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Just an enquiry on fuel pressures for a VR V8.

Firstly some specs:
- 383 stroker
- Walbro in tank pump 255lph.
- Cream top injectors
- Original fuel rails with pressure guage screwed into rail.
- Original fuel regulator.

Well I have been having some issues with inconsistant idle a/f ratios. Sometimes it will idle at around 13:1 and others at 16.9:1.

Findings:
At cold idle the fuel pressure is 40psi, at hot idle the pressure drops to 33psi.
When I remove the vacuum signal line to the regulator the pressure increases by about 4psi in both hot and cold situations.
If I clamp the fuel hose return to the tank the pressure will rise approx 80psi.
I have tried another second hand original regulator with similar results. Is this normal regulation control with the standard regulator?

The information I have with regards to fuel pressure specs are 280kpa(40.6psi) - 320kpa(46.4psi). With the info above it looks like my regultor is possibly faulty.
Anyone carried out similar testing to verify if what I have is normal?

Thanks.

There will be tune problem and possibly a fuel system issue, as a 4psi change should not change mixtures so radically. Maybe 1 afr point change, but you've nearly 4 !!!
So.........

Have you monitored injector duty to see if there is a marked change when your mixtures move from 13 to 16.9 ?.
Is closed loop/blm affecting your mixture ? (try disconnecting the sensors)
What changes have you made to the desired afr table, idle timing and ve table to reflect the changed engine characteristics?
Do you have low manifold vacuum from a big camshaft (i'd say so from the pressures below - How many inches of vac at idle ?)
What type meter and where is the AFR sensor located ? There is some glorified, worthless junk being sold to punters.
You have a real pump and not a dodgy copy ? (I don't like Walbro pumps. Period.)
You have no fuel leaks inside the tank or twisted hoses where the pump joins the fuel outlet tube ?
Was there a minor problem before and it's only showing up now ?
Are the injectors flow matched and clean ?
Is the engine idle stable while your AFR/s vary so much ?
Is the engine mechanically sound ?
Not misfiring from fouled plugs ?

Remember all things being equal, bigger pumps will flow more fuel and increase base fuel pressure somewhat. It's correct procedure to increase the flow capacity of the regulator and lines when a large change is made to overall flow.

Think of it this way.
A pump will flow 255 l/hr at a specific fuel pressure. Increasing the fuel pressure will reduce the maximum delivery volume of the pump. At idle you may only need 0.1 l/min (6l/hr). So the regulator needs to RETURN 249l/hr to the tank at idle - AT 0 PSI PRESSURE. If there is a restriction in the return line (like a small bit of hose, swarf, mud wasp nest, or restricted return inside tank etc...), there's nothing to force the fuel through and inconsistent pressures will be the bain of your life.
Many cheap liquid filled pressure gauges are too damped to see a rapid fluctuations. May I suggest you put a decent quailty (CODA, ABW etc) fuel pressure and FLOW gauge on the return line. If there is ANY noticable pressure present, you have a restriction somewhere. Remember that the return line should be very close to 0 psi.

I've had this before with a small sliver of rubber hose caught in the return tube in the tank - AND A BLOODY HARD TO FIND MUD WASP NEST.

Pressure should remain stable hot or cold, wet or dry, sun-up or sundown. Your pressure variation could be due to stabilization of fuel flow with the higher duty cycles at a cold idle - but I'm not convinced.

Obviously fixing a problem like this can be exceedingly hard via messages. Perhaps take it to your favourite w/shop and let them fix it.
Think about this -Would you call or message your doc saying "I'm sick, what's up with me?". No you'd go and he'd perform some tests to diagnose you!

As i say in the 1st week of apprentice training - ALWAYS CHECK THE BASICS FIRST

Good luck, I Hope this helped you out.

Thanks for a comprehensive reply.

Yes, I have no doubt that I have a tune problem as I have not yet finalised it.
My main concern was that the standard fuel regulator are supposed to be a 3 bar (43psi) unit and I am seeing less than this. The pressure gauge I have installed is new and glycerine filled (damped), I did not check it's accuracy before install.

On the weekend I will check for return line restrictions and carry out a good visual of the fuel lines and hoses ect.

Thanks,

a lot of the liquid filled gayges read wrong by 10psi or more and then they get hot and the oil in the gauge expands and makes them read out as well

Thanks for a comprehensive reply

Easy, I have many of these suggestions already written, as it frequently comes up again.

a lot of the liquid filled gayges read wrong by 10psi or more and then they get hot and the oil in the gauge expands and makes them read out as well

Absolutly correct !! They're sealed up tight not to leak but the heat screws them up as there is no expansion facility. Yep, as we said, cheap gauges - but not always... I remember many Autometer liquid filled guages did this....
As always, using cheap diagnostic/test equipment will lead you up the garden path! I've an extremely accurate and sensitive pressure gauge here that's worth many thousands of dollars new. Laboratory spec equipment is expensive for a reason - traceable, repeatable accuracy. Ask your supplier for a technical specification sheet on the gauge- you'll probably get a silly look or no reply..

It's a pity the world is going this way, but if there's a market for it and your able to convince people it's the ducks guts, they'll buy it (mostly on cheap price rather than quality) - because people won't admit they don't understand what they're looking at. The old addage of 'Stun 'em with science, baffle them with bullshit' is rather relevant..

The performance industry is loaded with lots of finest quailty brank new junk. Remember, Buyer beware !

But I should keep this message on track... Sorry about babbling on...

Stu

I might have a pressure regulator here you can have next week... lol.

I have heard the same thing about those gauges, but if your tight like me, I've also heard they are ok for testing differences like hot and cold and give you an idea of variation but just have the gauge out of the bay like a t piece just to do a test might be ok...

there was a run of shit walbros at one stage if you got one of the dud ones it might not doing its thing once it heats up

I replaced my pressure gauge that I have mounted into the standard fuel rails and have found idle pressure to be 40psi and with the vacuum hose removed it was approx 46psi.
The new gauge is accurate, I took it to work and pressure tested with a beamex pressure calibrator.
So far so good, I will check for heat soak over the next couple of day's.

If anyone is interested this is the gauge I installed yesterday: http://cgi.ebay.com.au/ws/eBayISAPI.dll ... 0455180235

if it reads 0 on the rail cold without pressure in the line and 0 when hot with no pressure in the line it is all you can ask for. I think the shit ones read different with temperature change of the gauge itself irrelevent of pressure in the line... but you prolly already know that but someone else reading this from somewhere else mightn't.

Heat soak has no influence on the new gauge.