Fuel pressure

[RR]

I have a fuel injected car, bosch system. When I put a fuel pressure gauge on to test, the lbs are correct at 55lbs while idling, however when I rev the engine the pressure drops?? Is this correct, I thought the pressure would increase with the need for more fuel? , and if it drops is this a leak in vacuum line or bad regulator. Any help appreciated.

[924Guy]

K-Jet or L-Jet? Or, even worse, D-Jet?

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Vaughan Scott
Detroit Region #280052
'79 924 #77 ITB/GTS1
www.vaughanscott.com

[JohnRW]

Don't make fun of D-Jet. In ITB, you'll be looking at a lot of 'Volvo' scripts on the backs of cars, until they get too small to read. That would be D-Jet.

[ITANorm]

Agreed - the D-jet on the 4AG Toyota makes ~12 more horsepower, stock, then the L-jet. The L was used over here for its superior emissions capabilities.

[mckeaaan]

RR,

The pressure should drop momentarily when you rev it up and then return quickly to nominal.

This is because the static fuel pressure is running through the regulator at 55 psi... then when you nail it; the injectors consume the volume of fuel in the rail more quickly than the pump can supply at that moment.

When you run at a constant rpm, let's say, 3000 rpm, you should have constant gage pressure a few psi lower than static.

On another note, you may be able to get a performance increase by increasing the rail pressure, as this will cause the injectors to flow more fuel.

Regards,
Matt

[ITANorm]

Bosch L-jet's (at least on the Toyota's) have a rising rate regulator. It senses vacuum and raises the pressure. Normal idle pressure on a 4AG is 38psi - with 45 psi at WOT.

That said - and if you have an L-jet similar to Toyota's - you may have a bad regulator, or a vacuum leak.

[dyoungre]

Hey RR,
What car, make model, year etc? Unfortunately ... the specifics (as you may now be realizing) may make the difference.

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Dave Youngren
NER ITA RX7 #61

[RR]

Thanks for the replies. The car is a porsche 944. I dont think the problem is a vacuum leak because the presure jumps up 5lbs when you disconnect. That also tells me the regulator is ok. Im thinking fuel delivery at this point, the filter is new, maybe the pump. In response to one of the above posters, the fuel pressure is not supposed to suddenly drop as you open the throttle, the regulator should immediatly close boosting the pressure and maintain the idle pressure.

[Ron Earp]

The fuel pressure should not move when you jump on the throttle. I've played with this, that is, watched this parameter on chassis dynos and if everything is working correctly and you are not atthe limit of your fuel system at high RPM then you will notice no change.

The pump is more than capable of suppling all the fuel needed for the motor at RPM. The regulator is there to make sure fuel pressure remains constant as demand changes, voltage changes across the pump, etc.

I would think you have a pump issue as you have suspected.



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Ron Earp
NC Region
Ford Lightning
RF GT40 Replica
White Jensen-Healey ITS
Silver "Skull" 260Z ITS
Email: "rlearp at gt40s.com"

[dyoungre]

RR,
I too have a fuel supply problem, and I went through a similar troubleshooting process, only to find out that I had a plugged pickup in the gas tank. Before buying a new tank, make sure you try pulling fuel from a second source (fuel filler can, large coffee can, or such) and see if the issue still occurs.

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Dave Youngren
NER ITA RX7 #61