Fuel rail pressure for 3.5

[Skiboy]

Hi all

I have a supercharged 3.5 and the after market fuel pressure reg diaphragm is shot/leaking (a lot)

So I have a new regulator (same brand) but I don't know what to set it to as old one not right obviously - engine dies at low idle - and the guy who set it up is out of business so can't ask him.

The manuals and posts I have seen suggest 36/37 psi with no vac (ie ign on but not running) for a 3.5 or 3.9 engine
At idle - with vac - this should drop to 29 psi

Is this enough for a forced induction engine?

Btw current pressure on gauge below 20 psi at idle!

All help and suggestions appreciated.

Ski boy

[russotouristo]

ign on but engine not running might result in lower voltage on battery hence lower performance of the fuel pump, hence incorrect pressure in the rail. i bet the best way to provide no vacuum condition is to remove the pipe from the manifold and plug it on a bolt - to prevent extra air to the manifold.

i would say, you need some higher pressure in the rail for your turbo setup (vs usual atmo intake). or, better, get another fuel map. do you use stock efi or some aftermarket model?

Sent from my JY-G4 using AULRO mobile app

[Skiboy]

I re tested yesterday as it was the gauge leaking fuel

Engine running it was About 40 psi then with no vacuum it was 30 psi

I will replace reg and set pressures at these levels initially and see how I go

[PhilipA]

The fuel pressure regulator has a vacuum input to ensure that the fuel pressure is constant compared to the air "pressure" in the manifold.

if you have a turbo blowing say 5PSI this has to be accounted for when setting a fuel pressure.
Most turbo engines run at higher fuel pressure than non turbo so that the pressure remains high enough for fuel atomisation during full throttle.

I would run a higher pressure than you propose eg you propose no vacuum eg atmospheric at 30 PSI. If you have 5PSI boost then it should be 5PSI Higher. You may end up rich at part throttle but factory turbo engines have unique maps to take account of this.
Regards Philip A

[Skiboy]

Thanks phil but the truck is supercharged not turbo - is that an issue in terms of your advice?

[PhilipA]

Thanks phil but the truck is supercharged not turbo - is that an issue in terms
of your advice?
No same same
You should really do it on a dyno to make sure you don't go lean with boost.
Regards Philip A

[Skiboy]

Ok thanks - given it runs ok now except it stalls at low revs when engine warm at worst moments (ie on brakes down hill) I think I will set it at same levels initially then dyno it later once on road

Then might increase say 5psi and see how runs

[PhilipA]

It depends whether it is a flapper 3.5 out of an old RRC or 14CUX 3.5 out of a early Disco 1.
The flapper injection had injectors with little pipes on top and these are really hard to find replacements for, although I read somewhere a while ago that Nissan 3.0 as per Commodore fitted.

If a 14CUX then the sky is the limit . The OEM injectors are Lucas and are black with a green band on them. Thye plug into sockets in the fuel rail with O rings.

They are old fashioned now and can be directly replaced by Bosch Gen11 Yellow with 4 laser cut holes as fitted to Mustang 4.6 and readily available on Ebay.
Regards Philip A

[Skiboy]

Thanks guys
- I replaced the reg and tested old before I did. Also noted amount of screw in on the adjuster.

Currently set at about 41/42 psi no vac and 34 psi with vac connected at idle

See how it run next outing. All o rings on connectors stuffed - one leaking.

Might not have been diaphragm in reg - not sure.

Found one fuel rail to injector hose the hose clamp not on hose - sitting on manifold - must have been missed when installed. Fuel leaking on manifold at that point. That won't be helping.

In process found thermostat housing leaking at bottom - lucky find - wonder why was slowly loosing water.

Needs a good run to check all this has fixed hot idle stalling! Fingers crossed.

Skiboy

[PhilipA]

From the posters description of "hose clamp" on injectors, the basis of his engine is a flapper.
If the injectors are stock, then I suspect that they would not be able to pass sufficient fuel for any high boost, so his boost must be modest.
Question for poster. What sort of supercharger is on the car? Is it a Jap centrifugal or a Roots type? I have a vague memory that 3.5 injectors are about 14Lbs per hour.

IMHO you cannot compare running a 4.6 on 3.9 injectors as 3.9 injectors are plenty big enough to run a 4.6. They are the same 19lbs per hour as Mustang 4.6 injectors.
The problem you have with a 14CUX is that the ECU has an inbuilt restriction to limit the duty cycle to approx 75% of their capacity. This can be changed with a chip with a better result than diddling with fuel pressures.

Regards Philip A

[Skiboy]

From the posters description of "hose clamp" on injectors, the basis of his engine is a flapper.
If the injectors are stock, then I suspect that they would not be able to pass sufficient fuel for any high boost, so his boost must be modest.
Question for poster. What sort of supercharger is on the car? Is it a Jap centrifugal or a Roots type? I have a vague memory that 3.5 injectors are about 14Lbs per hour.

IMHO you cannot compare running a 4.6 on 3.9 injectors as 3.9 injectors are plenty big enough to run a 4.6. They are the same 19lbs per hour as Mustang 4.6 injectors.
The problem you have with a 14CUX is that the ECU has an inbuilt restriction to limit the duty cycle to approx 75% of their capacity. This can be changed with a chip with a better result than diddling with fuel pressures.

Regards Philip A
Philip
The system runs a Powerdyne supercharger with Haltech computer system

Injector pic below
https://www.aulro.com/afvb/images/imported/2014/09/505.jpg (http://s835.photobucket.com/user/austskiboy/media/BreatherandPSshift23_zps0e9f54f1.jpg.html)

Skiboy

[PhilipA]

They are hose type injectors which came with 3.5 flappers, but I don't have any experience to know whether they are original or from some other car.

Perhaps someone old can post if they are from a 3.5 or something else like a Nissan 3 litre or XJS by the colour . Be careful that the hoses are in good nick and well held by clamps as many an XJ12 has burnt to the ground from a hose burst.


If it has a Haltech, you can tune the part throttle to be correct even with the turbo, as long as the injectors can flow enough for full throttle. So its probably even more important to tune it on a dyne with someone experienced with Haltech and who has the software.

The Powerdyne is a centrifugal supercharger which is like a turbo in operation although it has no lag as it is engine driven. That is it boosts more as revs rise and the impeller can fling more air outwards.
Regards Philip A

[Skiboy]

Thanks Philip

One of the hose clamps was sitting on the manifold - so I fitted it as in the pic above - you can see that there is a lot of fuel stain on the manifold so missing clamp leaking - and fire risk a worry.
So if a flapper motor what should the fuel rail pressure be?

[PhilipA]

So if a flapper motor what should the fuel rail pressure be?

Well AFAIR it should be 2.5 bar absolute ie about 37PSI no vacuum.

BUT that will not be true of a car with boost, so you have to adjust the PSI to take acount of the boost eg if 5PSI boost then raise by 5PSI and so on.

BUT even this is not absolute as you have a Haltech and supercharger so you have an experimantal setup. There is no stock spec and there are no rules to follow, except the rules you make yourself preferably on a dyno.

You could DIY by getting a wide band o2 sensor and driving and noting AF ratios at different loads etc, and you may be lucky that the AFs are pretty correct and you can adjust by increasing or decreasing fuel pressure. BUT if they are out you will need a Haltech tuning suite.
THE MOST IMPORTANT AF RATIO IS THE FULL BOOST ONE. too lean and you lose an engine.
Regards Philip A

[Skiboy]

Thanks for taking the time to have input on this Philip

Unfortunately the guy who set it up is out of business

I do however have the haltech software on a portable and can connect by a RS232 port and see all the mappings

BTW I have set the fuel pressure at about the same levels the gauge read on the old regulator plus screwed the adjuster to about the same point. So I doubt I am far off.

Would it be helpful to post a screen capture fro the diagnostic software with the engine running at different RPMs

[PhilipA]

Really not helpful without knowing the A/F ratios.
It will be helpful once you know the A/F ratios as you can then play with both pessures and injector bandwidths to get the engine running really well..
You need a wide band sensor and meter like this.
https://www.ngk.com/product.aspx?zpid=30503
Regards Philip A

[Skiboy]

I would love to learn how to do this so is it really something an amateur can achieve?

Even if I read the AF mixture level how will that help me without a dyno?
Or can you do this driving around the street?

I am keen just inexperienced - $299 is doable for the meter if it will achieve the result for an amateur like me.

[Skiboy]

Also the car already has an O2 sensor in the exhaust - does this not measure the AF mixture already?

[bee utey]

Also the car already has an O2 sensor in the exhaust - does this not measure the AF mixture already?
It does to a degree but you can't see it inside the car. A simple digital multimeter can be used as you just need to realise that below 0.4V is lean and over 0.6V is rich. Or buy a dash gauge to do the reading, eg:

NEW Autometer 2" AIR Fuel Ratio Gauge Silver 4375 2in 2inch 2 IN Inch | eBay (http://www.ebay.com.au/itm/NEW-AUTOMETER-2-AIR-FUEL-RATIO-GAUGE-Silver-4375-2in-2inch-2-in-inch-/231335659616?pt=AU_Car_Parts_Accessories&hash=item35dcae0860)

[Skiboy]

Wont the diagnostic software read that input so I can see it if I connect the laptop running the haltech diagnostic software up to the car?
I would imagine it will read the O2 sensor
Or am I miss understanding something here?

[Skiboy]

Not sure what wide vs narrow band means - ok looked this up and follow now.

I downloaded the instructions for the unit Philip posted the link for and it requires a boss to be welded into the exhaust to fit the sensor to then read the mixture

So if I install that I can then read the mixture at idle, coasting, acceleration, cruising etc and then adjust the fuel rail pressure to change that?

Hmm tempting to learn how to do all this. I note I would have to remove the sensor when not using it to read the mixture - inserting a plug in the boss.