There are two lines running parallel between the fuel tank and the engine' a supply line and a return line. I am trying to figure out which is the fuel supply line on my 2002 Cadillac Deville? Thanks! LaSandra

Only two are fuel lines, the main feed to the engine and the return line from the engine to the tank. One other gas tank line will be the EVAP line for sending gas tank fumes from the charcoal canister to the engine's intake manifold.
I think the other lines would be your brake lines to the rear wheels.

YouTube has many videos on repairing rusted fuel lines. Most people use the lines made of plastic in their repairs-takes special fittings though. These plastic lines have become very common now. In fact my '92 LeSabre has mostly plastic lines.

Can you identify or remember which pipe on the sending unit, the pump module, is the main outlet from the pump?
The three lines are the main feed or pressure line to the engine, the return line from the engine to the tank-this line is often smaller than the main line. And the third line is not a fuel line, but the vent line for fumes from the enclosed tank to the charcoal canister. This line may be identifiable by pipe diameter being smaller, or sometimes using rubber hoses with it, or maybe just a little offstanding from the other two lines. The main line (pressure line) will be the line that has the fuel filter in it. You can identify the main line this way, then just figure out where it connects onto the tank, and sort out the other two lines-the return line and the (?smaller?) vent line.

Well, you need a quick tutorial on your fuel system. Now to find a leak, stop trying to start the engine. You are only adding electrical charge to an already dangerous situation. Now stop and think this through. Modern fuel injection requires fuel under high pressure to operate. The injectors are immersed in this pressurized gas, as are the lines leading to them from the fuel tank's fuel supply line. So just by turning the key to ON, not start, you have initiated the process to start the engine, and that means the pump has operated for 2 seconds to put the fuel system under high pressure.

Now if you follow the fuel supply line (from the gas tank) to the engine connection, start here to find a leak. It is now under high pressure from here to the injectors on the engine- one for each cylinder-it should be dripping with gas somewhere. You could even turn the key to off after the pump has ran for 2 seconds. The fuel system is designed to hold pressure in the system for about 10 to 15 minutes, before slowly bleeding off. Enough time to search for a gas leak. A couple of other things associated with the system: there will be also a fuel return line from the injector rail, it goes back into the gas tank. both the supply line and the return line often run alongside each other. There is also a fuel pressure regulator, immersed in the fuel rail, a seal on that may have started leaking. If an air breather housing, or some cover prevents your seeing the fuel system entirely, then set it aside. You should be able to find the leak. Just be careful, have lots of rags handy, happy hands are learning hands.

id have to look that up in the FSM.
or if luckly , why not just go to car maker dot com.
and just look. its there. i tried, and they had no drawing for LINES. KIA bull.
Kia?

or get the FSM or try alldata.com
this a loop system.?
there are 4 lines. there.
2 to engine,?
2 to tank.
the 2 lines up from , one is return and one is supply.
it should be easy to tell which is which.
the one tied directly to the rail is Pressure supply
other is return.
at the tanks 2 fittings.

add a rubberhose two both.
run it to a bucket.
key on (or crank)
the one that shoots fuel is supply.
and now you know the color codes.

YOU HAVE FUEL SUPPLY LINE AND RETURN LINE,VAPOR LINE EXTENDS FROM FUEL TANK TO THE ENGINE COMPARTMENT.

You may further cause problems. I would recommend disconnecting the fuel line at the injectors, and disconnecting the fuel line before the gas tank, making sure there isn't a fuel filter along the way. If there is, then disconnect the fuel line at the filter. Use a compressor and blow air from the injector side, down to the filter end. Make sure you try and keep as much moisture out of the line as possible. You can do this with each segment of fuel line to flush out any particles that may have clogged your lines. Your filter may be your main problem. Try changing that first!

well i dont have a diagram but the run from the fuel tank along the drivers side frame to a frame mounted fuel pump then up to the engine. you can figure out what one is the return line by the fuel pump. the one that does not go to it is the sending not the return.

This line has to be hooked up for gas to go back to the tank. Gas gets sucked out of the tank then through the engine and then the excess has to return to the tank. This keeps air bubbles out of the system after it runs dry.

Sounds as though it may be your fuel pressure regulator. Easy fix and you can troubleshoot it easily also. Check the fuel pressure first. This is your part....


Fig. 13: Compression gauge and a combination vacuum/fuel pressure test gauge


TESTING Except Premier and Monaco NOTE: To perform this test, you will need a pressure gauge capable of reading pressures above 55 psi (379 kPa). The gauge must have a connection that will fit the fuel rail service valve. The gauge must be the equivalent of Chrysler pressure gauge No. C-4799 and the connector fitting compatible with pressure test adapter 6539. You may also need a T and the fittings necessary to connect the gauge into the fuel supply line at the tank, as well as a 2 gallon container suitable for collecting fuel.

1. Release the fuel system pressure as described in Section 1.
2. Disconnect and plug the vacuum line going to the fuel pressure regulator.
3. Remove fuel hose quick connector from the chassis line.
4. Install the adapter between the fuel supply hose and chassis fuel line assembly for the 3.0L engines or into the service port of the fuel rail for 3.3L and 3.8L engines.
5. Hold the gauge and have someone start the engine. Run the engine at idle speed in Neutral (manual transaxles) or Park (automatic transaxles). CAUTION
   Exercise extreme caution while under the hood with the engine running. Keep clear of all moving belts, fans, exhaust manifolds, etc.
   
6. Read the pressure. It should be 48 psi (330 kPa). If it is outside the range, take note of it.
7. Stop the engine.
8. Once again depressurize the system, disconnect the gauge and replace the protective cover.
9. If the pressure was correct, the test is complete. If the pressure is below the range, proceed with the next step; if it is too high, skip to Step 13. WARNING
   In the next step, note that fuel may drain from the lines as you disconnect them. Make sure all surrounding exhaust system parts are cool and that all sources of ignition are removed from the area. Collect fuel and dispose of it safely.
   
10. Connect the gauge into the fuel supply line running between the tank and the filter which is located at the rear of the vehicle. WARNING
    Make sure all connections are secure.
    
11. Have an assistant start the engine, then read the pressure gauge. If the pressure has risen more than 5 psi (35 kPa), replace the filter. If the pressure is now within range, allow the engine to cool and remove all sources of ignition; depressurize the system; disconnect the gauge from the lines; replace the fuel filter; and restore all connections.
12. If the pressure is still too low, gently and gradually pinch the fuel return line closed as you watch the gauge. If the pressure increases, the fuel pressure regulator is at fault. If there is no change, the problem is either clogging of the filter sock mounted on the pump or a defective pump itself.
13. If the pressure is too high, follow this procedure:
    1. Shut off the engine and allow it to cool.
    2. Depressurize the fuel system, then disconnect the fuel return hose at the chassis, near the fuel tank.
    3. Connect a 3 ft. (91cm) length of hose to the open end of the line running along the chassis. Position the open end of the line into a container suitable for collecting fuel.
    4. Have a helper start the engine, then check the pressure again. If it is now correct, check the in-tank fuel return hose for kinking.
    5. If the in-tank fuel return hose is okay, and the system still exhibits excessive pressure with the tank half full or more, the fuel pump reservoir check valve or aspirator jet may be obstructed and the assembly must be replaced.
14. If the pressure is still too high, perform the following:
    1. Shut the engine OFF, and allow it to cool.
    2. Depressurize the system once again and reconnect the fuel lines at the rear.
    3. Disconnect the fuel return hose at the pressure regulator. Collect all fuel that drains.
    4. Run the open connection into a large metal container.
    5. Reconnect the fuel gauge to the fuel rail.
    6. Start the engine and repeat the test. If the fuel pressure is now correct, clean a clogged return line or replace pinched or kinked sections of the return line. If no such problems exist, replace the fuel pressure regulator.

Premier and Monaco CAUTION
Wear eye protection when servicing the fuel system. Do not smoke or allow open flame near fuel system components during fuel system service.

1. Remove the fuel tank filler cap to relieve the fuel tank pressure. NOTE: System fuel pressure will be released when disconnecting the fuel supply tube. Have shop towels handy to absorb spilled fuel.
   
2. Use the quick disconnect tool 6182, or equivalent, to remove the black fuel supply tube from the fuel rail. Slide the tool over the nipple and up into the connector until the handle fits the connector. Pull the fuel supply tube off the fuel rail.
3. Install the fuel tube pressure adapter tool 6175, or equivalent.
4. Push the female end with the quick-connect fitting over the fuel rail until a click is heard. Pull the connector back to ensure that it is locked in place.
5. Push the male end with the nipple into the black fuel supply tube until a click is heard. Pull the nipple back to ensure that it is locked in place.
6. Attach a 0–60 psi (0–415 kPa) gauge from the fuel pressure test kit 5069, or equivalent, to the port on the fuel tube pressure adapter tool.
7. Start the engine and check the fuel pressure. It should be 28–30 psi (193–207 kPa) for early models and 43 psi (300 kPa) for late models. The fuel pressure regulator on these vehicles is non-adjustable.
8. If the fuel pressure is low, check for fuel flow through the return tube.
   1. If fuel is returning to the fuel tank through the fuel return tube, replace the fuel pressure regulator with a new one.
   2. If the fuel is not returning to the fuel tank or just trickling out, inspect for a faulty fuel pump, possible clogged fuel filter or other restriction between the fuel pressure regulator and the fuel pump.
9. If the fuel pressure is high, check the pressure regulator vacuum hose for leaks, and for a kink or restriction in the fuel return hose. If the pressure regulator vacuum hose is not leaking, the return tube is not kinked and does not have a restriction, the fuel pressure regulator should be replaced with a new one.
10. Remove the pressure gauge.
11. Disconnect the fuel tube pressure adapter tool with the quick disconnect tool.
12. Prior to installing the fuel tubes to the regulator and fuel rail the tube ends must be lightly lubricated with clean SAE 30 weight engine oil. Refer to the procedures describing the removal and installation of the fuel tubes.
13. Install the black fuel supply tube to the fuel rail and the gray fuel return tube to the pressure regulator.
14. Install the fuel tank filler cap.
15. Start the vehicle and check for fuel leaks. NOTE: When starting the engine, allow extra time for the system to pressurize itself.
    

I had a similar problem with my 95 Dodge with a cummins in it. Took forever to figure it out and so every morning, or if it sat 6-8 hours, would have to operate the primer pump under the hood to get it started for the day. Long story short. There is a fuel supply line to your engine and all diesels have a fuel return to to the fuel tank. After replaciing a section of fuel supply line with no results, discovered there has been a history of undetectable micro perferations in the steel fuel return line. The defective section of return line is normally the section in front of the firewall. I replaced this section with rubber hose. Its been several years since I did this repair... and if I recall correctly, hacksaw removed the firewall section of steel return tubing and hose clamped a section of rubber fuel line to replace the steel line I removed. I've never had a problem with engine starts since doing this repair. Hope this helps you out.
Regards Richard