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Weird fluctuation of fuel pressure.

Old 08-09-10, 02:08 AM
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Weird fluctuation of fuel pressure.

I have an issue that is quite annoying and we can't quite put our fingers on there.

This is what I have to begin with:

Bosch 044 pump mounted intank of an FB. The top of the tank has been retro fitted with a second gen top. All the feed and return is -6AN.

The fuel pressure regulator is a Mallory 4309, I know its meant for carbs but I've been using it for 3 years now or so, so I know it works.

The issue that I'm getting is that on long drives pressure starts to drop. I normally have it set at 60psi, but it will normally drop to 40 or so and start to spit and sputter.

I tried another pump, still the same thing, I had a hose going from the pump to the top fitting and thought maybe its loosing pressure through that, so changed it all to straight AN with only fittings and no hoses, still the same stuff.

Checked for kinks and what not and nothing there neither. It doesn't always do it neither which is kinda funny. But it seems to do it more on hot day or during the day time than night time. Like for example I drove it to Alabama for a race there, and it was mostly over night, didn't have an issue till the morning got around. Same thing on the way back, drove it through the day at first and i had to stock every 1/2 hour or so, but night time came around, I'd drive it from full to empty and nothing changes.

could it by any chance the fuel pressure regulator? Could the heat from the engine be weaking the spring on the hotter times, and reducing the pressure? Is that even possible?

Another thing to mention to i guess is that when the pressure drop to 40psi lets say, and I adjust it back up to 60. If I leave it overnight and turn on the car the next day, the pressure wouldn't be 60 it would be 80. I've tried it at various pressure level drops and it has always gone up from what i've adjusted it before. So if it drops to 50 and I add 10 for that day, the next morning its at 70.

Any thoughts would be great.
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Old 08-10-10, 03:15 PM
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Any chance it could be a tank venting issue? Maybe try running with the gas cap loose and see if it still behaves the same way?

Good luck man...





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Old 08-10-10, 06:09 PM
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i had a gsl-se that would do something like that. it was FAST on the way to work in the morning. the trip home in the afternoon though, it would start out ok, but by the time i got off the freeway, it had a hard time making it up the overpass.

the cause turned out to be the aftermarket fuel filter that had rusted internally and clogged itself. it looked like the inside of your old muffler.
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Old 08-10-10, 08:08 PM
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ill take the filter apart to see that, but venting is not the problem
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Old 08-12-10, 07:21 PM
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well took the filter apart with no good luck, it had a couple of small things but nothing serious. I still washed it and placed it back in and didn't change the pressure at all.
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Old 08-12-10, 08:39 PM
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I think that I'm going to order another brand new pump and see what happens. I'm really tired of all fuel issue, that's all I've been battling since the beginning of the year. I went through 3 walbro pumps in one month, then I upgraded to the Bosch ones and I already sent one under warranty, and who know maybe I killed another one in 3 months from the other one I just want this fixed, these pumps aren't cheap, and I'm getting afraid of doing long haul with it just Incase of it failing somewhere, or worse blowing an engine on the track running too lean.
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Old 08-13-10, 01:55 PM
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How is the fuel temp ?

I having issues with a A1000 pump with to hot fuel.
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Old 08-13-10, 03:28 PM
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well I've had it get pretty hot before, but I think its the pump heating it though, after tweaking a few things, I haven't had it heat up anymore. but even then if I fill up 1/2 the tank with fresh fuel, its still at the same pressure pretty much.
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Old 08-13-10, 03:46 PM
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check voltage at the pump when you first start the car and then again when pressure drops
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Old 08-13-10, 04:23 PM
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that's not so easy with the pump intank
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