fuel pressure drop

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diymirage

HP@idle > hondaHP@redline
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71 duster with a MRL build 408 dropped in

a while ago I took her to the dragstrip for the first time and noticed she was running out of breath so I decided to upgrade the fuel pump

I installed a carter M6270 mechanical fuel pump , a filter a pressure regulator and a gauge
(in that order)

I had the regulator set at 6 PSI but noticed that after a while it dropped to 3
I decided to clean the filter because I had "back bled" the tank vent line and i thought i might have some crud in there from doing that
it turned out I had a LOT of crud in there, so it looks like the filter did its job

then i realized i was being an idiot for installing the filter AFTER the regulator because probably had some crud in it aswell (which it did)

so, took the whole thing apart, cleaned it, and flipped the bracket around so i could put the filter between the pump and the regulator

hooked it all back up and fired it up...6PSI
let it idle for a while and it stayed at 6
so i went for a drive and when i got back the pressure was down to 3
i tried adjusting the regulator but could not got a different reading

i popped the tank cap off but didn't hear a "woosh" and no change to the gauge

then i jacked up the rear end to look at this flexhose i hear people talk about collapsing

what do you guys think, could this be my problem ?






and if that looks like it needs to be replaced with a slightly shorter piece, how do i remove it withouth getting showered with gas?

or is there something else that might cause this issue ?

i let the car cool down for about half an hour and when i fired it up, it read 6 PSI again
 
Is this a dampened gauge (fluid filled) and where is it installed? If it is in the engine compartment, your pressure is still 6 psi the temperature will cause the gauge to read low. I have a fluid filled gauge with a compensating valve, it will read low after warmed up, I open the compensating valve and it comes back up to my set pressure. I believe even the non fluid filled gauges are effected by heat under the hood.
 
It is a non fluid filled Guage, mounted on the fuel rail, where it tees of into two lines for the two bowls
 
Over the years we have all had our gas showers, and now it's your turn. :D

I could tell you how to avoid it, but you wouldn't learn it as well. LOL
 
Move your gauge outside from under the hood for best reading. The best can affect it as mentioned. Even if you move it just for diagnostic purposes.

If you do end up replacing that chunk. Which I would do anyways. Tie a small rag around each wrist/forearm area. That way any gas you get on your hands won't run immediately to your pits. Old mechanics trick.
 
Thanks

That's not a special 37 dollar preformed piece is it?
Just regulations 5/16 fuel level?

It almost looks like it is a litle too long, and that's forcing it to pinch

I guess one could run the gauge out from under the hood, for a test run...but I hate the idea of installing a rubber fuel line that long, and having the gauge flop around
 
Yep, it does look like its too long and needs to bend too much. Just regular fuel line will do it.
 
I guess one could run the gauge out from under the hood, for a test run...but I hate the idea of installing a rubber fuel line that long, and having the gauge flop around

Well I don't mean drive around with it flopping about. I'm thinking more just sitting in the driveway and watch the gauge. Although if you really wanted you could always rig up a hoseclamp around the fresh air vents, run the line under the hood. As a temporary way to diagnose. I wouldn't go more than around the block though.
 
I'm not sure 8f that would warm it up enough to affect it though

Maybe I'll get her hot tomorrow and just rub an ice cube around her to see if that rises
 
Thanks

That's not a special 37 dollar preformed piece is it?
Just regulations 5/16 fuel level?

It almost looks like it is a litle too long, and that's forcing it to pinch

I guess one could run the gauge out from under the hood, for a test run...but I hate the idea of installing a rubber fuel line that long, and having the gauge flop around

If you are running 408 then why do you have 5/16 fuel line? You need 3/8 from tank to carb.
 
the whole car is reverse engineered, meaning stuff broke and got replaced with performance parts before i intended too

my plan was to build the car from the back forward
8 3/4 rear end
then build the transmission
then eventually build the 408

i ended up spinning a rod bearing a few months after i dropped in the 360, and was "forced" to build the 408 well ahead of plan

that's why i never upgraded the fuel line
 
That line will be a problem at the big end, but it should not be a problem under your described conditions.
For the gauge to read low, assuming there is nothing wrong with it. The fuel either has to be going somewhere, or the line does not have pure fuel in it.
If the engine idles, obviously the fuel is not running into the intake. So that just leaves the return line.
Bourdon tube mechanical pressure gauges are heat sensitive, for sure.

It may also be that the pump only puts out 3 psi at idle. Simply reving it up should then show the pressure jump up. You may have to block the regulator closed to see it. I have been running that pump since 1999,without a regulator, and it causes me no grief whatsoever.
 
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No I don't think that. Actually I would suspect stuck open first. And plugging the outlet or pinching it off could prove that, when the revs go up.

But like the guys said before me , that style of gauge is temperature sensitive, and I have seen a few guys get fooled by them.
That's one of the reasons I have no aftermarket gauges on my car, except a tach, and I only got that, several years after the build was complete.If I don't have gauges telling me bad stuff than I don't have any worries.If I don't have any worries than I can be having fun. I like having fun....... Most of the time I don't even watch the tach. I mean it doesn't tell me anything I don't already know.....It's just there to impress a passenger, who has never seen a Mopar go into the 7s......
 
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That line will be a problem at the big end, but it should not be a problem under your described conditions.

Like this?

a while ago I took her to the dragstrip for the first time and noticed she was running out of breath so I decided to upgrade the fuel pump


The fuel line wont be an issue unless you are using more than 3/4 throttle. BTW steel line is less than $20 for enough to do an A body, I just got a roll the other day for our 67 Vert to replace the 5/16" since we installed an injected Magnum which doesnt need it now but it will get a built engine this fall after we have some fun with it. I would start with the kinked looking rubber line first and put a filter back by tank until you can afford a new tank sans grit, rust and crud. Just my $.02....
 
where would i get a roll ?
i asked a buddy of mine who works at advanced auto, but they didn't carry it
 
Don't forget to upsize the sender hardline too. I installed a big EFI filter at the back,just in front of the spring perch. There is kindof a pocket there.The filter wasn't cheap, but it is still in there 16 years later;so still less than a buck a year,lol. I have no filter on the pressure side,nice clean install under the hood. EFI jumper hoses.I stuffed the hard line into the front stub frame, and brought it out through a hole about 6 inches behind the pump. Then I extended it to about 1.5 inches from the pump. Just far enough away that engine movement won't tear the EFI jumper. The motor is chained down, but it does move around a bit.
 
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Don't forget to upsize the sender hardline too. I installed a big EFI filter at the back,just in front of the spring perch. There is kindof a pocket there.The filter wasn't cheap, but it is still in there 16 years later;so still less than a buck a year,lol. I have no filter on the pressure side,nice clean install under the hood. EFI jumper hoses.I stuffed the hard line into the front stub frame, and brought it out through a hole about 6 inches behind the pump. Then I extended it to about 1.5 inches from the pump. Just far enough away that engine movement won't tear the EFI jumper. The motor is chained down, but it does move around a bit.

I have absolutely no idea what that means....and I am pretty sure OP's is carbed.
 
EFI hose is thicker, more robust and somewhat harder to kink. The EFI filter I chose had a finer filter medium and a huge capacity. It has a hose-barb on the inlet side, and a screw-in fitting on the outlet. I ran steel from the pump to the dualfeed, where I installed another short EFI jumper. If I have to run screw-clamps, I run two per connection, with the screw-heads rotated 180* from eachother and then not over-tightened.
It's my understanding that you can now buy senders with 3/8 fuel-pipe. In 99 I could not find one and so,built my own. The 3/8 socks were still available at the Chrysler dealer,so I bought a spare.
 
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ok, dumb question here
I'm getting an order together for some parts and I was going to add some 3/8th steel fuel line

Amazon.com: OE Zinc Automotive Steel Brake Fuel Transmission Line Tubing 3/8" OD Coil Roll: Automotive

however, when I look at rubber fuel line, it is listed as 3/8 ID
and the steel one is listed as OD

so, just to be clear
the stock steel line is 5/16 OD which needs to be replaced with 3/8th OD so that the rubber 3/8th ID line will fit over it?

( I think I just needed to think this out, it makes sense now)
 
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