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post #1 of 18 (permalink) Old 01-12-2016, 05:23 PM Thread Starter
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Fuel pressure question

I have a fuel pressure gauge with the sending unit connected to the fuel rail. Normally it reads 35 psi steady I noticed lately it's more like 30 psi. When I went to leave work today the car was hard to start and stalled once on me. Once she was running I had no issues and the fuel pressure was at 30 psi. They way I see it there are 3 possible problems and looking for input on how to diagnose.
1. Fuel pump
2. Fuel Filter
3. Regulator
Any input on my likely problems and where to start? Thanks.
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post #2 of 18 (permalink) Old 01-12-2016, 05:58 PM
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I'd start with the easiest of the three and go with the fuel filter, especially if it hasn't ever been changed.

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post #3 of 18 (permalink) Old 01-12-2016, 06:58 PM Thread Starter
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That's what I figured. I changed it about 30k miles ago.
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post #4 of 18 (permalink) Old 01-12-2016, 08:32 PM
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Fuel pressure should fluctuate with manifold vacuum. You should see ~28-30 psi at idle and ~38-42 psi at WOT (assuming we're talking about a NA setup).

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post #5 of 18 (permalink) Old 01-13-2016, 07:57 AM Thread Starter
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It is NA, thanks. It doesn't seem to go above 30 PSI. She started fine this morning. I'm going to put a new filter on her and see what happens.
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post #6 of 18 (permalink) Old 01-13-2016, 08:00 AM
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Unless you have an adjustable FPR on there to adjust pressure, a permanent fuel pressure gauge is "too much information", ie: something to watch and worry about. A temporary one to diagnose an issue is a better idea.

Like mentioned, the pressure constantly adjusts with throttle input. Your random issue (one stall) could be anything. Just cause you have a gauge on there doesn't mean it's fuel related. But a filter is cheap enough to replace.

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post #7 of 18 (permalink) Old 01-13-2016, 08:31 PM
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Quote:
Originally Posted by Kidd-7 View Post
It is NA, thanks. It doesn't seem to go above 30 PSI. She started fine this morning. I'm going to put a new filter on her and see what happens.
Even at KOEO? If so, there is most definitely a fuel delivery issue.

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post #8 of 18 (permalink) Old 01-14-2016, 08:55 AM Thread Starter
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What is KOEO?
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post #9 of 18 (permalink) Old 01-14-2016, 10:10 AM
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Key on, engine off.
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post #10 of 18 (permalink) Old 01-14-2016, 10:44 AM Thread Starter
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Ahhh, yes KOEO it has been around 25 - 30 psi.
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post #11 of 18 (permalink) Old 01-14-2016, 04:20 PM
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That signifies a problem. If you leave it KOEO for a bit (30-60 seconds), does the fuel pressure remain steady, or does it drop? Does the fuel pump only prime for two seconds at initial key on, does it remain on, or does it cycle?

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post #12 of 18 (permalink) Old 01-14-2016, 06:31 PM
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If the fuel pressure doesn't vary with throttle, check the vacuum line; if there's fuel in the vac line, the regulator is bad.

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post #13 of 18 (permalink) Old 01-15-2016, 08:55 PM
No, Mr. Lemmywinks, No!!!

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Quote:
Originally Posted by mcm51 View Post
...he said it runs a 30 psi all the time, that pressure should be moving around...
Those are the "Hard line" iirc; they crumble where you can't see them.

If it doesn't vary, the first and cheapest thing to check is the vac line.


Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.


"War Is Peace. Freedom Is Slavery. Ignorance Is Strength." - George Orwell.
Truth Isn't Truth. - Rudy Giuliani, 2018 Award winner, “Most Outrageous Diversion” by MSNBC for this quote

Last edited by Grog6; 01-18-2016 at 06:53 PM.
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post #14 of 18 (permalink) Old 01-18-2016, 01:06 PM Thread Starter
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It seems to have been the regulator. I replaced the filter and no improvement. I had a spare regulator and threw it on, driving to work today she stayed right around 35 PSI. Thanks for your assistance.
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post #15 of 18 (permalink) Old 01-18-2016, 01:19 PM
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Pressure at the rail should vary with manifold vacuum. You should see 38-42 psi at 0 inHg. Subtract ~1 psi for every 2 inHg of vacuum. If pressure doesn't fluctuate with throttle position, the regulator isn't working correctly.

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post #16 of 18 (permalink) Old 01-19-2016, 11:25 AM Thread Starter
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I attached a photo of my gauge. I don't think it provides the resolution for a 4 PSI difference. With this new regulator the pressure is staying around the 35 line. Whereas before it sat around 25 to 30 psi, like whats in the picture. Thanks for all the help.
Attached Images
File Type: jpg 2016-01-06 17.32.22.jpg (30.8 KB, 32 views)

Last edited by Kidd-7; 01-19-2016 at 11:28 AM. Reason: picture
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post #17 of 18 (permalink) Old 01-19-2016, 04:41 PM
No, Mr. Lemmywinks, No!!!

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Check the vacuum with the engine running; looking at the gauge with the engine running, pull off the vacuum line, and see if it has vacuum by sticking your thumb to it.

The fuel pressure should change; I forget which way. I think it goes up.

The engine will probably stumble; idle has the most vacuum. (OK, slammed closed throttle butterfly at 7k rpm has the most vacuum, but... )

That's probably the best you can do with that gauge.

I prefer gauges like this:
Our FP doesn't go over ~40psi; a 60psi gauge supports more boost than you will ever use.



ymmv.

Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
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post #18 of 18 (permalink) Old 04-04-2019, 09:44 PM
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Quote:
Originally Posted by Kidd-7 View Post
It seems to have been the regulator. I replaced the filter and no improvement. I had a spare regulator and threw it on, driving to work today she stayed right around 35 PSI. Thanks for your assistance.
I'm serious, what technique did you use to just "throw" on a fuel pressure regulator? Don't you have to peel everything off the top of the engine to just get to the rail so you can remove it to get to the regulator?
MDW
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