Fuel pump/pressure question

97MarkJA

97MarkJA

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This past summer I replace my fuel pump with the Walbro 255, I drove out to Denver in October and a rough idle and die twice. I was thinking bad fuel or the high alt was treating the car very well but now I wonder if I have a bad pump... relay... something.
The car is up north for storage, I've been working on it when I can get up there. I just finished up my cobra intake swap, I have cobra rails with an automotive fpr. I set it at 41 pounds, ran perfect. A few hours later I started it up and it was at 26 pounds, now this weekend right before I left I pulled it back into the garage and it was at 50 pounds.
I know these pumps are variable voltage which will vary the pressure but it shouldn't be down at 26 pounds.
Could the pump be going bad? Or the pcm?
On a side note.... if you run a fpr with a return system what is the need for variable voltage?
 
Are you checking the fuel pressure with the vacuum disconnected?

Regardless of the fuel pump voltage, the pressure should be the same, after all that's what a fuel pressure regulator is for.
 
I do have a FPCM on the way, hunting for a VLCM. I'm not one to throw parts at the car but its 250 miles away in northern MN.
Any suggestions on what the problem might be or something else I could check would be great.
 
Does the cars PCM know what size pump you are running? I had small issues before I corrected the tune. I used to see 33-36 under vac and when removed would see over 45psi. Should be 30ish with vac and 40 with no vac. Could have a FPR issue.

You dont need 50psi unless you have very large injectors and pump and are running boost.
 
LaserSVT said:
Does the cars PCM know what size pump you are running? I had small issues before I corrected the tune. I used to see 33-36 under vac and when removed would see over 45psi. Should be 30ish with vac and 40 with no vac. Could have a FPR issue.

You dont need 50psi unless you have very large injectors and pump and are running boost.
Click to expand...

I still need to change the intake volume in the tune, Does anyone know where the fuel pump size or volume is in the SCT software?
 
Static leakdown test should be less than 5 psi per minute after shutdown.

I had a bad fuel pump which caused an almost immediate pressure drop to zero - running pressure was fine though.

Sometimes the fuel pressure regulator may be the cause of erratic loss of pressure.

Have you checked for the presence of fuel at the vacuum connection of the FPR? That would indicate a compromised diaphragm. Depending on vacuum conditions it may not leak all the time, only at a certain regulator bypass rate.
 
I haven't found any fuel at the vacuum line, I checked it several times to see if I maybe got a bad regulator. I also checked the regulator and gauge on a variable rate MG fuel pump w/gauge to make sure it was accurate after I had to adjust it 3 times.
I'm guessing by static leak down you mean without the return line blocked off. I've only done one other leak down test and it was on a nissan, on that one I had to block off the return line.
 
Yes, the leak down test simply checks how long the fuel system remains pressurized after shutdown. On the return type system such as ours, it is simply testing the fuel system integrity from the pump to the pressure regulator. The pump discharge check valve and fuel pressure regulator seat are common causes of excessive leak down and will demonstrate intermittent pressure fluctuations during normal operation.

I personally think that a leak down rate of 5 psi per minute is excessive, but that's what is allowed per the manual.
 
This may be a dumb question but is the check valve built into he fuel pump? I don't remember seeing a check valve unless it was built into the hat for through basket.
 
97MarkJA said:
This may be a dumb question but is the check valve built into he fuel pump? I don't remember seeing a check valve unless it was built into the hat for through basket.
Click to expand...

I think the check valve is internal on the fuel pump?
 
That's the same pump I have just with the accessories. If you look at the application guide it only works for cords up to 97 ( fixed voltage output ) not Ford/Lincoln with FPDM. I didn't have a problem with mine until I drove straight through to Denver.... and hadn't for sometime after that until I drove up to Fargo. Seemed to start having issues at about the 2 hour mark.
 
Don't know what to say.
Works for me, Im just a lucky bastage I guess.

It would suck to pull the tank, install a SVT focus pump and have the same problem.
But if I was to do it again I would get the SVT focus pump because its a easy install.
"no hacking of the basket"
 
It worked great for me too..... for awhile. I'll be dropping the tank as soon as I can get back up north, if the snow would ever stop.
 
I am still uber confused. I had a Walbro 255 in my 97 and pulled it out and dropped it in my 98. Running a blower for 10k miles now and now fuel issues. Wonder what I really have in there then cause it fit both cars the same and by the numbers on the dyno it is 240-255lph due to duty cycle at the cars applied HP level.
 
LaserSVT said:
I am still uber confused. I had a Walbro 255 in my 97 and pulled it out and dropped it in my 98. Running a blower for 10k miles now and now fuel issues. Wonder what I really have in there then cause it fit both cars the same and by the numbers on the dyno it is 240-155lph.
Click to expand...

Wasn't the gas tank on your 98 physically different from the 97? This thread has me wondering as well.
 
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