Erratic Fuel Pressure

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Erratic Fuel Pressure

Unread postby gamh44 » Fri Feb 08, 2019 5:49 am

Been making basic adjustments to my tune (MAF curve, Injector data) and have noticed inconsistencies in the fuel pressure.

Today, I started the engine cold and the FP was 40PSI (vac on). Once warm this dropped to around 35PSI. I adjusted it back up to 40PSI and started on making some tune changes. After about 10 mins and a few changes, the pressure had dropped to around 25PSI (vac on). Played havoc with the FE% values I was looking at.

Crook regulator? Fuel filter? Pump?
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Re: Erratic Fuel Pressure

Unread postby decipha » Fri Feb 08, 2019 4:05 pm

u have to remove the vac line to check the base fuel pressure, 40 cold should be about 53 base.

25 with vac on should be about 34 base.

sounds like a bad fuel pressure regulator
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Fri Feb 08, 2019 6:59 pm

Thanks, decipha

I assume there is some rubber diaphragm in the regulator. The car has on occasion in the past 10 years sat for months on end without running. I imagine it may well have dried out at some stage so wouldn't be surprised if that could create a failure that would cause the fluctuations.

I'll have another go at it today and see if it does the same thing. Then a new reg will be on the cards.
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Sat Feb 09, 2019 4:30 am

Well, it did it again. Started around 45PSI base and 38 with vac off. Made some tune adjustments, collected some data and but the time I had finished it was down to 32 base and 26 without vac.

I pulled the regulator and split it apart. Diaphragm looks fine. No fuel on the spring side. Valve seat has no blemishes. Not much else to go wrong. I'll give it a wash and have another go. All looked pretty clean as well.

There is nothing that I can see that would make the pressure drop after 10-15 minutes.

What is there that fails in a FPR? I am guessing that the problem is either due to time running or engine temp.
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Re: Erratic Fuel Pressure

Unread postby decipha » Sat Feb 09, 2019 11:06 am

it sounds like the spring has gone bad
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Wed Feb 13, 2019 3:12 pm

Thanks for that.

Do these springs fail often? My Mallory (now MSD I believe) unit has two concentric springs that are pretty hefty units. Look more like valve springs than for a regulator.

Is there any chance that a crook battery could drop the pressure that the pump is sending. Just trying to eliminate all other possibilities before I fork out 300 beans for a new reg.

I ask because in a recent log while idling (attached), I noticed the FE% (graph below) started rising from 1.1 to 1.2 right at the time the FP started dropping. All this started at a time when VBAT dropped from about 13.8 to 13.6V, which was when the thermo fan kicked in.

I did another test and the voltage at the pump dropped from 13.5V to 13.1V at the time the thermo fan kicked in.

My battery is a few years old but hasn't had much use. Quite a few cycles to dead flat and back. Probably worth replacing anyway.
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Re: Erratic Fuel Pressure

Unread postby decipha » Wed Feb 13, 2019 5:47 pm

why not use stock fuel rails and a stock replacement fuel pressure regulator?

i would suggest a kirban regulator over all others

no that little voltage fluctuation wont affect fuel pressure like that

doubtful the battery is bad
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92 (2x) 5spd & auto - Bianqa, Andrea
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94 5spd Rionda
95 auto Aisha
Vehicle 2 Information: Others:
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00 Explorer 5L - Bernyce
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Wed Feb 13, 2019 11:13 pm

Thanks again.

I have the original rails and regulator on the shelf. I considered patching the regulator in just to confirm where the problem is but hadn't thought to drop the whole lot back on. Might be easier.
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Sun Feb 17, 2019 4:51 am

Righto, well...

Machined up this assembly (see below) to fit the stock regulator to the PRO-M fuel rails. Looked at replacing the rails and all but thought this might end up easier. Not so sure now!

Got it all fitted up and fired up the fuel pump and ... while the assembly looked all good, the old reg started leaking out the crimped join between the top and the bottom sections. I dropped it out and gave it the berries with some vice-grips and a couple of gentle taps with the hammer and put it back in. This time no leaks.

Fired the engine up and it ran as would be expected. Pressure was around 37PSI with vac and 42PSI vac off. Cold.

There was a small weep from one of the AN fittings I machined. Might not have quite been exactly 37* so I will wait for the fuel pressure to bleed back to the tank and then add a heap of thread tape and see how we go.

Image
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Tue Feb 19, 2019 3:48 am

Got it all running with the stock regulator and ...

Same story.

Cold start, pressure was 37 with vac and 41 vac off. Numbers started dropping as soon as there was some heat in the engine - eventually to 19 with vac and 25 vac off.

So doesn't look like the reg was the problem.

Hadn't considered it before but could the gauge itself be stuffed? Car didn't really run noticeably different at 19PSI and FE% was much the same as when it was 37.

I can probably test it with a heat gun but it might be just as easy to just replace it.

EDIT:

Worth adding that the pressure actually climbed a couple of PSI for each 10 minute interval after the engine was shut off.

And ... just found this on Summit: A change in ambient temperature can cause case pressure to change as much as 7-12 psi. As a result, gauge readings can fluctuate from 40 psi cold to as little as 28 psi hot.

What?!?! Have I been chasing my tail all this time. Has the pressure always been low on the gauge when hot and I just haven't noticed it?!??! I think I might just set it cold and ignore it.
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Re: Erratic Fuel Pressure

Unread postby decipha » Tue Feb 19, 2019 6:39 pm

sounds like u have a crap gauge. I couldn't even begin to tell you how many vehicles Ive set the fuel pressure on cold and it stayed the same when it was warm

Ive never seen nor heard of a gauges accuracy falling off 12 psi thats just lunacy
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Vehicle Information: Supercoupin' x10
90 (4x 5spds) - Dante, Ruby, Daja, Ava
91 4r70w - Skarlett
92 (2x) 5spd & auto - Bianqa, Andrea
93 auto - Danika
94 5spd Rionda
95 auto Aisha
Vehicle 2 Information: Others:
00 Lincoln LS - Luanda
98 Camaro SS - Bounquisha
02 Harley F-150 - Sasasha
03 Marauder - DyShyKy
00 Explorer 5L - Bernyce
07 GMC 2500HD 6L - Veranafer

Re: Erratic Fuel Pressure

Unread postby gamh44 » Wed Feb 20, 2019 2:43 am

Crap gauge indeed. Just used a small bag of ice to see if it made a difference. Let the gauge drop then once engine was warm applied the ice. Pressure climbed from about 24 to 36.

What a useless piece of equipment. Not sure that disclaimer was on the Summit site when I bought the gauge 8 years ago.

It appears that this is not unheard of with liquid filled gauges near hot engines. I would guess that plenty of gauges you have used would have been liquid filled and in the heat of the engine bay so I reckon I will just get another one and see how we go. Nothing to lose.
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Re: Erratic Fuel Pressure

Unread postby red5.0fogger » Wed Feb 20, 2019 7:53 am

liquid filled can be a pain since they need to hold the liquid in they can build pressure. If you need to take a hot reading you can burp it. There should be a small rubber plug on the side somewhere. just pry it out slightly with your fingernail. I would suggest taking reading with it either hot or cold and stick with one or the other. burp it from time to time as a reality check. Or get a regular air filled one.
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Re: Erratic Fuel Pressure

Unread postby gamh44 » Thu Feb 21, 2019 2:48 pm

Brilliant, thanks.

Just checked and yep - there's a bung on the case. I'll give it a go on the weekend.
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Vehicle Information: Cobra replica. 302 HO from Mustang. 8LD ECU with chip. Massflo (Pro-M) EFI. Trickflow heads and cam. QH Jaybird and F3v2 chip. TunerPro RT.

Re: Erratic Fuel Pressure

Unread postby gamh44 » Fri Feb 22, 2019 5:55 pm

Well, that was a distraction I could have done without.

red5.0frogger, you nailed it. I started the car to warm and the gauge went down as it has done. Popped the bung and it snapped right back to 40.

Now back to the regular scheduled program ....
gamh44
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Posts: 116
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Vehicle Information: Cobra replica. 302 HO from Mustang. 8LD ECU with chip. Massflo (Pro-M) EFI. Trickflow heads and cam. QH Jaybird and F3v2 chip. TunerPro RT.

Re: Erratic Fuel Pressure

Unread postby decipha » Fri Feb 22, 2019 7:45 pm

ah yea hows that go

garbage in == garbage out
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Posts: 17801
Joined: Mon Jul 15, 2013 5:29 pm
Location: New Orleans, LA
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Vehicle Information: Supercoupin' x10
90 (4x 5spds) - Dante, Ruby, Daja, Ava
91 4r70w - Skarlett
92 (2x) 5spd & auto - Bianqa, Andrea
93 auto - Danika
94 5spd Rionda
95 auto Aisha
Vehicle 2 Information: Others:
00 Lincoln LS - Luanda
98 Camaro SS - Bounquisha
02 Harley F-150 - Sasasha
03 Marauder - DyShyKy
00 Explorer 5L - Bernyce
07 GMC 2500HD 6L - Veranafer


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