Juhatee

I thought I knew all the tricks already when it comes to C5 fuel issues but now I'm puzzled:
Last autumn I ran out of fuel when the gauge showed 1/2 tank. I took out the passenger tank hose (the one the fuel flows back to drivers side) and emptied the tank to a can.
I took out the siphon system, tested with compressed air and it worked. The air got through. I disassembled the orifice part, cleaned it and put it all back together. I filled the tanks and let it sat for the winter.
This spring I ran out of fuel again when the tank read 1/4. I filled it up and got home.
I tested by taking the return line to a can, forced the fuel pump(s) to operate until the tank was empty. The passenger tank remained full. The passenger side fuel level showed 1,6V on HPtuners log.

This far it seems ordinary. Just a problem with a siphon system. Perhaps a small particle in the orifice...

I emptied the passenger tank again. I took out the siphon orifice again. I didn't find any debris in there. I cleaned all the parts carefully. I tested the siphon with compressed air and it worked.
I directed the pressure line (the one going to the siphon) to a can and let the fuel flow by pressurizing the pump(s) for half a minute. The fuel in the can seemed clean.
I put everything back together, filled the gas and started driving.
I drove more than 250 miles and got 2 times "check fuel gages". Now the gauge was again showing 1/2 tank. I knew the passenger tank was the issue again and connected HPtuners which showed 1V on drivers side tank and 1,6V on passenger side. So the passenger side was full again and drivers side was almost empty.

I'm puzzled. Is there something else than just siphon line not flowing?
The car has twin fuel pumps and the system has been working for more than ten years now without this issue. The fuel pressure line is original, so the Y-connector which feeds the siphon is original. I have a return line fabricated from fuel pressure regulator in the engine bay to the drivers side tank. This car is 1999 so it didn't have the return line originally. It's also boosted application but then again, it has been working for more than ten years with this setup.

Juhatee

Today I studied this again. I drained the passenger tank. Removed the siphon system and disassembled the check valve from the lines.
the check valve was flowing to both directions, so I got to a local hydraulic store and bought a new one.
I put parts back in place and tested the siphon with compressed air.
I also tested the line from passenger tank to driver side tank with compressed air. It wasn't blocked.

I put enough fuel to the tanks so that passenger tank was showing 1.9V (~85%) and driver side tank was full (2.5V).
I put the return line to a can and started the fuel pump by jumping the relay connection points.
After 9.5 gallons the pump starved for fuel. Passenger tank showed still 1.9V, so it is full.
I'm very confused what to try next.

Edit: Learning something new while I'm on it: The small cylindrical thingy in the line before the orifice is not just a check valve. It must be a "second fuel pressure regulator". I was thinking that what would maintain the fuel pressure in the main line after the pump has been shut down and this line has as much to do with it as the main line with the fel pressure regulator #1.
I wasn't able to disassemble it, I pried the metal open but there seems to be hard glue inside. I ordered another (very pricey) second tank pump assembly just because of this part but I'm not very hopefull the problem will be solved with that.
Edit 2: The T-connector in the main pressure line seems to be before the check valve, so the theory of losing the fuel pressure from line due to passenger tank regulator fail is busted.

Juhatee

Today I received the new passenger tank siphon system. It was'nt plug&play since the mounting plate is with different bolt pattern in these late C5 tanks compared to my '99. I used my original mounting frame and took the siphon system from the new one.
No luck. The fuel does not move from right tank to the left one.
I have pressure in the siphon line, I have free passage in the return line from right to left and the siphon is brand new. Perhaps I should change the car to another brand next.

jcgunn

No one is posting an answer because you have done EVERYTHING that any of us could suggest. It has to be something simple, right?

wrmiles

One thing you didn't list but should have been obvious: perhaps the motive flow line from the tank panel to the jet pump is broken/disconnected.

Juhatee

The line from main line Y-fitting to right side tank siphon line seems to be ok, since I get fuel out of it when pumps are on.
The return line from right side to left I have tested with compressed air and it flows.

Today I gathered T-fittings and I will build pressure gauges in the siphon line and the return line. I suspect I should read about 50-60 psi on the siphon line and close to zero on the return line. If I don't then I'll look into left side tank things.

lionelhutz

The service manual says the return line flow went through the siphon pump, so with the return running to a can you get no transfer.

Try running the drivers sidetank half empty then reconnect the return line. Let the pump run again and it should re-fill the drivers tank.

Juhatee

Thanks. The return line I'm referring to is the return from right side tank to the left side. Not the return line from engine bay to left side tank. The siphon line is an Y-connector from fuel main pressure line to right side tank and the "same" line continues from there through the orifice to the left side tank.

Now I'm really confused: I tested the right side lines. The black line (pressure from main line to the orifice) shows 60 psi, so the orifice gets the fuel pressure. I also tested the return line from right side to left and it shows no pressure.
Then I filled the tanks with 2 gallons of fuel. Disconnected the return line between tanks at left side tank and put the hose in a can.
I put the pumps on and the can filled fast with 1/2 gallons of fuel until the fuel pump didn't get fuel anymore. So 1 1/2" of fuel was now in the right side tank and was left there. But my hose was from the right side tank, so how can I have so much fuel from there and still get the level rising on that side.

Juhatee

One theory came to my mind now: The fuel filter is after the Y-connector in the main line. My measurements show something like 65 psi in siphon line and 55 psi in fuel rail.
So the pressure drops 10 psi at the fuel filter. If the filter flows as good as new, the drop will be smaller, however the pressure at the fuel rail will be the same since the pressure regulator is in the fuel rail.
This leads me to think that the fuel pressure before the filter (and in the siphon) is higher than usual when the filter gets more dirty.
Perhaps the siphon doesn't work as it was meant to work when the pressure is higher than original(?)
I have read earlier that in S/C aplications with more fuel supply the transfer between the tanks wouldn't work properly especially at road course. I have seen this with my car on track days but not on public roads. Perhaps that higher pressure before the filter is the culprit on this too.
I'm thinking of putting an adjustable choke in the siphon line and by choking it more and looking at the fuel level change I could find the proper flow to the siphon.
Any thoughts on this?

Juhatee

New fuel filter arrived today.
Side note: WIX 33529 is a piece of sh*t. It doesn't have the frame attachment and the other end of it doesn't have the quick fitting which you can pinch by fingers. It needs the removal tool and C5/C6 space is limited for those kind of tools.

The fuel pressure dropped just a hair before the filter but it didn't cure the problem. I will test tomorrow with a adjustable choke in the siphon line.

Juhatee

Today I tested this with a choke in the siphon line. I adjusted it from 7 psi with ~7psi steps until it had the full fuel pressure. No luck.
Fuel is flowing from right tank to left one but exactly th

e same amount as goes into the right tank through siphon line. So the end result is dry left tank and full right one.


A pic to help understanding the terms.

oelarse

I think you should check out your right tank jet siphron pump .
If no vacuum is caused be the jet pump in the right tank no fuel will be transfered to the left tank .
The last picture show before and after cleaning

Juhatee

I have cleaned it and eventually changed the whole thing to a new one. So the problem is not the siphon itself.

oelarse

This is the way I understand the system :Fuel from driverside causing a vacuum in the venturi and fuel from the right tank is due to this drawn into the line and mixing with the fuelfeed from the electric pump on lefte side.
Are you able to verify if any vacuum/suction force is caused by the venturi ?

69

Looks like you have done everything in troubleshooting this problem....Just spit balling here but you may want to remove the drivers side fuel pump module and perform a detailed visual inspection on the jet pump fuel line where it exits in the top or lid of the pump Canister or Housing...


Cheers,
Goose

Juhatee

This is exactly the idea this seems to work. I have no clue of how to measure how the venturi sucks the fuel except measuring the output from the fuel line. Now it seems the output is the same as input, the venturi flows through and doesn't take any fuel with it from right side tank.

Juhatee

Thanks. But my driver side fuel pump looks like this:

I have RSI double pump setup there. It has been working fine for ten years, otherwise I would look the issue from the design side.
But you're right, I should take it out and see if everything is in order inside.

Juhatee

More info and findings from today:
I found out that I had only 1 fuel pump working. The other on had broken ground wire. I fixed that and expected this to cure the problem. Although I didn't see the reasoning since with 1 pump the system is like stock. It didn't work after this either.
I filled tanks somewhere 1/2 and started driving. Right side tank was showing 2.29 V in the beginning and after +60 miles it was 2.12 V. So a coffee cup of fuel was transferred to left side. Meanwhile the left tank dropped from 2.04 V to 1.1 V. So something like 3 gallons of fuel was consumed.

Now I did the GM test for this system. I disconnected the fuel transfer line from driver side tank and put a hose from the line to a measuring container. The spec says it should be 332,6 ml in 8 seconds. It was almost a double of that: 575 ml.
I expected the siphon/ejector line to be double also but no: I disconnected passenger tank fuel supply line (the one going into siphon) and let it flow to the measuring container for 8 seconds. It was supposed to be 300 ml but it was 370 ml.
This test is somewhat questionable since when I stop the 8 second test, it will drain some of the fuel from the pressure line to the container so the 300 ml might be just what this system was flowing in exactly 8 seconds.

The test shows that a massive amount of fuel is transferred to left side, at least compared to the fuel feed line to the siphon. But it doesn't do that in real life. The passenger side stays full while the driver side goes empty.
I also blowed air to the transfer line just with my mouth and there was no blockage in that line at the driver side tank.

Juhatee

Finally a possible culprit has been found! I assumed the blue tagged line leaving from the passenger side tank is also labeled blue on the driver side. No it's not. It's labeled green on driver side. The blue one is fuel return from fuel regulator. I have been measuring that wrong line all the time. I wonder how much fuel pressure I 've had when I plugged that line on earlier tests...

Now I measured the green line and there was no flow at all. I could blow through it to passenger tank either. So now I'm trying to figure out what's blocking it.

Edit: Seems like it's not flowing like I expect it to flow. I can get pressurized air through either direction and I can blow with my lungs through it but it feels limited somehow. I order some GM quick fittings for 3/8" pipe and build a new line in between tanks. We'll see how this helps.

Juhatee

696 readers have been anxiously waiting for me to close this thread. I'm close, I can tell you already.
Today I visited hydraulic shop and they made me a 6 AN teflon steel braided line (in fact out of hydraulic parts, so it's not AN). I put it between the tanks (blue one on the passenger side and green one on the driver side).
It's not on it's final routing yet, I just routed it above the transmission and to a can on driver side on tank height.
Fuel is moving now!
I'm waiting for a GM quick fitting to the line so I can start to route the line properly and then this story will end.