Bypass charcoal canister? Return Lacking Fuel System information

[xtrattitude4u]

Where IS the charcoal canister on your car? Though we have a similar model, I think we may have a different emissions system, which is why I was hesitant to comment on any of your posts before.

The charcoal canister is right in front of the driver side front wheel, same as polskamafia mjl's car. You can see it through the front bumper, since I have no fog lights.

One of the vacuum lines visible to the left of the canister runs up to the intake. The other runs across to the passenger side, then turns a corner and goes under the car back to the FPR just forward of the passenger rear wheel.

On a related incident: today my car started to lose power and die. I was close to my destination (thankfully) and pulled into a parking spot sputtering like running on 3 cylinders.

It died and went into winter mode, wouldn't switch out. CEL came on. I tried to start it, it sputtered and died again. I am now thinking my transmission is toast.

I reluctantly started checking plug wire connections, knowing that I will not find anything since I secured everything properly. I then inspect the canister lines since this was the last thing I messed with. I'm now thinking: "what have I done to my car?"

I plug the vacuum line back into the canister the way it was before. Now it runs fine! But why would this matter? The charcoal canister is clogged, what would that change? I drive off very confused.

After pulling into Schuck's and discovering their Scantool was stolen, I opened the hood again and discovered this:

The vacuum line that was attached to the purge valve had a huge rip in it. As soon as the valve opened, it let air into the intake.
I took the hose in and the only thing they had that size was rubber fuel line. Perfect! That hose is used to purge fuel vapors anyway. It smelled of gas and fuel line should be great.

[whoa]

The FPR just forward of the right rear wheel? Can we have a picture of that? You sure it's not the rollover valve?

[xtrattitude4u]

The FPR just forward of the right rear wheel? Can we have a picture of that? You sure it's not the rollover valve?

You can see that there is a return fuel line attached to the FPR, and only one supply fuel line all the way to the front from here. If fuel pressure was to be regulated, it would have to be here. The vacuum line is attached to a "T". One side goes to the Charcoal canister, the other goes to the gas tank.

Just to prove it, here's a pic of the cover just barely visible in the bottom corner of the previous picture. You can see the driver rear wheel across from the point the picture was taken.

[whoa]

I can't really see what's going on in that picture; you don't really show the valve itself but only hoses.

Right at that spot is where the roll-over valve goes, on a fuel vent line between the tank and the charcoal cannister. The vacuum line to the FPR (which is supposed to be near the steering gear on your car), is not supposed to "go through" the cannister.

If there is indeed a FPR in your picture, it might be that someone at some point put in a new one in a more accessible position. Be careful tracing hoses, though; there should indeed be a hose from that area to the cannister, but it should come from the roll-over valve and not a FPR.

[xtrattitude4u]

The FPR is the copper colored object just low and left of the words "vacuum line" in the first picture.

Here is a little better view. It's difficult to get a camera under there with proper lighting, forgive me for the crappy pictures.

[xtrattitude4u]

I looked above the steering gear and there was nothing in line with the fuel line anywhere near that area. I still know of only one continuous fuel line from the device in the picture to the fuel rail.

[whoa]

Well, you're right; that's the "three way unit with pressure regulator". I was looking at info for the turbo model. Sorry!

The Motronic 4.4 changes to the NA included a new "tailored return lacking fuel system". Here is the description:

Last year the Return Lacking Fuel System
(RLFS) was introduced on the B5254S with
Motronic 4.4. The fuel pressure regulator is
connected so that fuel pressure is retained at a
constant level (300 kPa (43.5 psi)) above the
pressure in the EVAP line to the fuel tank (≈
atmospheric pressure).

This gives an accurate enough control with
today's fast processors in the engine control
module (ECM) of a naturally aspirated engine.
On a turbocharged engine on the other hand
with larger pressure variations the intake
manifold, this does not provide enough
accuracy.

The advantage with not having a fuel return
line from the fuel rail is that the engine does
not heat the fuel, the temperature in the fuel
tank remains low and the amount of
evaporated fuel is reduced.

”Three way unit”
The fuel flow is controlled as before via a three
way unit which is integrated with the fuel
pressure regulator. The following are
connected to the three way unit:
The fuel line from the fuel tank.
The fuel line to the fuel rail.
The return line to the fuel tank.


In short, you're right not only that that is the FPR, but also that there is a hose connecting it to the canister (it tees into the fuel-tank-to-canister vent hose). That's not, however, a vacuum hose; that is, it's not a hose meant to bring manifold vacuum to the FPR. It's simply the tank vent hose, which is close to atmospheric pressure and is used as a reference to base the fuel pressure on.

So if you connected the vent hose directly to manifold vacuum, then I guess you lowered your fuel pressure by exactly the amount of your manifold vacuum. That would mean that you were running a leaner-than-designed mixture (depending on whether everything else, including your injectors, is in good shape).

Have you checked whether any EVAP (or fuel trim, for that matter) codes are stored?

[jblackburn]

Last year the Return Lacking Fuel System
(RLFS) was introduced on the B5254S with
Motronic 4.4.

Ah ha! That explains why our cars are different Mine's got the regular 850 setup with 2 fuel lines up and back.

[xtrattitude4u]

No check engine light is on, other than that, I have not had it scanned.

So if you connected the vent hose directly to manifold vacuum, then I guess you lowered your fuel pressure by exactly the amount of your manifold vacuum. That would mean that you were running a leaner-than-designed mixture (depending on whether everything else, including your injectors, is in good shape).

Have you checked whether any EVAP (or fuel trim, for that matter) codes are stored?

So I guess I'm a little confused about the operation of the charcoal canister, the FPR, and their relationship.
Here is what I considered to be the tank "vent" hose.
The two circled tubes are the two vacuum connections I was referring to. One of them (I believe the larger one) connects to the manifold. (also inline is the small charcoal filter and the purge valve) The other runs back to the fpr on the passenger side. The large hose that's visible is the vent hose (I think) and I have never disconnected it. To clarify, this large hose runs back along the DRIVER side to the tank, whereas the small line to the FPR runs along the PASSENGER side.

So if I'm understanding this correct, even though the vacuum line for the intake connects to the canister, vacuum is only drawn from the VENT hose side, not the connection that leads to the fpr?
The vacuum line connects to the Canister, so air has to be pulled from somewhere. If I may think out loud:
The vacuum line that connects to the Canister from the intake has the purge valve. So the vacuum is only drawn when the valve opens, and only from the tank vent hose to purge vapors. In that case, what is the other, smaller line for?

I'm really disappointed. I thought I had solved a problem that got me great gas mileage. If it turns out I cheated and my car's not supposed to run like that, I'm gonna be quite unhappy.

[whoa]

The smaller line you talk about is the tank vent hose. It runs through a roll-over valve near the fuel filter, then past the FPR (where there is a tee connection to the FPR on your car), and then to the canister. On a warm day, the fumes in the tank expand and get pushed up that line into the canister, where the fuel is absorbed by the charcoal and the cleaned air is vented to the atmosphere (through that fat line, probably). When the purge valve opens, it sucks the fuel out of the charcoal, and the atmospheric vent replaces the lost volume with air.

If this really did improve your mileage, that might be a sign that you had been running rich, maybe because of leaky injectors, bad front O2 sensor, bad MAF, . . . maybe even that vacuum leak that eventually got so bad it hobbled your car (you'd think that would cause a lean condition, with unmetered air getting in, but who knows how the ECU compensates).