Fuel pressure deficiency...in-tank actual and hpfp specified

Hello all...I have a 2007 GTI. First off - Everything is in fine shape.

Recently, as some may be aware of, given bits and pieces of this issue have been brought up in other posts or threads of mine, I switched back to my GIAC K04 HO(HO=RS4 fprv and a 140 BAR request for rail pressure) from a trial period of two years with a DriverMotorsport K04+ tune. My GIAC tune was fine a few years ago...but not the case when it was reloaded last year. The rail pressure request should be 140 BAR but only hits 110 BAR specified even though the actual exceeds that.

I have a USP High Flow lpfp(in-tank pump) and the mentioned condition is present with either the USP or OEM pumps installed...with the OEM pump currently installed. I cannot believe that both of these lpfp's are failing but cannot figure out what the problem is. Again, there are no fault codes and the filter and follower are nearly new. Additionally, with both lpfp's, the actual low pressure will drop below specified as much as 0.5 BAR sometimes when I collect data for group 231 in the upper revs...6k to redline mostly. Earlier in the revs the pressure will match requested low pressure.

I've had recommendations to replace nearly everything like: cam follower, filter, hpfp, lpfp, fuel pressure relief valve, and the controller. But I think everything is in good shape other than the controller which I haven't heard of being the culprit to this. Only I haven't heard of anything being at fault for this, so....any others here ever experienced this? "This" being, everything runs fine...it is just that the high fuel pressure request is the stock 110 BAR rather than the tune limit of 140 BAR...

Tomorrow I go have another GIAC K04 HO tune installed and then plan to look at the data again. I've done this twice now and don't expect it to be fixed by trying a third time. Hoping one of you have experienced this and can point me to a fix rather than more suggestions of what it could be when I am aware of the condition of the related bits currently installed...mostly :thumbsup:

Well I'm nearly done trying to figure this out...GIAC sent the same tune that's already loaded. I can just settle on there is something wrong that doesn't show in the data....hope that the tune is fine...and see if when I replace something that the fuel pressure request rights itself some time in the future because I just don't want to pay for the tune loading anymore.

Depending on what the actual low pressure is.. will determine on how much your requested high pressure is.

I still think you have a low fuel pressure problem since you said it's dropping below requested.

Yep...very possible. Just need to figure out what to replace because both my oem pump and the USP hi-flow pumps do this. I don't want to replace another 3rd pump to find I needed to do the controller...and vice versa. The issue existed before buying the USP Hi-flow lpfp and didn't get corrected by it...DM had to adjust the tune to correct it. The pressure tests with vcds look correct for both high and low...but those are done at idle. So, there is either a problem with the controller or a restriction somewhere in the feed line that has always existed...←my thoughts at the moment.

BTW...the low pressure actual always begins to fall below specified the moment I go full throttle and throughout the revs during a data pull. Sometimes it only falls 0.2 BAR and the most it falls is 0.5 BAR...and it is a gradual decline and later in the revs.
For example,
rpm = Sp.___Actual
3k = 4.15___4.07
4.5k = 4.15__3.91
6k = 4.11___3.94
6.8k = 4.11__3.48
coasting afterwards
2.4k = 5.60__7.01

Then, as you can see above, when I let off the throttle following a data pull and coast...the low actual goes more than 1 BAR above specified and I assume this is normal.

Plus, the specified high pressure specified will only exceed 110 BAR after I let off, but not when at full throttle.

High pressure during a pull...redline is 6900rpm
rpm = Sp._____Actual
3.5 = 109.99___110.5
5k = 109.99___114.29
6.5k = 109.99__132.25
then coasting...
5k = 122.00____109.17
3.5 = 113.33___113.33

The increased low pressure is normal when letting off at wot. What is the duty % of your low pressure pump at those rpms u listed?

Here are the things I would check starting from the tank:

- low pressure pump
- controler and wiring for lpfp
- fuel filter
- fuel line
- metal fuel line on top of mani
- rubber fuel line on the bottom of hpfp (this is where my issue was)
- low pressure sensor on hpfp
- wiring for solenoid on top of hpfp
- hpfp
- the check valve on fuel rail
- sensor on fuel rail

That's all I can think of off the top of my head.

Your low pressure isn't keeping up with specified...that's for starters...


What's injection timing on the injectors on that same pull? How's lambda? What's fuel trims?

high_octane3541 said:

The increased low pressure is normal when letting off at wot. What is the duty % of your low pressure pump at those rpms u listed?

Here are the things I would check starting from the tank:

- low pressure pump
- controler and wiring for lpfp
- fuel filter
- fuel line
- metal fuel line on top of mani
- rubber fuel line on the bottom of hpfp (this is where my issue was)
- low pressure sensor on hpfp
- wiring for solenoid on top of hpfp
- hpfp
- the check valve on fuel rail
- sensor on fuel rail

That's all I can think of off the top of my head.

Click to expand...

Before 5k rpm it is in the 70%'s
at 6k rpm it gets into the 80%'s...and thanks for the list. It seems quite reasonable and should help :drinking:

I've replaced both the low and high pressure sensors on the hpfp and the rail...the RS4 fprv isn't suspect...lower rubber fuel line is new and it's proper sized and meets pressure specs.

donjuan1jr said:

Your low pressure isn't keeping up with specified...that's for starters...


What's injection timing on the injectors on that same pull? How's lambda? What's fuel trims?

Click to expand...

Inj timing at:
4k is 8.?
5200 is 9.? and doesn't exceed10.0.

Lambda is:
0.95 to 3k rpm...then it is 0.84 specified and actual stays between 0.82 and 0.84.



Fuel trim .....going to look at this and get back :thumbsup: https://www.youtube.com/watch?v=yM3iarhSP68

ROH ECHT said:

Before 5k rpm it is in the 70%'s

at 6k rpm it gets into the 80%'s...and thanks for the list. It seems quite reasonable and should help :drinking:

Click to expand...

What's the duty % at idle?

Your injection window is too large, they're staying open too long...and kinda lean for my liking as well.

OK...I took this class :lol: : https://www.youtube.com/watch?annot...&feature=iv&src_vid=yM3iarhSP68&v=oRX2V6_a3dc

...and watched this vid: https://www.youtube.com/watch?v=yM3iarhSP68

I will get back real soon...going to look at fuel trims, lpfp duty % at idle, and report.

donjuan1jr said:

Your injection window is too large, they're staying open too long...and kinda lean for my liking as well.

Click to expand...

high_octane3541 said:

What's the duty % at idle?

Click to expand...

It is possible it is making up for trims...the long term should show why the timing is as it is. I'm having my coffee right now, but will head out to read data in a moment :thumbsup: GIAC never mentioned fuel trims or if they did they didn't say, "well we really need to look at that". That would've pushed me to learn it...so I thank you two for staying connected and doing so.

Does the inj. timing conflict with the lambda? Because the O2 reports are rich numbers...with the mixture going from 1.0 to 0.95, then to 0.85. The specified O2 drops to 0.78 in the upper revs above 6k rpm but the actual remains at 0.84 to redline. Or, lambda just looks good and it is more likely that the inj. timing is a correction for stored long term fuel trim?

I have been thinking about what DM did to the tune...they told me the adjustments made to their old K04+ tune was a open loop tune...regarding fuel and I wonder if that had a residual effect when I loaded the GIAC last year...and if it did, what can I do other than clear codes now it is back to a closed loop?

LPFP duty at idle is 50% ± 1-2%

Fuel trim - ...short term idle bounces from 5% to -5%.
....long term - idle is -1.6% and under partial load is 13.3%. ←This value stored for long term under load means 13.3% correction for lean condition right? And, is data stored since the last time I cleared codes correct? It also says the fuel trim is 13.3 - 1.6 = 11.7, yes? And this is also why the inj. timing/pulse width is lengthened yes?

After clearing codes and waiting for short term to bounce between + and -...the long term at idle settled on 0.9%

So, the possibilities, lpfp deficiency is present and the data shows its decline. Or could it be the injectors not delivering as they should and is this why the actual high pressure exceeds specified and the lpfp actual is low? When I did the rebuild last year the techs showed the injectors to me and said they would replace them even though they didn't look terribly caked with deposits...only a bit dirty. Could it just be the lpfp controller or the lpfp itself? Without codes, right? :iono:

And finally the questions...does any of that ↑ have anything to do with the 110 BAR high pressure request? Is this separate from the fuel trim issue? Does the lpfp deficiency limit the hpfp request? TBD!

Ltft of 13% is you're running lean and to stay in line for specified lambda it's adding fuel. I dunno if you have a hardware issue or a tune issue seeing you had this same problem with two tuners?