Thread: Fuel rail pressure for 3.5

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

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?

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

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

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

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.

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

Originally Posted by Skiboy

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