Fuel pressure hold

anthony · Post by **anthony** » Sat Apr 21, 2012 2:45 am

While at idle fuel pressure reads 42 psi

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After engine off for 60 seconds pressure drops to 21 psi and stops there.

How long should it hold pressure?

Engine runs and starts right up.

Jeff Dennis · Post by **Jeff Dennis** » Wed Apr 25, 2012 9:12 pm

Anthony,

My understanding of all modern injection systems is they are designed to maintain pressure all the time. Not sure if 21psi is correct but others may know here. I like your FP gauge can I ask where you bought it? Jeff

Blaise · Post by **Blaise** » Thu Apr 26, 2012 11:21 am

I think the M30B35 wants 3 bar (43.5 psi), so you are pretty close. Standard E12 M30's wanted 2.5 bar (36 psi). I imagine that some of that is maintained by the fuel pump, as the pump is not really a sealing valve.

What is your concern?

anthony · Post by **anthony** » Fri Apr 27, 2012 12:33 am

The fuel gauge is an Auto meter that I purchased online but can't remember the site.

About a month ago I replaced my fuel pump because it was starting to work intermittently . However, I did not replace it with a new pump, but one that I had around. I must have gotten it a pick n pull thinking it may come in handy, and it did. This replacement is loosing pressure in minute what took the original a couple of hours.

I was thinking of getting a new pump but wanted to be clear if I was replacing a pump be cause it was a proplem or just didn't trust it. Other than the gauge I would have no other reason to be suspect.

anthony · Post by **anthony** » Thu May 10, 2012 3:07 am

I have been experiencing poor engine performance so much my mpg dipped below 20mpg for the first time. So I add techron and noticed an improvement.

But the surprise is it is now holding fuel pressure for an hour and a half. That is a bigg increase from compared to not holding pressure for one minute.

So I believe injectors can be a cause of lack in fuel pressure holding.

OcCoupe · Post by **OcCoupe** » Thu May 10, 2012 1:47 pm

anthony wrote:I have been experiencing poor engine performance so much my mpg dipped below 20mpg for the first time. So I add techron and noticed an improvement.

But the surprise is it is now holding fuel pressure for an hour and a half. That is a bigg increase from compared to not holding pressure for one minute.

So I believe injectors can be a cause of lack in fuel pressure holding.

A shot in the dark here but is it possible that you have a leaking cold start injector? This could explain the pressure drop and the drop in fuel economy.

Blaise · Post by **Blaise** » Fri May 11, 2012 4:09 pm

OcCoupe wrote:
anthony wrote:I have been experiencing poor engine performance so much my mpg dipped below 20mpg for the first time. So I add techron and noticed an improvement.

But the surprise is it is now holding fuel pressure for an hour and a half. That is a bigg increase from compared to not holding pressure for one minute.

So I believe injectors can be a cause of lack in fuel pressure holding.
A shot in the dark here but is it possible that you have a leaking cold start injector? This could explain the pressure drop and the drop in fuel economy.

I don't think motronic 1.3 uses the cold start valve.

Mike W. · Post by **Mike W.** » Fri May 11, 2012 10:11 pm

anthony wrote:I have been experiencing poor engine performance so much my mpg dipped below 20mpg for the first time. So I add techron and noticed an improvement.

But the surprise is it is now holding fuel pressure for an hour and a half. That is a bigg increase from compared to not holding pressure for one minute.

So I believe injectors can be a cause of lack in fuel pressure holding.

Injectors could be the cause. On my E28, the ones in it when I got it had terrible leak down, more than about 15 minutes of sitting and it would be hard to start due to being flooded. Overnight was actually a little better, kind of like an extra duty cold start valve. I went thru about 3 sets before getting ones that didn't leak. I think my driving pattern at the time contributed to it, but hey, it was how and where I needed to go.

OcCoupe · Post by **OcCoupe** » Sat May 12, 2012 12:13 pm

Blaise wrote:I don't think motronic 1.3 uses the cold start valve.

Sorry, I didn't read your sig, I assumed you were running stock.

alotawatts · Post by **alotawatts** » Sat May 12, 2012 2:06 pm

Mike W. wrote: I went thru about 3 sets before getting ones that didn't leak. :

Were any of the bummers sourced from AutoHausArizona ?
I have some still packaged......

Mike W. · Post by **Mike W.** » Thu May 17, 2012 10:17 pm

alotawatts wrote:
Mike W. wrote: I went thru about 3 sets before getting ones that didn't leak. :
Were any of the bummers sourced from AutoHausArizona ?
I have some still packaged......

Nope, all were used. And all were better when I installed them, but started leaking before long. Until the last set which were -714 instead of -203. I don't know if the later ones were better or if I just got a good set.