L-Jetronic Fuel Pressure Testing

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Randy in Maine
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L-Jetronic Fuel Pressure Testing

Post by Randy in Maine » 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
79 VW Bus

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vwlover77
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Re: Testing L-Tronic Fuel Pressure

Post by vwlover77 » Tue Sep 05, 2006 10:22 am

If I remember correctly, for a fuel injector to fire, what matters is the differential pressure between the tip of the injector and its fuel supply. If the injector tip is experiencing a pressure of -30 inHg (a vacuum of 14.7 psi), and 26 psi differential pressure is required to fire the injector, then the supply pressure only needs to be 11psi.
Don

---------------------------
78 Westy
71 Super Beetle Convertible Autostick

"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen

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Amskeptic
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Re: Testing L-Tronic Fuel Pressure

Post by Amskeptic » Sat Sep 09, 2006 12:41 am

vwlover77 wrote:If I remember correctly, for a fuel injector to fire, what matters is the differential pressure between the tip of the injector and its fuel supply. If the injector tip is experiencing a pressure of -30 inHg (a vacuum of 14.7 psi), and 26 psi differential pressure is required to fire the injector, then the supply pressure only needs to be 11psi.
For a fuel injector to actually deliver, it must overcome its own cutoff pressure. The injectors have to maintain a static pressure when the engine is off to prevent vaporization during heat-soak. That pressure is at least 14 psi.

The running pressure differential between fuel supply and manifold vacuum (absolute pressure)is kept constant with the pressure regulator, but as Randy noted, once it drops below the injector cut-off, party's over.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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Amskeptic
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Post by Amskeptic » Sat Sep 09, 2006 1:28 pm

vwlover77 wrote:Yes - I was forgetting about the injector's internal mechanical cutoff.

I'm still curious to know: At a steady 3000 rpm, no load, with a much higher vacuum signal to the pressure regulator than at idle, what should the fuel pressure gauge read? I'm guessing psi in the low 20s????
The threshold nature of the fuel pressure regulator coupled with the lack of modularity in the vacuum signal gives us more of an on/off fuel pressure variation. The regulator with full 21 in/hg vacuum should not drop the fuel pressure below 28. You are describing Off-Throttle Overrun above. . . note-to-self that thar is a glossary term.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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airkooledchris
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Re: L-Jetronic Fuel Pressure Testing

Post by airkooledchris » Mon Jan 10, 2011 6:14 pm

thanks for posting this writeup, there are some other tests listed here that I haven't yet performed, but will bookmark this for next time.

im trying to figure out why im averaging 11.25 MPG on my bus. this number is the average from the last 7 fillups (most done with at least .25 of a tank remaining) - most of it highway driving through the hills and probably 20% of it in the city.

anyway, one of the tests I performed this weekend was for the fuel pressure.
I disconnected the vacuum hose from the pressure regulator and plugged the hose
fuel pressure at idle (1000 RPM) was 40 PSI, which is 5 higher than spec and 3 higher than the tolerance listed

With vacuum hose connected, at idle (1000 RPM), fuel pressure was 32 PSI, which is 4 higher than spec and 2 PSI higher than the tolerance listed

At full throttle and with the vacuum hose connected, I saw exactly 30 PSI

what im wondering is, how much is too much? could these slightly higher numbers give me poor fuel economy?
is it more likely that the Fuel Pressure Regulator is failing or just that something is blocking the return line? (I need a decent gas can to run the test mentioned above, but ill find one and try it this weekend if that seems the likely culprit.)
1979 California Transporter

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Amskeptic
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Re: L-Jetronic Fuel Pressure Testing

Post by Amskeptic » Tue Jan 11, 2011 1:01 pm

airkooledchris wrote:thanks for posting this writeup, there are some other tests listed here that I haven't yet performed, but will bookmark this for next time.

im trying to figure out why im averaging 11.25 MPG on my bus. this number is the average from the last 7 fillups (most done with at least .25 of a tank remaining) - most of it highway driving through the hills and probably 20% of it in the city.

anyway, one of the tests I performed this weekend was for the fuel pressure.
I disconnected the vacuum hose from the pressure regulator and plugged the hose
fuel pressure at idle (1000 RPM) was 40 PSI, which is 5 higher than spec and 3 higher than the tolerance listed

With vacuum hose connected, at idle (1000 RPM), fuel pressure was 32 PSI, which is 4 higher than spec and 2 PSI higher than the tolerance listed

At full throttle and with the vacuum hose connected, I saw exactly 30 PSI

what im wondering is, how much is too much? could these slightly higher numbers give me poor fuel economy?
is it more likely that the Fuel Pressure Regulator is failing or just that something is blocking the return line? (I need a decent gas can to run the test mentioned above, but ill find one and try it this weekend if that seems the likely culprit.)
Blame the crappy gauge for those numbers. They are laughably casual.

You need . . . a LM-1. WHY DON'T YOU HAVE AN LM-1? You need to adjust the wiper at full-throttle at 55 mph for 12.5. Then you check at full throttle for 12.8- 13.0 at 70 mph. That slope is determined by the black cog. I am still working out the procedure that can be bullet-proof for the non LM-1 owner.
Colin :bom:
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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airkooledchris
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Re: L-Jetronic Fuel Pressure Testing

Post by airkooledchris » Tue Jan 11, 2011 1:56 pm

I left an LM-1 around here somewhere's :scratch:


OK, so no concerns over the very slightly high fuel pressure, ill leave a fuel pressure regulator off of the parts list for now.

thanks, I want to keep ruling out bits and pieces along the way and this was one of the items that wasn't in spec according to Bentley.

im replacing the temp 2 sensor today, then I think it's best that I run through at least half a tank of gas to see where im at, so I don't mess with too much before I get a new reading.
1979 California Transporter

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vwlover77
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Re: L-Jetronic Fuel Pressure Testing

Post by vwlover77 » Tue Jan 11, 2011 7:01 pm

Are you running oversize tires thereby making your odometer inaccurate?
Don

---------------------------
78 Westy
71 Super Beetle Convertible Autostick

"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen

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airkooledchris
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Re: L-Jetronic Fuel Pressure Testing

Post by airkooledchris » Tue Jan 11, 2011 9:03 pm

vwlover77 wrote:Are you running oversize tires thereby making your odometer inaccurate?
well, yes, they are larger than stock tires. I have this app on my iPhone for biking that tracks your route and measures your distance traveled and when I did a test run around the neighborhood 1 mile on my bus odometer equaled 1.03 miles on the trip computer. In my Vanagon, where I last ran these tires I had the same question, so I took it up on the highway until I found a mile marker and 1 = 1 on the odometer.


you think I should be putting more effort into making sure this isn't my source of poor MPG numbers?
1979 California Transporter

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Randy in Maine
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Re: L-Jetronic Fuel Pressure Testing

Post by Randy in Maine » Wed Jan 12, 2011 5:28 pm

Question:

Is your 79CA system all 79CA?

ECU, AFM, distribitor/ignition module, coil, DR? When I had a Federal AFM unit, mine ran really rich.

By the way I have some spare exhaust sytems for that set up from the heaterboxes to the cat if you need them.
79 VW Bus

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vwlover77
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Re: L-Jetronic Fuel Pressure Testing

Post by vwlover77 » Wed Jan 12, 2011 7:18 pm

airkooledchris wrote:you think I should be putting more effort into making sure this isn't my source of poor MPG numbers?
Well, if the odometer is only 3% off, then no....

What do your spark plugs look like?
Don

---------------------------
78 Westy
71 Super Beetle Convertible Autostick

"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen

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airkooledchris
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Re: L-Jetronic Fuel Pressure Testing

Post by airkooledchris » Wed Jan 12, 2011 9:34 pm

Randy in Maine wrote:Question:

Is your 79CA system all 79CA?

ECU, AFM, distribitor/ignition module, coil, DR? When I had a Federal AFM unit, mine ran really rich.

By the way I have some spare exhaust sytems for that set up from the heaterboxes to the cat if you need them.

The AFM, Distributor, ignition module, and coil are all def CA79 specific, though I haven't checked the numbers on the ECU yet and I can't think of what the DR? is for?

The exhaust looks to be all there, but the cat could be messy inside, ive never had this exhuast off and all of the connects from the various bits TO the cat look pretty well rusted together.
If I remove anything from the current setup ill need all new gaskets/etc to get it back together again for sure. Im contemplating removing it in order to check the state of that CAT further...

The heaterboxes look great, and the crossover/muffler look alright, it's just that bit from the elbow/o2 sensor/cat that looks corroded currently.
vwlover77 wrote: What do your spark plugs look like?
not bad. if anything, they look a BIT on the lean side.


Edited to add:
I went on a longer drive to really make sure the odometer was on and in the end I need to add .0414 to every mile

On the most recent fill up, the worst one, instead of 9.41 MPG it would have been 9.8 using 10.488 gallons in 102.818 miles VS 98.7

I think I can rule out overstock tires for now, or grossly inaccurate odometer
1979 California Transporter

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Randy in Maine
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Re: L-Jetronic Fuel Pressure Testing

Post by Randy in Maine » Thu Jan 13, 2011 7:26 am

The 79CA double relay has 13 "prongs" as opposed to the federal version that has 11 "prongs".
79 VW Bus

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airkooledchris
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Re: L-Jetronic Fuel Pressure Testing

Post by airkooledchris » Thu Jan 13, 2011 10:58 am

Randy in Maine wrote:The 79CA double relay has 13 "prongs" as opposed to the federal version that has 11 "prongs".
AHh, of course, DR. Thanks. :thumbright:

confirmed my Dr has 13 prongs. I also double checked the coil, which is the correct part number for 79 CA model.
I was going to write down the number on the ECU quick, but it isn't as easily accessible as on the Vanagon so ill save that for later.
Im guessing I either need to pull it from the mount by the battery, or maybe you can see the part number by removing that inspection plate thing by the gas filler?
1979 California Transporter

Psucamper
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Re: L-Jetronic Fuel Pressure Testing

Post by Psucamper » Tue Dec 04, 2012 2:10 pm

Overnight mystery. I have a 79 2 liter bus (hydraulic) modified to accept 73 exhaust system. No funky attachments or anti-smog devices present. Newly installed fuel filter. Had a hard start problem a month ago and fixed it by cleaning TS2's corroded/head threads. All has been well since. Yesterday had a little rapping at #1 lifter and ran engine to load lifter. After warming it was very quiet and smooth.
Did not like the 'underneath everything' position of the TTS and that miserable mess of white ground wires that hook on the spades to the block. Can't see them and can't move them. So I made up a new 'harness' of ground wires for everything including the brown wires from the double relay and sent them all to a common point on the block. Yesterday, started engine and it immediately went into cold start at fast idle and finally came to curb idle (1000rpm/autotrans). Thinking all was well,I quit and did other things. Today started engine and it ran but the cold start speed was low and after maybe 15/20 seconds of run it died.Very smooth run during those 30 seconds. Almost certain the are no vacuum leaks and brake booster line is plugged at plenum. So my guess was no gas or too much gas. Easiest test was to check fuel pressure at rail and I did it. Gauge pressure was no more than 15 psig! That pressure was held in the fuel system for many minutes after run showing good check valves at both ends. This is a new Bosch FP with essentially no run time on it. My current suspicion is the FPR is partially stuck/faulty open and won't permit a pressure buildup to a nominal 28 psig. Hate to pull the FPR if there's a diagnostic that will clarify the situation. Next? Bob L.

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