Help understanding my rail pressure (and other UG items)

[bigceltic]

I just installed/hooked up my ultraguage and have it set to monitor engine load and rail pressure (FR PSI). I also display Ambient Air temp and Intake Air temp.

When at idle, in neutral, the rail pressure reading fluctuates a lot (not in sheer numbers/quantity but never stops), up and down and up and down. Load stays at 0.00 but the RP goes from a low of 6589 to a high of 6907 (was watching it this morning after 30 mi drive to work). When driving it fluctuates a lot as well up to 19007 (17000-19007), with load going slightly up and down (12%-22%) - cruise set for 60 but this is CO and the roads are not flat. Are these good numbers? Is slight fluctuation ok or is should I be looking at other tests. What is an allowable range. It is kind of annoying with the RP constantly moving - if this is not indicative of a prob, I may have to change my main display!

I checked my oil and I am not making oil, the truck idles really smoothly, starts perfectly, runs smoothly, no complaints. I have about 300 mi before I hit my 10K fuel filter change interval (3 filters: SDFK, Factory, GDP MK2). No complaints on performance at all.

On the Ambient v IAT, from what I have read, there is often a diff between the two so their divergence is no big deal (IAT often higher - I am seeing about 10-15 deg at the moment) - does IAT tell me anything useful?

Thanks in advance,
George

 

[AH64ID]

Rail pressure moves a lot and it is perfectly normal. With a constant flow there will be minor pressure changes even with a static engine load.

Temps don't tell you much other than how hot the air is. Ambient is pre-turbo and IAT is in the manifold, and 98% of the time ambient is lower.

Here is the oem rail pressure map. As you can see it moves a lot. 7-23K.

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[AllianceDiesel]

^^^^^He's smart.

But like he said, my pickup is all over with rail. There is a set point and it is constantly trying to adjust to hit that set point. There are too many variables to be able to hold a constant pressure

 

[bigceltic]

Holy Moley, I am supposed to understand that? ;-) Thanks, John. Is there a RP that I should be aware of - say below X,XXX that tells me I need a new LP or FCA - or if at speed I am not getting to 19K or something? I have read (if I understood it correctly) that if I am mashing the go fast pedal and my RP drops, it may be my LP or CP3. Or an upper bound that I should be aware of that tells me I may have bad injectors? This could be your first article in TDR! Understanding RP!

George

 

[AllianceDiesel]

The way I personally have found and understood a bad fca was a rythmic miss/tic that will come and go. Now if you have the ability to watch your set point and actual, you can find bad cp3, lp, and even fca. Lp can easily be checked by a flow test and so can the cp3.

My pickup is getting horrible mileage and I am starting to suspect a fuel problem even though my rail stays with the set point. I need to make some time and test the lp and cp3

 

[bigceltic]

Thanks. I have no issues (knock on wood) to date, but am at almost 110K so I am trying to increase my awareness and monitoring.

 

[AH64ID]

For nearly all issues with rail pressure you will get a code when it deviates from the set point too much.

I watch it but I just like to see what it is, same thing with ambient and intake temp.

 

[p-Bar]

You will probably find the rail pressure guage is more for troubleshooting than anything else . my .02