I did a lot of searching for this but did not really confirm anything.

I installed my Quad rail and fuel pressure gauge yesterday. At idle, the rail pressure is 6800-7000 psi. It increases steadily until it flatlines at about 18k psi around 2000 rpm. From there, as I increase throttle input, it actually drops slightly to 16-17k psi as the rpms increase. The pressure will not increase at all unless I go to WOT at which point it jumps immediatly to 22-23k psi. LP pressure maintains 8-9psi except at WOT when it drops to 5-6psi.

So is the rail pressure flatline normal above 2000 rpms? The reason I ask is because it coincides with the sluggish problem I have. The truck falls on it's face above 2000rpm unless I am at WOT, and my mileage is dismal around 15mpg unloaded.

 

IT sounds to me, like there is either a MAP sensor problem or something telling it to drop rail pressure. My rail pressure goes consitantly with the throttle. Your's sounds a little high at idle as mine is only around 5500 or so. Do you have access to a scanner? It could be a APPS problem, map sensor, rail pressure sensor, FCA. Tons of things that can affect it. Any codes? I am assuming no, or you would have mentioned it. There could also be a boost leak, which would affect the map sensor and in return affect the rail pressures. Need more info.

 

I have read that a lot of idle pressures are in the 5500 range, so that might explain the rough idle problems I have been having as well. I saw as high as 7500 at idle this morning after starting it cold.

I have not been able to see any codes either. Even yesterday after I had a little install problem with the RP gauge, the CEL came on but no codes, just 'done'.

I don't have access to a scanner either. I am trying to prepare myself for the next trip to the dealer as they seem to be lost on this problem. I am trying to correlate something to the problem and it sounds like these pressures might point somewhere. Is there anyway I can test some of these sensors?

 

Guide me on a hunting trip , or just room board and a liscense, and I'll bring my scanner! LOL Best of luck to ya, but it sounds like a scanner is the next step unfortunately.

 

I'm tempted to take you up on that, but I am a terrible hunter so I think you would get the short end of the stick on that deal. I tell you what, I have a nice dwell meter I will trade for your scanner. I will even throw in my analog multi-tester for free!

Seriously though, I think I will take out the FCA and disassemble for cleaning. If it doesn't fix it at least I know there isn't a piece of trash stuck in it. The way my pressures are out of whack (esp. idle) I am really suspect of that. Cold idle went to about 8,900 this morning before it finally settled into the 6,800-7,000 range. Once in a while it will dip to about 5,500 but it is very sporatic.

I think I will also check hose clamps to make sure I am not getting a boost leak. I might also pull the MAP and RP sensors to see if I can find anything visibly wrong.

 

Probably gonna be hard to see anything visibly wron with those sensors, but hey........use your multimeter to see if there are any "glitches" at the rpm's you are having trouble at! There is 3 wires to the map sensor. One will be ground, one 5V and the other is reference. Backprobe the reference and see if it drops or does anything funny at your trouble rpm's. The rail pressure sensor would surely set a code if it was at fault. That and what you are seeing with your rail pressure guage is just a "result" of whatever other sensor is giving the ECM "bad" information. The only other thing would be to backprobe the APPS sensor to see if it is dropping off as well. I would almost bet, from what you are describing , that either the APPS or the MAP is at fault.

 

Well here is an update. I removed and checked/cleaned the IAT, MAP, and RP sensors, as well as the FCA. I also double checked all electrical connections. Nothing out of the ordinary other than the MAP sensor was a bit sooty. I could not get the end cap out of the FCA without mangling it, so I cleaned it the best I could. It rattled when I shook it hard so I think it's okay. I haven't had a chance to do any signal testing yet, but maybe this weekend I will have some time to fiddle with it some more.

I did try the APPS reset procedure tonight and if anything my problems are worse. When I rev the engine in neutral it will not let me rev between 900-1200 rpm. As soon as I hit 900 and try to go further it jumps up to 1200. It also takes a lot of throttle input to get it to rev to 2500.

This thing is getting sicker. I filled up today and calculated 14.7 MPG. That is all highway running empty at 75 mph. I also confirmed the RP drop. I run a solid 20K psi cruising on the highway. With moderate throttle input, the RP drops to 16K and stays there unless I got past about 3/4 throttle. Someone on another forum said the drop is the ECU strategy for moderate acceleration - it only uses increased injector duration until heavy acceleration. Somehow this just doesn't seem right but maybe it is. Several others have said it should increase steadily - not drop.

I guess I will have to run it in to the dealer and have them tell me that they can't find anything. Maybe they will throw yet another batch of parts at it.

 

Try one more thing before the dealer if you would.....just to humor me! Try running with the O/D off and see if you can definately relate it to a throttle position, or an RPM. With taking it out of overdrive, you should change the rpms at a given speed. You might have just figured out your own problem by trying the apps reset! I'm now thinking more of an APPS problem.

 

I have spent a lot of time doing just that. It seems like no matter what gear I'm in, it always goes flat about about 2000 rpms, and really doesn't want to accelerate after that without a lot of extra throttle input. Even with WOT the turbo feels awfully laggy. In fact it seems worse (or more noticable) in 2,3,4 versus 5 and 6. Sometimes I can also feel some light surging in the upper rpm's if I hold a steady speed in any gear. At lower RPM's (under 2000) the response seems okay.

But the jury is still out on the APPS reset. I put on 275 miles today and I am showing just over half a tank remaining. DTE says 300 to go. If that is the case, I think my mileage might have gone back up to where it was before. It also seems like it might be running a bit better, but I might just be driving myself crazy now.

Edit: I forgot to mention last night that when I got home I didn't have any trouble getting it to rev between 900-1200rpm. How long does it take for the ECM to re-learn the APPS? Maybe it was just having a good day?

 

Well it runs pretty good until it gets to full operating temp, and then it starts acting up, getting sluggish and idling rough. Also my mileage only went from 14.7 to 15.6 on this last tank so something is still not right.

 

my 05 6 speed is doing the same thing 900-1200 it just jumps i dont have a clue