Fuel Pressure Test Result Help

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Dec 18, 2009 | 04:41 AM

dailo

Thread Starter

Racer

Joined: Dec 2003

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From: Hong Kong

Fuel Pressure Test Result Help

Hi,

I have a 91 Vert.

I just had my fuel pressure diagnostics done, as there is hesitation under WOT in lower RPMS, and in higher RPMS it sort of cuts a little.

The only thing abnormal was that the fuel pressure took 3-4 seconds to reach 46PSI. Pressure held steady with the pump off, so I assume the FPR is OK.

Im just not sure it should take a few seconds to reach 46PSI, I saw a youtube video where the pressure was instant with the key turned on.

Just replaced the fuel filter last week, and the same problem persists, could the fuel pump be tiring out? Injectors are new from FIC.

Any ideas?

Thanks!

Dec 18, 2009 | 05:21 AM

94vettelover2

Burning Brakes

Joined: Oct 2009

Posts: 773

Likes: 2

From: Minneapolis MN

Quote:

Originally Posted by dailo

Be more specific.What do you mean 3-4 seconds?While starting?Why do you think hesitation is from fuel pump/issue?Tape fuel gauge to windshield & go for a ride while observing fuel pressure(steady driving,WOT,at idle,etc).When is last time a tune-up was done?Pull vacuum hose off fuel pressure regulator after car has been sitting & not running to see if fuel is leaking at vacuum port on regulator.Check for codes stored in computer.How long has car been hesitating & does car seem idle roughly?

Dec 18, 2009 | 08:51 AM

jmrl98

Drifting

Joined: May 2002

Posts: 1,509

Likes: 3

From: Hazel Park MI

When my vehicle exhibited those symptoms, my FP was too high. My FSM stated the symptoms were from too low of FP, but the opposite was true in my case.

Dec 20, 2009 | 10:30 PM

dailo

Thread Starter

Racer

Joined: Dec 2003

Posts: 349

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From: Hong Kong

The 3-4 seconds was from key on to full pressure. After the car was started, pressure was steady at 46psi. Will try taking the car for a drive to see, with the but it only seems to happen once the car is warm.

The car had a slightly rough idle, which went away after cleaning the TB and IAC.
It is well tuned. (oil and filter, air filter, spark plugs, plug wires, coil, IAC, are new, TPS is fine)
No fuel in the vacuum port. No codes in the computer.

Have been having this hesitation problem for a few months now. It is most noticeable when going uphill, I can practically floor the pedal, (but not enough for a downshift from 2nd to 1st) and the engine will very very slowly pick up, and once it revs past 4k sometimes will drop rpms for a split second.

Thanks for the help

Dec 21, 2009 | 03:12 AM

Frizlefrak

Race Director

Joined: Dec 2000

Posts: 17,551

Likes: 26

From: El Paso Texas

Quote:

Originally Posted by 94vettelover2

Be more specific.What do you mean 3-4 seconds?While starting?Why do you think hesitation is from fuel pump/issue?Tape fuel gauge to windshield & go for a ride while observing fuel pressure(steady driving,WOT,at idle,etc).

Do this and report back. We need to know precisely what the fuel pressure is doing when the symptoms occur. If your fuel pressure is within spec when the issue is occurring, we can rule out the fuel pump and regulator. When the fuel pressure is tested, does the pressure come up quickly at first and then take a couple of seconds to reach 46 or does it come on slowly and build steadily?

When you changed plugs, how did the old ones look? Any blistering on any plug(s) that might indicate a lean condition in that cylinder? After you've run the car and got it to replicate the symptoms, hit each exhaust port with an infrared thermometer and post the exhaust temps for each. I'm looking for a cylinder leaning out due to an injector issue.

If your fuel pressure is holding steady during an event, and the exhaust ports are all uniform temp and the plugs all looked similar when pulled, I think it's time to move on to the ignition / emissions systems.

Tape the gauge up to the windshield, replicate the problem, and tell us what your fuel pressure reading is.

Dec 22, 2009 | 10:49 PM

dailo

Thread Starter

Racer

Joined: Dec 2003

Posts: 349

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From: Hong Kong

OK will have to make a hose thats long enough to reach the windshield.

As for the pressure, it comes on slowly but builds steadily.

Will report back once I have driven with the fuel gauge attached to the windshield.

Dec 23, 2009 | 12:31 AM

AGENT 86

Team Owner

Joined: Jan 2003

Posts: 20,316

Likes: 246

From: Summerland B.C. Canada

The gauge on windshield would be most useful, if it's a delivery problem. Pressure can be good at idle or static, but once demand comes, can it supply the volume to keep up the pressure.

Dec 23, 2009 | 09:50 AM

pcolt94

Le Mans Master

Joined: Jan 2006

Posts: 5,621

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From: Orlando FL

Quote:

Originally Posted by dailo

Hi,

The only thing abnormal was that the fuel pressure took 3-4 seconds to reach 46PSI. Pressure held steady with the pump off, so I assume the FPR is OK.

Im just not sure it should take a few seconds to reach 46PSI, I saw a youtube video where the pressure was instant with the key turned on.

Just replaced the fuel filter last week, and the same problem persists, could the fuel pump be tiring out?

I agree the pressure should be instant when the key is turned on as mine is. Make sure your gauge is not the problem.

I would pinch off the return line at the tank area and try it again to see if there is any difference in the response of pressure building. Possibility the FRP is just slow to regulate and passes fuel out of the return line.

Its also possible the pulsator at the pump has a restricting and has poor flow or the sock is clogged. I would check all the easy stuff first but then you could end up at the fuel pump as last on the list.