I think I've found the culprit to my fuel woes... but I'm not sure I understand... (1 Viewer)

lelandEOD

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So, long and short: my 3FE FJ40 has been giving me grief for some time now. Some of the symptoms I'm experiencing are: loud fuel pump, hesitation and stumbling under load when the engine gets hot on a warm day, and occasional knocking/pinging while driving up grades at highway speed.

So, tonight, I'm playing around with it and as I'm laying under the truck, I'm watching the fuel pass through the glass pre-filter I've got just upstream of the fuel pump. There are bubbles mixing with the fuel and the pump was making it's usual labored whine.

I decided to make sure I didn't inadvertently do something goofy with my connections to the Pollak valve that switches from the front to rear tank so I used a pair of vice grips to pinch off each line to make sure I had the supply and return lines hooked up as I thought.

Here's the interesting part: When I pinch off the return line the oddest thing happens... the bubbles in the glass filter disappear, the fuel pump instantly goes silent, and the engine sounds like it's idling better. I watched the screen in the fuel filter and can even see it flexing or distorting, as if under strain, as I pinch the return line. When I pinch the return line, it relaxes and when I un-pinch the return line, I can see it suck inwards as the fuel pump strains to suck fuel through it.

I took it for a test drive around the neighborhood and it is running much better if I leave the fuel return line pinched. The stumbling/hesitation is gone and although I won't know if this solves the pinging until I drive up a long grade, my suspicion at this point is that the pre-detonation was being caused by fuel starvation affecting the fuel/air ratio.

Now, I'm completely perplexed because I don't know why the fuel system is behaving this way. Everything is new... pre-filters, fuel pump, engine fuel filter, fuel damper, fuel pressure regulator, benched, flowed, and rebuilt fuel injectors, etc.

It would seem to me that there is a problem with the fuel pressure regulator not allowing the pump to build enough pressure but I have absolutely NO idea why I'm getting air bubbles in the fuel supply unless I pinch the return line off.


Please help.
 
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65swb45

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Where does the return line originate? I know virtually nothing about fool infection, but I do know that on the 73-78 carbed cruisers that have the return line at the carb, you can physically SEE the restricter at the head of the return line.

Hth

Mark
 
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Electric pumps don't suck worth a :censor:, the fuel needs to flow into them. Fuel has a pretty low vapor pressure, when sucked, under vacuum, it's even lower and can vaporize, foam, bubble, the warmer it is the worse it is. This is why most EFI pumps are in the tank. If that isn't possible, at minimum needs to have big supply tubing, with as few restrictions as possible, filters, connections, etc.
 
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Your logic is sound. If the pump were bad it wouldn't run any different with the return blocked. It sounds to me lie a faulty pressure regulator.
 
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So, tonight, I'm playing around with it and as I'm laying under the truck, I'm watching the fuel pass through the glass pre-filter I've got just upstream of the fuel pump. There are bubbles mixing with the fuel and the pump was making it's usual labored whine.

You should not be picking up air right out of the tank, so perhaps your fuel pickup hose/sock/system in the tank is defective. Is your return line to the tank far from the pickup line, so that it's not possibly stirring up trouble in the tank? The question to ask is 'why is air EVER getting into the feed stream from your tank'.

I presume you are working with an in-tank pump custom-fitted into an FJ40 fuel tank?
 

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