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Kjet fuel return volume

302ftw?

302ftw?

Well-known member
Joined
May 7, 2010
Location
Goshen, NY
Hi all, checking my work here. Is this the correct orientation for the return hose? The other banjo is coming from the pressure valve on the side of the fuel distributor. I ask because in 30 sec I only got about 500ml or less than half expected volume according to K-jet.biz. if this is correct I am honestly surprised the car even ran.

kLxTj1U.jpeg
 
Thanks @PCH I knew that the line I had connected to was the return, was just asking if having the frequency valve line attached to it affected flow at all. My guess is no.
 
So after changing my fuel filter and witnessing the return fuel flow, I am seeing a constant string of bubbles. Even after 3+ liters letting the fuel pumps run it seems that the bubbles are not abating. I haven't even tested pressures yet but I have a feeling that this is part of my problem. @dl242gt @hiperfauto is this indicative of a fuel pump or accumulator failure in your experience? I actually have a replacement for all of the above including in tank and external as I knew this day would come eventually.

I have verified both fuel pumps are running testing them separately with a momentary switch.
 
Assuming you still have an external pump, I'd check the little rubber hose inside the tank. Especially with a partially-filled tank, fuel starts its journey from a wet bath, then travels through air at the top of the tank. A crack in that hose can allow air to get sucked into the line almost forever.
 
Excellent. Thanks @Toybox and @hiperfauto. I had a hunch that the replacement pump I installed a year ago was just crap quality and I ordered the ipd 740 upgrade kit, which now is in my possession. Going to install tonight!
 
Update: installed new in-tank pump, definitely a secure connection between pump and pickup tube... What else could it be?
 
Update 2: pulled the feed line to the external pump and ran the intank- definitely passed the 1l in 30 sec test and 0 bubbles. I reached out to Kjet.biz and they suggested the external was cavitating and to change it. Installing a new accumulator and external pump and back flushing all the feed lines. After pulling the high pressure line from the accumulator there were chunks of stuff that came out of the line. Probably a combination of dying external pump and crap from 40 years of gas. Testing tonight - fingers crossed!
 
Welp, I'm stumped. New accumulator and external pump. Back flushed all the lines. Fixed the crumbling wires feeding the external pump. STILL HAS F$&@#$& BUBBLES. The last thing I have not changed is the feed line to the external pump. It is old and I did find a crack in it. Perhaps it is pulling in air? If that doesn't fix it the next step is to light the whole thing on fire and get a BMW.
 
Excellent choice. A BMW will never give you any problems with rotten rubber hoses pulling in air. :-P

Kidding aside... yes. Any time you have a rubber hose that is before a pump, a crack will allow the pump to suck air. A crack after the pump will allow it to spray fuel. (Which performance it gives you just depends on pressure relationships to atmospheric.)
 
Toybox said:
Excellent choice. A BMW will never give you any problems with rotten rubber hoses pulling in air. :-P

Kidding aside... yes. Any time you have a rubber hose that is before a pump, a crack will allow the pump to suck air. A crack after the pump will allow it to spray fuel. (Which performance it gives you just depends on pressure relationships to atmospheric.)
Click to expand...
Clearly the words of a madman. I ordered 12mm hose from the local BMW dealer originally for an e21 320i.
 
Toybox said:
Any time you have a rubber hose that is before a pump, a crack will allow the pump to suck air. A crack after the pump will allow it to spray fuel.
Click to expand...

All true. The system in question, though, should not have any hoses that are under suction. The pre-pump picks up fuel directly from immersion in it. The large hose running to the main pump should be under pressure. There is no reason for that pump to cavitate if it has adequate supply, which, by your test, it does.

Sorry, I know that is no help, but there is this: I don't remember seeing any indication that the system pressure and control pressure have been checked during this episode. Did I miss it?
 
What's the pressure that the in-tank "lift pump" delivers all by itself? I thought it wasn't much, hence why you need the external pump.
 
Harry Tuttle said:
All true. The system in question, though, should not have any hoses that are under suction. The pre-pump picks up fuel directly from immersion in it. The large hose running to the main pump should be under pressure. There is no reason for that pump to cavitate if it has adequate supply, which, by your test, it does.

Sorry, I know that is no help, but there is this: I don't remember seeing any indication that the system pressure and control pressure have been checked during this episode. Did I miss it?
Click to expand...
No pressure testing - I was advised to get this right first before testing. I have the right rig to do it, but if volume isnt correct there is no point
 
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Question for the kjet sages: in Feb I pulled the fuel head off and cleaned the main fuel valve because it was sticking a bit. There is an oring between the fuel metering head and the air flow meter. Upon disassembly that original oring crumbled and replaced with a generic replacement. Is there a chance that air could be pulled in from a leak there? @hiperfauto @dl242gt @PCH @Harry Tuttle
 
302ftw? said:
Question for the kjet sages: in Feb I pulled the fuel head off and cleaned the main fuel valve because it was sticking a bit. There is an oring between the fuel metering head and the air flow meter. Upon disassembly that original oring crumbled and replaced with a generic replacement. Is there a chance that air could be pulled in from a leak there? @hiperfauto @dl242gt @PCH @Harry Tuttle
Click to expand...
A crumbled oring could make for an air leak. If the new o ring is generic and not a fuel resistant o ring it might be damaged by the fuel residue that builds up in the air flow box. That spot isn't generally an air leak though.
 
I am not sure about that o-ring allowing air infiltration. Having a hard time seeing fuel on either side of it, especially with suction. I would, however, recommend a good one. I like Viton for o-rings that are around solvents or gas.

There is in my opinion never a bad time to check Kjet pressures. When the fuel system is suspect, it is useful info. The engine doesn't have to be running, even (edit- if you have a low-flow situation it would be good to compare pressures when running and not). If you have the rig, I can't see any excuse for not putting it on there. Just leave it on until the issue is resolved. I sometimes leave the gauge on for weeks, while driving around.
 
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