the stock tune tonight and report back. I have been eyeing the FPR all day and from what I can tell it is just the one located on the CP3, right? Is there one also on the rail in the LMM? Tons of videos on LBZ and LLY, but cant find much for LMM.
As I am reviewing the logs in more detail it just doesnt make sense. It will sometimes show a drop in demand on the graph, but the numbers it is reflecting dont align to the graph. ODD.
it sure does. Sounds like mixed reviews on the race plug. Thinking of shelling out for a single stage. Still wouldn’t explain why the actual pressure is so much higher than demand, would it??
you said you’ve seen 28k rail pressure? I don’t think a stock FPRV would allow for that high of pressure. I would replace the rail pressure sensor but I could be wrong.
ahh I thought you were talking valve. Good call on the sensor. Any idea where it is?
Yes I am. The Fuel Pressure Relief Valve should be opening a little above 26k so I would find it hard to believe you’re actually getting to 28k rail pressure. That makes me think the rail pressure sensor is going crazy. Could be wiring also but I would think you would get a code. The rail sensor is on the back of the passenger side rail.
ahhh gotcha. Just ran stock time test drive and bottle test at the same time. Zero fuel in the bottle so it’s not blowing off. Think that further confirms your theory on the sensor.I haven’t reviewed the log yet but it is also strange that the edge reflects what appears to be different numbers than the log from what I recall yesterday. We will see as I focused on demand and it capped at 26.1k. Previous test never exceeded 13k on the log. Appreciate your help. Will report back on the sensor
I was about to ask what kid of a scope you were using to see your fishbite, but I see where you got your information. I think there is likely a circuit problem for the FRP Sensor. It seems to work and then just drops out entirely, as if it were shorting. That's where I would spend my time in analysis and testing.
Appreciate it Ron. Will be spending this week on testing and will ensure I post back with updates. Appreciate the perspectives from all. :drink: