Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Log of differential fuel pressure

General Tuning Discussion

Forum Posts



Tech Articles

Discuss all things tuning in this section. News, products, problems and results. 

= Resolved threads


We've been chasing fueling issues on a 300zx with Haltech ECU, Tomei FPR and AEM 320lph in-tank fuel pump. The car had been setup on the stock fuel pump wiring with the pump controller bypassed. We were seeing that when we got to 1 bar of boost the engine would fuel starve. Believing this to be the 18 gauge wiring to the pump we've run a dedicated 14 gauge wire circuit for the pump from the battery via a relay and also added a 10 Bar AEM fuel pressure sensor into the ECU.

Today when we log the injector differential pressure we're seeing it fall off unexpectedly (see attached graph). What could be the cause of this? Do we look for further wiring issues inside the tank or could it be something else?

Attached Files
  • 12751970-10155191971211959-466859093-o.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

Could very well be too restrictive fuel lines and/or fuelfilter. Stock ones?

Also make sure you got a good reference point on both map and fuelpressure sensor. Are those two hooked up to the same cylinderbank or one on each side? If one on each side do you log AFR on both sides (another way of asking if the wastegates are propperly synked.) Important cause benchsynking them will not be good enough. You will need to do pulls and adjust WG rods to get it right. And ESPECIALY important if you dont have stock STYLE intercoolers anymore where the right turbo feeds air into the left bank and vice versa, but instead have one of those FMIC`s that lets the right banks turbo feed the right bank and so on..

Hi kickerzx, do I guess by your username that you have a similar car?

The fuel lines are OEM with a new OEM fuel filter, but there seems plenty of evidence of these making much more power than we're currently seeing..

There is a par of lambda's - one per bank. AFR reference was on the left bank but has also been tried today on the balance bar.

Turbos are GT525 with .54 a/r turbine, larger manifold and 3" downpipes. The intercoolers are larger yes, but still one per turbo and not a single FMIC.

I'm not sure the WG actuators are adjustable will have to check tomorrow - it's all very tight under there :)

There could be numerous causes for this, can you add throttle position to the log? It looks like the reference pressure source has dropped off causing the fuel pressure to drop off. It looks like the throttle has shut off and caused the manifold to go into vacuum.

Your guess just might be absolutley correct. :)

Ok, your fuelsystem should be up to the task then.

Try hooking the fuelpressure regulator up to the same place as the MAP then. Just T it in or something. See if that helps.

Too me, since fueldeliveryproblems theoreticaly is out of the way, i am thinking there must be a difference in what the FPR and MAP sees. Either unballanced turbos or a boostleak maybe?

There arent any intercoolers made for this car that just dumps all the air into one chamber. There are frontmounted ones thoug that if you look at them are in reality two coolers stacked on top of each other. Theese might have inlets on one side and outlets on the other. This destroys the oem settup where some balancing is done as one turbo feeds air to the other side bank and vise versa. This stops one turbo from going bananas if the other wont as there wont be enough exhaust to drive it.. If you have sidemounted intercoolers youre allright though.

you can reach teh left hand WGrod for inspection and adjustments. The right hand one, not so much..

Scraggles, could you explain where you're taking your pressures values from? Pictures would be good too. What size of injectors are you using?

Something just occured to me...

You are running the pump at full tilt all the time right?

If so this will more than likely lead too regulator overrun at idle where you would sett Your fuelpressure, and up to a certain load. When your engine actually eats up enough fuel through the injectors to get the pressure Down to what the regualtor ACTUALLY is set to.

This is my bet now if both banks really are in ballance.

You would probably be better of PWM`ing the fuelpump.

Attached is a drawing to try and show where the FPR ref and MAP are taken from.

FPR is taken from the factory location, we've also tried the centre of the balance tube with no noticeable difference.

Kickerzx, we suspect the factory controller to be a source of problem for a higher current pump so the car is modified to run the pump without controller and at constant voltage - the same as many others cars.

We've installed a fuel pressure sensor and are logging fuel pressure differential following the fuel startvation issues we were getting at only 0.7 bar boost. We expect to see that the rising rate regulator is accurately tracking intake pressure and hence fuel pressure differential remains constant but we don't it steadily drops as we increase boost. We guess we could map an additional fuel compensation table to increase injector pulse width (off a table of fuel pressure diff vs pressure), but that's only working around the issue without understanding/solving it.

Hi Chris, injectors are Nismo 740cc side feeds. The graph shows where the throttle was let out at around the 2/3 point to illustrate that the injector pressure ratio is flat, as opposed to falling as when boost is increased.

Attached Files

Here's an engine bay picture if it helps visualise..

Attached Files
  • 12699044-10155193673036959-873367014-o.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

so from your first picture the 3 lines are:

Fuel pressure (measured)

Manifold pressure (measured)

fuel pressure minus manifold pressure (calculated) ?

Where are you measuring your fuel pressure from, the start of the fuel lines (fuel filter side) or FPR side or the line between the banks?

Yes that's right Chris. Fuel pressure sensor is in the FPR.

Is there any way that you could fit another fuel pressure sensor to the filter side?

I'm not familiar with the layout of the fuel lines on the 300ZX but on the EJ in the Impreza's they suffered pressure drop whilst using side feed injectors, our way to sort it was to feed both rails independently instead of in serial, running them as parallel rails.

It could be with the same with what your seeing here, it would be possible to map around the pressure drop as it looks to be repeatable but personally I would fix the mechanical issue first.

Yes, I believe the Subaru problem is due to the large distance between the fuel rails. The 300ZX rails are very close together and theres no other report of a similar problem with HP way in excess of what we're shooting for. Non the less it is something we're discussing.

There was a marked increase in fuel delivery after I put a dedicated wire circuit out to the tank. I had to butt into wires outside the tank as I currently don't have a way of getting wires through the tank straight to the pump and the connections are quite corroded on the outside.

Don't really have uh opportunity of fitting another fuel pressure sensor, we don't have one and the plugin ecu is out of inputs.

I'm curious fro what I've read about measuring differential fuel pressure; where everything seems to suggest an atmospheric referenced boost pressure versus an absolute referenced fuel pressure sensor, whereas ours are opposite, I guess the Haltech is doing the maths internally but also an unknown.

With "a marked increase in fuel delivery" do you meen the pressure went up?

I understand why you are looging differensial pressure. I do it myself on my own car alos. Great tool. That is why i am suggesting regulator overrun. I to have dropping differensial pressure, but this is from too small fuel lines. Just tuned around it until i get around to fix it.

Can yo provide a log with rpm also?

He went out of inputs heh? This is the one of the reasons i try to talk people out of buying pnp ecus... They rearly listens tho..

It just happens to be that i got an upgraded fuelpump controller for the Z32 that i dont use. It is designed to be able to run 2 pumps without crapping out.. This one


I also got a Vipec i88 wire in ecu for sale if he wants more inputs and configurability

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?