fuel pressure dropping over time

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diymirage

diymirage

HP@idle > hondaHP@redline
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been able to take the duster on some short trips around town, and ive noticed something annoying...after a while the fuel pressure drops


starting at the back there is a new (so clean socked) 3/8 fuel sending unit
running through a 3/8 fuel line to a carter M6270 mechanical pump
then into a Malory 4309 return style regulator (using the stock 5/16 line as a return)

when i fire it up, it reads 6-7 PSI on an autometer electronic fuel pressure gauge (looks like a Z-series, but not sure)


after i drive for a few miles and everything gets up to temp, it drops down to 3 psi, at which point it seems to struggle keeping the carb filled (if i let the pedal down to far, that is)

last time i put the car away, it was reading 4 PSI, and removing the gas cap did not register a change, so i think we can rule out a bad tank vent causing a vacuum



any other things to look for?
 
Make sure the fuel lines are run away from heat sources.
 
RustyRatRod said:
Make sure the fuel lines are run away from heat sources.
Click to expand...


its a 408 LA, the fuel lines run in the stock position
the regulator is mounted on the passenger fender and from there feeds into the carb, pretty standard

i would expect the fuel pressure to drop a little with under hood temps rising, but not this much
specially not with a return system and fuel constantly circulating
 
diymirage said:
its a 408 LA, the fuel lines run in the stock position
the regulator is mounted on the passenger fender and from there feeds into the carb, pretty standard

i would expect the fuel pressure to drop a little with under hood temps rising, but not this much
specially not with a return system and fuel constantly circulating
Click to expand...

Well, you have the gauge that's telling you it's losing pressure for "some" reason. Heat is always the first reason I look for, especially if there are "times" when the fuel pressure is good. Have you pulled the dipstick and smelled the oil to see if maybe the fuel pump diaphragm has ruptured?
 
TT5.9mag said:
How’s the fuel filter?
Click to expand...


A little dirty, but not terrible
(Of course it is clean now...I'll have to see is that helped)

20210615_125101.jpg


20210615_130435.jpg



And cleaned



20210615_131252.jpg



RustyRatRod said:
Well, you have the gauge that's telling you it's losing pressure for "some" reason. Heat is always the first reason I look for, especially if there are "times" when the fuel pressure is good. Have you pulled the dipstick and smelled the oil to see if maybe the fuel pump diaphragm has ruptured?
Click to expand...

Oil smells and looks great

Here you can see the way its routed...about as good as it gets, I'd say


20210615_140514.jpg


(pay no mind to that VDO gauge on the fuel log, it is just there to plug the hole)
 
diymirage said:
A little dirty, but not terrible
(Of course it is clean now...I'll have to see is that helped)

View attachment 1715751899

View attachment 1715751900


And cleaned



View attachment 1715751901




Oil smells and looks great

Here you can see the way its routed...about as good as it gets, I'd say


View attachment 1715751902

(pay no mind to that VDO gauge on the fuel log, it is just there to plug the hole)
Click to expand...
The braided lines will do a pretty good job of keeping the heat away. I would suspect the regulator is funky, or the fuel pump is weak. Even if your parts are new, it can happen. I would bypass the regulator temporarily and see what that does for you.
 
Ricks70Duster340 said:
The braided lines will do a pretty good job of keeping the heat away. I would suspect the regulator is funky, or the fuel pump is weak. Even if your parts are new, it can happen. I would bypass the regulator temporarily and see what that does for you.
Click to expand...
Not sure if I can bypass it (at least not easily)
But I bet I could "gut" it and run it as an open T

I'll see if cleaning the filter helped, and if not, I'll try that
 
diymirage said:
A little dirty, but not terrible
(Of course it is clean now...I'll have to see is that helped)

View attachment 1715751899

View attachment 1715751900


And cleaned



View attachment 1715751901




Oil smells and looks great

Here you can see the way its routed...about as good as it gets, I'd say


View attachment 1715751902

(pay no mind to that VDO gauge on the fuel log, it is just there to plug the hole)
Click to expand...
That filter looks like it has enough surface area to support maybe 300hp. Maybe. Might just be your pic but I can’t see a 408 making good power with a filter that size.
 
TT5.9mag said:
That filter looks like it has enough surface area to support maybe 300hp. Maybe. Might just be your pic but I can’t see a 408 making good power with a filter that size.
Click to expand...
Good point

It made 420 and change on the builders dyno

So your thinking is that even when it's clean it will restrict fuel flow?

I can see that, at full bore, but the pressure drop in seeing is during regular traffic
 
It’s hard to tell really but aeromotives page shows pressure drop testing on filters very similar to that one. I’ll see if I can find a link. I had a similar problem. Maybe when clean yours is just adequate and a small restriction causes a big problem.
 
diymirage said:
Good point

It made 420 and change on the builders dyno

So your thinking is that even when it's clean it will restrict fuel flow?

I can see that, at full bore, but the pressure drop in seeing is during regular traffic
Click to expand...
I don't recall you saying if it has ever worked properly. I would bypass or open up the regulator and give it a whirl. My son had a similar problem where his truck with a 360 would fall on its face after getting into it. We found the fuel pump was bad & after replacing it all is good again. When we took the pump out, the pump arm had slop in it.
 
Ricks70Duster340 said:
I don't recall you saying if it has ever worked properly. I would bypass or open up the regulator and give it a whirl. My son had a similar problem where his truck with a 360 would fall on its face after getting into it. We found the fuel pump was bad & after replacing it all is good again. When we took the pump out, the pump arm had slop in it.
Click to expand...

yeah, it works fine for the first 20 minutes or so
then temps rise, fuel pressure drops and issues occur (not saying they are all related, but they do show up together/at the same time

TT5.9mag said:
View attachment 1715751955
View attachment 1715751956 View attachment 1715751957
Hope this is readable.
Click to expand...

was hard to read, but enough info there for me to google it and find a bigger version
it does look like my filter element is about the size of bottom one in the last picture..not very big indeed
 
I have that same pump with a 3/8s line; no regulator, straight into the 750DP.
And no gauge.
so no worries.
since 1999

The first thing I would do is defeat the return line and see what the pump can actually put out.
And the second thing I would do is run a pump volume delivery test.
 
What does your mechanical gauge (plug) on your fuel line do during this decrease in pressure?
 
And another question, will the engine rpm in high gear?
 
TT5.9mag said:
What does your mechanical gauge (plug) on your fuel line do during this decrease in pressure?
Click to expand...


Hard to tell exactly ,with it being under the hood and out of sight

I have noticed it seems to follow the electronic gauge though it reads consistently a few pounds low

I took her for a spin right now and it seemed to do the same thing

so i pulled over and adjust the fuel pressure up a bit

gave her a good blast and it jumped from 7 to 10 PSI

so i pulled over, ran her down to 5 again

after i did that, it seemed to hold at 5 on the gauge during normal driving, but the moment i get on it, the pressure starts to drop and as long as i keep my foot in it, it keeps dropping
so at 2 PSI (and a hair north of speeding) i decided to call it quits for the night and putted home




[UOTE="TT5.9mag, post: 1973498144, member: 46323"]And another question, will the engine rpm in high gear?[/QUOTE]

well, i could wind it up in second, but with the rear gears i have i dont have enough road before i really get in trouble
 
Sounds like you have a regulator that needs rebuilt. Or at least taken apart and cleaned.
 
diymirage said:
And falling back in line when it cools down?

What would cause that (other then heat, of course)
Click to expand...

Open to out of the box? Slap an ice pack wrap around the suspect component. Maybe it can help isolate a heating up issue.
 
TT5.9mag said:
Sounds like you have a regulator that needs rebuilt. Or at least taken apart and cleaned.
Click to expand...

im gonna try that today


Bewy said:
Check that the fuel tank vent is working.
Click to expand...

one of the first things i did was pop the tank cap off while it was low on the gauge
and no difference

IF the tank vent were to be the problem that gauge would have shot right back up, wouldnt it?



Daves69 said:
Open to out of the box? Slap an ice pack wrap around the suspect component. Maybe it can help isolate a heating up issue.
Click to expand...

thats not a bad idea
ive wrapped things in cardboard to help pinpoint an oil leak before, so why not?


ill rebuild/clean/inspect the regulator and if the issue persist ill try the ice pack
 
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