Puzzling 3FE fuel supply issue - Please Help!

[lelandEOD]

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 played around with it and as I'm laid under the truck with the engine running, I watched the fuel pass through the glass pre-filter I've got just upstream of the fuel pump. There were 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 correctly.

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 and un-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.

 

[LAMBCRUSHER]

check for loose connections in the return line...it could be sucking in air???

 

[klinetime574]

If the glass pre-filter is not needed, remove it. I think this would be okay in a carbureted scenario, but with an EFI system you need to maintain fuel pressure. If you really want to keep it, is the filter rated for fuel injection, and are you using EFI hose clamps?

Just to verify the order: tank --> pre-filter --> fuel pump (?)

 

[lelandEOD]

Yes: tank, pre-filter, fuel pump, stock FJ62 fuel filter, fuel rail, then back to the tank.

I do need the filters because even though I tried to rinse the tanks before use, I'm still getting a lot of sediment and junk particles in the glass pre-filters. I will swap out the double banded pinch clamps in favor of screw-type hose clamps to see if that helps.

Thanks for the replies!

 

[orangefj45]

I would use a larger pre-filter that can handle the volume of fuel you're trying to push thru it. If the filter is too small and you're dealing with contamination issues, it won't be able to pass enough fuel thru it to supply the volume needed.

Georg @ Valley Hybrids

 

[jcj_78FJ40]

I would put a fuel pressure gauge on it. The loaner one I used from autozone had the correct "banjo bolt" to attach to the starter injector. My guess is that your fuel pressure is too low. When you pinch the return you are increasing the fuel pressure by forcing more fuel to the injectors.

What pump are running?

 

[lelandEOD]

Just read through this old thread of mine and realized I left it open-ended...

The problem I was having here was that I was using el-cheapo Autozone pre-filters that were not designed for EFI applications. The seals in the filters were sucking air into the lines which was causing all sorts of problems.

 

[cruiser88]

Cap