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Fuel and Ignition Fuel Pumps and Systems, Ignition and Spark Systems

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Old 05-14-2018, 09:42 AM   #1  
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Fuel pump vs regulator - help deciphering numbers

Hi all - I have a 97LT1 stock original fuel system. Its a road race car so I had to take it in for routine dyno certification. last dyno 6 years ago the AF ratio was perfect at 12.5 throughout. This time power is exactly the same but AF went up to 14-14.25. I checked for vacuum leaks, fuel is a little old but not enough for this I think, so Im checking my pump, here are the numbers:

With the fuel pressure regulator still hooked up to vacuum:
Idle 40psi, at throttle tip in it goes to 43 for a split second, then falls with more throttle to a low of 32 above 3000 RPM.

With the fuel pressure regulator unhooked and vacuum plugged:
Idle 46 PSI and no change at all with throttle.

It still has the original fuel pump and regulator. So I think this means the pump is good and it needs a new regulator? Im going to town to get a new fuel filter just to see if it makes a difference. Thoughts?
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Old 05-14-2018, 10:43 AM   #2  
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Re: Fuel pump vs regulator - help deciphering numbers

New fuel filter made no difference.
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Old 05-14-2018, 09:50 PM   #3  
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Re: Fuel pump vs regulator - help deciphering numbers

Is there something I'm not clear about? I'm just trying to find out if the problem is with the pump or the regulator based on the results of my testing. Thanks for any help!
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Old 05-14-2018, 09:54 PM   #4  
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Re: Fuel pump vs regulator - help deciphering numbers

Quote:
Originally Posted by SSTAT View Post
#1
With the fuel pressure regulator still hooked up to vacuum:
Idle 40psi, at throttle tip in it goes to 43 for a split second, then falls with more throttle to a low of 32 above 3000 RPM.

#2
With the fuel pressure regulator unhooked and vacuum plugged:
Idle 46 PSI and no change at all with throttle.
Unclear.... we’re the first test and second tests performed with the engine under load, or with the engine idling in neutral?

With the engine under max load, above 5,000 RPM, the fuel pressure should not drop more than a few PSI below the “no vacuum” value.

How much the pressure drops with vacuum at idle depends in part on how big the cam is. Stock cam vacuum will typically drop the fuel pressure about 8 PSI at idle.
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Old 05-15-2018, 06:37 AM   #5  
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Re: Fuel pump vs regulator - help deciphering numbers

Thanks! I gave results at idle and revving the motor (in neutral) both with and without vacuum above. It fell to 32 while revving to anything over 3K with vacuum, without vacuum it was 46 and didn't budge no matter how high I revved it. I have read it should never be below 40 psi. What would you do next to sort it out? Yes it has a big cam.

Last edited by SSTAT; 05-15-2018 at 06:45 AM.
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Old 05-15-2018, 03:52 PM   #6  
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Re: Fuel pump vs regulator - help deciphering numbers

Without the vacuum line attached, the pressure should stay at the “no vacuum/idle” value, in this case 46 PSI. Should never change. GM spec is 43.5 PSI, but anything in the range of 41-47 PSI is acceptable.

When you reattach the vacuum line at idle, the pressure should drop proportional to intake manifold vacuum. In your case, 36-38 PSI would be expected with a stock cam. Maybe only 4-6 PSI drop with a more aggressive cam.

Revving the engine with no load on the engine is of little value. You would expect the pressure to rise as the throttle blades are opening, level off as the engine reaches steady state RPM, and even drop below the 36-38 PSI value as the throttle closes and the revs drop off. All of that happens very quickly. 32 PSI is suspect, but not conclusive.

The real test is taping the gauge to the windshield, taking the car out on the road (or on a dyno), and go WOT, taking it above 5,000 RPM. At peak RPM, under full throttle, the pressure should approach the “no vacuum/idle” reading, in your case 42-44 PSI. If pressure falls below that at any time during the WOT/under load process, the fuel pump may be on its way out.
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Old Yesterday, 04:21 PM   #7  
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Re: Fuel pump vs regulator - help deciphering numbers

I think Im on to it. I ffound that the custom pipes/check valves (from the air pump) that came with the my old AS&M headers were rocking back and forth in their fittings, and one header collector was missing 2 nuts and I could move the gasket back and forth with my hand. I think exhaust air leak was telling the computer it was getting too much air so the engine was dumping gas it to correct it (my actron scanner showed trims at +25% at idle) Thanks for the help all-
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