Fuel pressure

[Pete Revenidis]

Adjusted fuel pressure today up to 42psi due to knocking once in a blue moon it was at 38 mounted gauge on hood at part throttle it goes up to 48psi on full throttle it drops to 38 and then knock? ??? It has hotwire walbro and accufab regulator. What could be wrong????
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[earlbrown]

Dieing fuel pump, clogged fuel filter, internal tank hose falling apart (or pulsator leaking like hell), faulty regulator, bad signal line feeding regulator, clogged or curled up fuel sock, warped up stock vacuum block......

 

[Pete Revenidis]

Dieing fuel pump, clogged fuel filter, internal tank hose falling apart (or pulsator leaking like hell), faulty regulator, bad signal line feeding regulator, clogged or curled up fuel sock, warped up stock vacuum block......

But why is it ok at part throttle

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[earlbrown]

if the pump is tired, or the tank hose is hemorrhaging it might get you by at part throttle but when the pressure goes up, the leak (or weakness) might move your pressure into a severe lean condition.

There's no way to accurately troubleshoot over the interwebs until you can eliminate some items on that list then go from there.

 

[Pete Revenidis]

if the pump is tired, or the tank hose is hemorrhaging it might get you by at part throttle but when the pressure goes up, the leak (or weakness) might move your pressure into a severe lean condition.

There's no way to accurately troubleshoot over the interwebs until you can eliminate some items on that list then go from there.

I figured I was going to have to start throwing parts at it to figure out what's wrong the funny thing is the car feels faster a part throttle then full throttle
The fuel pressure will actually go up to 50 psi at part throttle and as soon as I put my foot to the floor it drops down to 38
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[earlbrown]

You don't have to start throwing parts at it, you just have to start inspecting and testing parts, then replace or fix what is bad.

If your base fuel pressure is 45 with the line off, you should be seeing that 50#s at part throttle 5#s of boost. Verify there are no vacuum leaks on the throttle body block, short hose on the block and the hard line on the FPR regulator. If the regulator line starts to leak at 5#s it can do exactly as you describe. For example.


If you just start tossing untested (even new) parts at it, you'll have a pile of untested parts, a light wallet, and might not even get it fixed before a head gasket shoots out the side of the block.

 

[Pete Revenidis]

You don't have to start throwing parts at it, you just have to start inspecting and testing parts, then replace or fix what is bad.

If your base fuel pressure is 45 with the line off, you should be seeing that 50#s at part throttle 5#s of boost. Verify there are no vacuum leaks on the throttle body block, short hose on the block and the hard line on the FPR regulator. If the regulator line starts to leak at 5#s it can do exactly as you describe. For example.


If you just start tossing untested (even new) parts at it, you'll have a pile of untested parts, a light wallet, and might not even get it fixed before a head gasket shoots out the side of the block.

Ill look for leaks but im suspecting the Fpr because i had to turn it quite a bit to get 2psi out of it and I've never had to do that with any other ones i adjusted

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[Pete Revenidis]

One more thing if i take off the vacuum line it should stay steady at 42 the whole time correct? ?? No matter how much throttle if it drops at full throttle again it should have something to do with the pump?

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[earlbrown]

It should be steady with the line off as long as you don't rev the engine. That line will pull the pressure down under vacuum and raise the pressure under boost. If that line has a piece of trash in it acting like a check valve or a hole that vents boost it will screw up your pressures (and your fueling)

 

[Pete Revenidis]

It should be steady with the line off as long as you don't rev the engine. That line will pull the pressure down under vacuum and raise the pressure under boost. If that line has a piece of trash in it acting like a check valve or a hole that vents boost it will screw up your pressures (and your fueling)

I'll check it now see what i come up with

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[Pete Revenidis]

Well no vacuum leaks and since i have a 20ft roll of silicon vacuum hose i replaced both ends of the rubber line they were a little hard no change. It goes to 50 at part throttle but where is it supposed to be at full throttle ??? 42 were i have it set at? ?

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[earlbrown]

at full throttle it should be EXACTLY base pressure + lbs of boost.

 

[Pete Revenidis]

at full throttle is should be EXACTLY base pressure + lbs of boost.

So 42+15

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[Pete Revenidis]

So i have not narrowed it down yet and now my fuel pressure gauge bounces around at part and full throttle i got my new TT chip and injectors with a scanmaster i didn't put chip and injectors in but here are my numbers at full throttle
O2 1050
Knock 8.7°
At 62 mph
those are no good
If i crimp the return line would it make the fp regulator show if its bad i have done this on other cars to test and if there bad pressure stays stedy

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[earlbrown]

crimping the return line with show you deadhead pressure of the pump. I. E. the most amount of pressure it can generate (at idle voltage). It will also make your car go RICH as hell.

 

[Pete Revenidis]

crimping the return line with show you deadhead pressure of the pump. I. E. the most amount of pressure it can generate (at idle voltage). It will also make your car go RICH as hell.

Earl I know that but if the regulator is bad the pressure shoudnt drop if it still drops that means my pump is not flowing correctly

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[notacarlo]

It won't rise either.

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[earlbrown]

Earl I know that but if the regulator is bad the pressure shoudnt drop if it still drops that means my pump is not flowing correctly

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Pinching the return line takes the regulator out of the equation. When it's pinched the pump will pressurize the feed side with deadhead pressure.

A gauge up front will read deadhead pressure minus any pressure drops from flow or leaks between the pump and the gauge.