Fuel pressure readings/system diagnosis

1997 Riviera - 3.8 Liter, L36 - 139,794 miles


Have recently developed a problem with start-ups, and after discussing with service guys at the Buick dealer, decided to do some Fuel Pressure measurements.


Here are a few questions: 1) What does it mean if the FP gauge is jittery (fluctuating 6 to 10 psi) while idling? It should be a steady pressure, yes?

2) Why does the FP drop ~2 psi (in 1 min) after ignition is turned ON, yet it stays at 51 psi (for 5 min) after ignition is turned OFF?

3) 'Ignition ON' readings started low (30 psi) then increased to 47 psi after several tests. Is this a weak/bad fuel pump?

Here are the pressure readings

Fuel Pressure (PSI)

Trial Ignition After FPR Vac Engine After
Num On Start Removed Off 5-min
-------------------------------------------------
1 30
2 34
3 40->38(1)
4 37->35
5 43->39
6 37
7 37 41-45(2) 47-51 48
8 46->45 41-45 47-51 43 51
9 45->44 39-48 47-50 44 51
10 47->46 45 51
11 44->42 33-45(3) 40
-------------------------------------------------

(1) Initial pressure, drops to ->P after ~1 min.

(2) Pressure jittery, gauge swing Min-Max typical.

(3) Starts at 42 psi, drops to 33 and chugs, smooths
out to 43, then jitters 41-45.The issue is "unstable idle after warm start-up."


Thanks,

-Ed

The drop is ok, 30 is low under any conditions, I would pinch off the return line and see what press. you have, running. I assume the fuel filter is new or clean? It does sound like the pump is failing. BTW always post the year of car, engine and mileage. Just looked at your other thread, I would replace the fuel filter no matter what and retest.

The drop is ok, 30 is low under any conditions, I would pinch off the return line and see what press. you have, running. I assume the fuel filter is new or clean? It does sound like the pump is failing. BTW always post the year of car, engine and mileage. Just looked at your other thread, I would replace the fuel filter no matter what and retest.

Ok thanks.. i need to understand the system better to interpret what 'pinching return line' does and means. Also.. was in the LR wheel-well last weekend and not sure there are any 'soft' rubber fuel lines, they were hard plastic IIRC. The fuel filter was replaced in 2007 at 100,410 miles. See: http://www.automotiveforums.com/vbulletin/showpost.php?p=7082253&postcount=13


Pressure Snap

What do you make of the tiny video clip below? It shows the Pressure gauge 'snap' up to ~42 psi during Ignition On. Then 'snap' down very quickly: http://home.comcast.net/~edwinn/97_Riv_FP_ignition_on_click.mp4 (300 KB)


I think the Fuel Pump Relay is heard in the background.. then the check valve 'catches' the pressure from dropping? But what's worrisome... is the gauge flutters like crazy! It that a cheap gauge? with an under-damped movement? or is that normal? I need a gauge that shows pressure and doesn't 'flip' all over the frikin' place. :eek7: Lol


-Ed

Borrow or get another gage and check that fluctuation. 6 years on the filter, repl it as I said, just eliminates a possible issue.

Borrow or get another gage and check that fluctuation. 6 years on the filter, repl it as I said, just eliminates a possible issue.
Thanks MW,

I got a reply from Tech Support at Actron (otctools.com) and they said: "You will see this depending on the vehicle. The gauge is fine and you will see the same with another gauge. If the vehicle has a fuel pressure regulator with a vacuum line remove it and see where the average pressure is at, or refer to a manual to see what the recommended wide open throttle (WOT) fuel pressure should be. Make sure you get the same recommended pressure or pressure with vacuum off the regulator when at WOT under full load. If it does not get to the same pressure or gets there and drops while under load then it most likely is a fuel pump.

You are seeing 38-50 fluctuation , so average by splitting it, so I would consider this to be about 44 psi at idle, if you disconnect the vacuum line (if equipped) or do a WOT run under load you should see it go up about 5 psi."

So that says the gauge response is normal. NAPA has the exact same pressure gauge product that i bought at Sears (a few beans less at sears). Sounds like the problem is going to be the fuel pump. I'll put a new fuel filter on before buying a replacement fuel sender. The expectation is the pressure will be stable with no more gauge flutter or engine stumbling.

Will do another set of pressure measurements to diagnose.. with a little better planning next time.


Since you guys like photos, here's a snapshot from the Ignition-On video above.

http://home.comcast.net/~edwinn/engine_video-snapshot.jpg



-Ed