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Old 09-13-2008, 03:15 PM
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Fuel Pressuer Guage Jumping Around

I put a fuel pressure gauge on my camaro and when it's idling the gauge jumps back and forth for 0 to 15 real fast. When I snap the throttle, it will go up and stay constant at 6 or 7. Is this jumping back and forth normal? If it isn't what does it mean?

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Old 09-13-2008, 03:27 PM
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I am thinking the jumping from 0 to 15 at idle is being caused by a cracked fuel supply hose, possibly back at the fuel tank. JMO
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Old 09-13-2008, 03:29 PM
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mechanical or electric guage??????
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Old 09-13-2008, 03:31 PM
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A cheap pressure gauge will do that. The actual pressures seen at the gauge vary wildly at idle because of huge pressure changes. If its a carbed car, the diaphragm pump squirts fuel once per revolution so it fluctuates big time. On an EFI car, the pump supplies pretty steady pressure, but you see the pressure change because of the injectors opening and closing.

Get a good oil-filled pressure gauge and it will help things out a lot. It absobs those wide fluctuations and gives you a more average reading.
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Old 09-13-2008, 04:03 PM
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Quote:
Originally Posted by squeeezer
mechanical or electric guage??????
It's a mechanical guage.
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Old 09-13-2008, 04:05 PM
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Quote:
Originally Posted by curtis73
A cheap pressure gauge will do that. The actual pressures seen at the gauge vary wildly at idle because of huge pressure changes. If its a carbed car, the diaphragm pump squirts fuel once per revolution so it fluctuates big time. On an EFI car, the pump supplies pretty steady pressure, but you see the pressure change because of the injectors opening and closing.

Get a good oil-filled pressure gauge and it will help things out a lot. It absobs those wide fluctuations and gives you a more average reading.
The gauge is a $15 Mr. Gasket guage. The engine is carbed.
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Old 09-13-2008, 05:14 PM
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so do you have to use an isolator or is it just a fuel line mounted unit???
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Old 09-13-2008, 05:17 PM
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Quote:
Originally Posted by squeeezer
so do you have to use an isolator or is it just a fuel line mounted unit???
It's a fuel line mounted unit.
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Old 09-18-2008, 03:32 PM
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I have the mister gasket pressure gauge on my vette and it jumps also. I was told it was becuase i have a mechanical fuel pump.
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Old 09-18-2008, 04:32 PM
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That makes sense. I had an electric fuel pump on my other project and I didn't have this issue.
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Old 09-18-2008, 05:16 PM
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Say you idle at 750, the fuel pump strokes at half that . . . . 375 RPM. That's about 6 times per second. These pulses are slow enough that it's easy to believe the gauge pulsing. Even if only because the needle is so rapidly accelerated to 6-7 PSI, it just keeps going on each pulse. I bet you have a lot of hard line between the pump and gauge. If you use a little hose in there it might help cushion the pulses just a bit.

Pat
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Old 09-18-2008, 05:23 PM
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borrow a guage from somebody and compare readings....

to confirm the guage is junk and the pump is fine....
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Old 09-18-2008, 10:13 PM
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Spend the dough and get a nice liquid filled gauge as suggested earlier. You get what you pay for and 15 bucks aint much for a gauge.
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