L-Jetronic Fuel Pressure Testing
Posted: Sun Sep 03, 2006 7:11 am
Pressure Test of L-Jetronic
Procure a pressure gauge that will measure between 0-60 psi.
If you are stuck in the middle of nowhere, you can make one
out of a "0-100 psi water pressure tester gauge" from Home Depot
and some fittings from the "air compressor isle" that will allow you to attach it to the 7mm pressure tap on the 3/4 fuel rail for about $20.
You can sometimes find a good gauge at the FLAPS for about $50
but you might have to cut off the end of the 5/16" hose and
clamp it securely onto the fuel rail.
A good quality gauge looks like this one from German Supply.
http://www.germansupply.com/home/custom ... 300&page=1
To relieve residual fuel pressure that may be present, open the gas cap to remove pressure in the tank and apply a little suction on the vacuum line that goes to the fuel pressure regulator (located at the right front of the engine, attached to the front tin) for a couple of seconds to release the pressure in the fuel rail. Regardless, when you remove that first fuel hose in the engine compartment, always have a rag to catch any possible spray.
Install the pressure gauge on the fuel rail test port. Mount it securely, but you do not have to reef on the clamp. The fuel rail test port is a little bolt (with a 7mm head) in the fuel rail between #3 and #4 injectors. Left side of engine. . .
A 3 foot length of 7mm FI hose allows you to stand clear of the engine while you take readings. Do not leave the gauge on the rail when you are done. Remove it and reinstall the 7mm plug with a copper washer to keep it sealed.
Start engine. (if you are diagnosing a no-start, someone needs to check gauge for pressure during cranking) At idle, the gauge should read 2 bar or about 29 psi +/- 3 psi. Still idling, remove the vacuum line to the fuel pressure regulator and the pressure should go to about 2.5 bars or about 36 psi +/- 3 psi. The injectors won't inject if pressure is less than 26 psi.
Reconnect that vacuum line to the regulator to see if you get a pressure drop of 0.5 bar or about 7 psi (back down to about 29 psi) If you do not get drop in fuel pressure, make sure you have vacuum. It is a short trip to the plenum where it comes from. There are other components sharing this vacuum circuit, so make sure they are not leaking either.
Rev the engine up to about 3000 RPM a few times and see if the pressure varies within 29-36 psi. Getting "on" the accelerator drops the vacuum signal to the regulator.
A dying pump or clogged fuel loop, will behave as though you are running out of gas, because you are.
If the pressure is low on both parts of the test, either:
1) the pressure regulator is returning too much fuel to the tank or
2) the fuel pump is not delivering properly (could be a bad pump, plugged fuel filter, rust in the tank or a plugged screen in the tank but most of us do not have a screen in the tank).
Pinch shut the return fuel line on the right side of the engine SLOWLY FOR ONLY A FEW SECONDS.If pressure climbs to about 4 bars (60 psi) but no more, that will show you that the fuel pump/ relief valve is indeed working correctly. If the pressure drops below 26 when you release the return hose your regulator is toast. If the pressure does not rise, there is a problem with the fuel delivery.
If the fuel pressure is too high, you need to shuff off the engine and remove the fuel return line from the regulator and direct it into a gas can. Run the pressure test again and if the pressure is OK, there is something blocking the return line. Low pressure compressed air or a non-sparking wire might help clean it out. The gas pumping around the fuel rail is also what keeps the fuel pump cool.
The pump should put out about about 1 liter of fuel in about 30 seconds if it is clear and getting the 12 volts it wants. Drink down that 1 liter Coke bottle in the fridge and you have a graduated cylinder for your fuel delivery test. Disable the ignition by pulling the black wire that shares the harness with the oil pressure switch from the (+) (15) terminal of the coil. Pry the black plastic cap off the top of the air flow meter, takes a bit of effort but be persistantly gentle. Turn on ignition. You should not hear the pump running! If you do, you must check the safety contacts in the air flow meter. There is a long silver rod on the wiper that cuts off the fuel pump when the engine is not running. If the pump is whirring away with only the ignition on/engine off, you must bend the silver rod clockwise until you see it pry open the little copper contacts and shut off the pump. Then, when you just move the wiper counter-clockwise, the pump should run. You want to be able to turn the pump on and off at will with the air flow meter wiper! Shut off ignition. Unscrew the gauge from the long hose you decided to use at our recommendation, and place the hose in the "graduated cylinder" for this test. Turn on the ignition switch, babysit the hose/container and gently move the wiper under the square plastic cap on the airflow meter. It will click the double relay and turn on the pump. Release the wiper to stop the pump. Correct volume in the specified time? Good. No? Check filter and hoses for obstructions/kinks, and your pump ground, double relay, and starter connections for correct voltage.
Residual pressure test - when your bus won't start when warm:
With the gauge installed on the rail test port, run the engine to get it all pressured up, then shut it off. After 20 minutes the pressure should not fall below 1 bar or about 14.5 psi.
If it falls below that check:
1) all fuel line connections for leaks
2) to make sure that none of the injectors are leaking including the cold start valve and don't forget the injector seals.
The fuel pump also has a check valve to allow for only one way traffic for the fuel. To test the fuel pump check valve, run the engine and shut it off and immediatedly clamp shut the 1/2 side of the fuel loop just before the regulator. Use a pair of needle nose vise grips with a short piece of fuel line on each jaw to prevent real damage, or a fancy-shmancy fuel hose clamp tool. If the pressure drops below the specification after 20 minutes, the fuel pump check valve is faulty. New fuel pump time.
Remove the clamp on the return line, start it up again and repeat the test by clamping the fuel line upstream of injectors 3 and 4 and if the pressure is now within the specification of 14.5 psi, then the regulator is faulty.
One more thing...change out the fuel lines when you first get your bus so you will know when they were last done and then inspect them every year. We suggest changing them out with the correct FI fuel line every 7 or 8 years on your birthday and do them all including the vent line connections. Hate to see a nice baywindow bus burn down.
Richard's website has the fuel lines you need to come up with and where they go. Thanks Ratwell again for doing that! http://www.ratwell.com/technical/FuelHoses.html
German Supply has a nice diagram and has a kit that actually fits and it is of excellent quality. Thank you Scott for your good work!
http://www.germansupply.com/xcart/custo ... ctid=16400
http://www.germansupply.com/Scotts_Fold ... %20Bus.jpg
RandyinMaine & Amskeptic
Procure a pressure gauge that will measure between 0-60 psi.
If you are stuck in the middle of nowhere, you can make one
out of a "0-100 psi water pressure tester gauge" from Home Depot
and some fittings from the "air compressor isle" that will allow you to attach it to the 7mm pressure tap on the 3/4 fuel rail for about $20.
You can sometimes find a good gauge at the FLAPS for about $50
but you might have to cut off the end of the 5/16" hose and
clamp it securely onto the fuel rail.
A good quality gauge looks like this one from German Supply.
http://www.germansupply.com/home/custom ... 300&page=1
To relieve residual fuel pressure that may be present, open the gas cap to remove pressure in the tank and apply a little suction on the vacuum line that goes to the fuel pressure regulator (located at the right front of the engine, attached to the front tin) for a couple of seconds to release the pressure in the fuel rail. Regardless, when you remove that first fuel hose in the engine compartment, always have a rag to catch any possible spray.
Install the pressure gauge on the fuel rail test port. Mount it securely, but you do not have to reef on the clamp. The fuel rail test port is a little bolt (with a 7mm head) in the fuel rail between #3 and #4 injectors. Left side of engine. . .
A 3 foot length of 7mm FI hose allows you to stand clear of the engine while you take readings. Do not leave the gauge on the rail when you are done. Remove it and reinstall the 7mm plug with a copper washer to keep it sealed.
Start engine. (if you are diagnosing a no-start, someone needs to check gauge for pressure during cranking) At idle, the gauge should read 2 bar or about 29 psi +/- 3 psi. Still idling, remove the vacuum line to the fuel pressure regulator and the pressure should go to about 2.5 bars or about 36 psi +/- 3 psi. The injectors won't inject if pressure is less than 26 psi.
Reconnect that vacuum line to the regulator to see if you get a pressure drop of 0.5 bar or about 7 psi (back down to about 29 psi) If you do not get drop in fuel pressure, make sure you have vacuum. It is a short trip to the plenum where it comes from. There are other components sharing this vacuum circuit, so make sure they are not leaking either.
Rev the engine up to about 3000 RPM a few times and see if the pressure varies within 29-36 psi. Getting "on" the accelerator drops the vacuum signal to the regulator.
A dying pump or clogged fuel loop, will behave as though you are running out of gas, because you are.
If the pressure is low on both parts of the test, either:
1) the pressure regulator is returning too much fuel to the tank or
2) the fuel pump is not delivering properly (could be a bad pump, plugged fuel filter, rust in the tank or a plugged screen in the tank but most of us do not have a screen in the tank).
Pinch shut the return fuel line on the right side of the engine SLOWLY FOR ONLY A FEW SECONDS.If pressure climbs to about 4 bars (60 psi) but no more, that will show you that the fuel pump/ relief valve is indeed working correctly. If the pressure drops below 26 when you release the return hose your regulator is toast. If the pressure does not rise, there is a problem with the fuel delivery.
If the fuel pressure is too high, you need to shuff off the engine and remove the fuel return line from the regulator and direct it into a gas can. Run the pressure test again and if the pressure is OK, there is something blocking the return line. Low pressure compressed air or a non-sparking wire might help clean it out. The gas pumping around the fuel rail is also what keeps the fuel pump cool.
The pump should put out about about 1 liter of fuel in about 30 seconds if it is clear and getting the 12 volts it wants. Drink down that 1 liter Coke bottle in the fridge and you have a graduated cylinder for your fuel delivery test. Disable the ignition by pulling the black wire that shares the harness with the oil pressure switch from the (+) (15) terminal of the coil. Pry the black plastic cap off the top of the air flow meter, takes a bit of effort but be persistantly gentle. Turn on ignition. You should not hear the pump running! If you do, you must check the safety contacts in the air flow meter. There is a long silver rod on the wiper that cuts off the fuel pump when the engine is not running. If the pump is whirring away with only the ignition on/engine off, you must bend the silver rod clockwise until you see it pry open the little copper contacts and shut off the pump. Then, when you just move the wiper counter-clockwise, the pump should run. You want to be able to turn the pump on and off at will with the air flow meter wiper! Shut off ignition. Unscrew the gauge from the long hose you decided to use at our recommendation, and place the hose in the "graduated cylinder" for this test. Turn on the ignition switch, babysit the hose/container and gently move the wiper under the square plastic cap on the airflow meter. It will click the double relay and turn on the pump. Release the wiper to stop the pump. Correct volume in the specified time? Good. No? Check filter and hoses for obstructions/kinks, and your pump ground, double relay, and starter connections for correct voltage.
Residual pressure test - when your bus won't start when warm:
With the gauge installed on the rail test port, run the engine to get it all pressured up, then shut it off. After 20 minutes the pressure should not fall below 1 bar or about 14.5 psi.
If it falls below that check:
1) all fuel line connections for leaks
2) to make sure that none of the injectors are leaking including the cold start valve and don't forget the injector seals.
The fuel pump also has a check valve to allow for only one way traffic for the fuel. To test the fuel pump check valve, run the engine and shut it off and immediatedly clamp shut the 1/2 side of the fuel loop just before the regulator. Use a pair of needle nose vise grips with a short piece of fuel line on each jaw to prevent real damage, or a fancy-shmancy fuel hose clamp tool. If the pressure drops below the specification after 20 minutes, the fuel pump check valve is faulty. New fuel pump time.
Remove the clamp on the return line, start it up again and repeat the test by clamping the fuel line upstream of injectors 3 and 4 and if the pressure is now within the specification of 14.5 psi, then the regulator is faulty.
One more thing...change out the fuel lines when you first get your bus so you will know when they were last done and then inspect them every year. We suggest changing them out with the correct FI fuel line every 7 or 8 years on your birthday and do them all including the vent line connections. Hate to see a nice baywindow bus burn down.
Richard's website has the fuel lines you need to come up with and where they go. Thanks Ratwell again for doing that! http://www.ratwell.com/technical/FuelHoses.html
German Supply has a nice diagram and has a kit that actually fits and it is of excellent quality. Thank you Scott for your good work!
http://www.germansupply.com/xcart/custo ... ctid=16400
http://www.germansupply.com/Scotts_Fold ... %20Bus.jpg
RandyinMaine & Amskeptic