Limping Home with L-Jet

by Richard Atwell
(c) Copyright 2005-2011

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When the L-Jet fuel injection system works, it works amazingly well. Tune-ups are few and the engine stays in tune much longer than carburetors. This is the great advantage over carbs besides better fuel atomization under all operating conditions.

When something does go wrong you should be be aware of a few tricks that may get you going again. Some people think that fuel injection makes it harder to limp home when trouble occurs on the road simply because you can adjust a carburetor with a screwdriver. Unlike modern FI system, there is no logic in the L-Jet ECU that will let you drive back to the dealership in a manner that is non-destructive to the engine but L-Jet doesn't leave you hopelessly stranded either.

In the system the two most important parts are the air flow meter (AFM) and the temp sensor II (TS2). The itself ECU is practically failsafe. Both the AFM and TS2 are dumb devices but this works to our advantage when problems arise. The other parts, aux air regulator (AAR), cold start valve (CSV), decel valve, pressure regulator and thermo time switch (TTS) don't have to be as precise for the FI system to work.


Trouble idling:

Like a carburetor the there is a bypass screw for setting the idle rpm. When the engine runs ok at speed but can't idle for some reason you can turn the screw CCW to let a little more air bypass the throttle.

The ECU doesn't care what the idle rpm is set to. It will try to deliver the appropriate quality of fuel based on the airflow measured in the AFM and the speed measured at the distributor coil in the same way that the carburetor is measuring the fuel based on the air speed/flow in the venturi.

Don't be fooled into thinking that because the throttle linkage connected to the pedal determines fuel quantity in a carburetor and the same cable only controls the throttle plate position in a FI system that the carb is more failsafe. All you need to understand is how to alter the way the fuel is metered: the engine doesn't care what kind of induction system is attached to it.


Fuel not pumping:

After the engine has started the double relay (DR) power circuit takes over from the starting circuit and powers the fuel pump via a contact in the AFM. If this contact fails you will not have any fuel delivery. The symptom is that the engine will fire then die right after.

The easy workaround for this situation is to bypass this switch at the double relay by jumping pins 86b and 88a together. This will power the fuel pump as long as the ignition key is turned on. Unlike a carb system that has been modified using an electric fuel pump without any safety shutoff circuitry the FI fuel pump can continue to pump while the engine is off and the key is on because the fuel pressure regulator (PR) simply circulates extra fuel back to the tank.


Not enough fuel:

1) The AFM tells the ECU how much air is entering the engine in order to meter the correct amount of fuel. If the AFM track partially wears out the engine may have trouble running at certain range of driving speeds. Two workarounds exist for this.

The first is to enrich the fuel mixture by disconnecting the hose line from the pressure regulator (PR) and plugged the hose. If you disconnect the vacuum hose without plugging it you will introduce a vacuum leak and the engine will not idle.

Under normal conditions the fuel pressure regulator maintains a specific pressure in the fuel lines. L-Jet works by varying the amount of time the injector is open under a constant fuel pressure. However, this pressure is not constant in the fuel rail because the pressure must be constant at the tip of the injector only. In other words, as the pressure on the tip side from the engine vacuum increases, the pressure in the fuel rail must decrease to maintain this constant relative tip pressure.

When we disconnect and plug the vacuum hose we are artificially increasing the fuel pressure at the tip and this can help to deliver more fuel under limp home conditions.

2) Inadequate fuel delivery can be compensated for by replacing the TS2. Not many people realize that when this part goes bad, the engine dies not because of lack of fuel but because of too much fuel: this over-rich condition is what kills the engine.

If the engine is in a really bad state, disconnecting the TS2 may allow the engine to run. A better solution is to carry a $3 5K Ohm Linear potentiometer from Radio Shack. The purpose of using this over simply replacing the temp sensor II (TS2) is to artificially enrich the mixture.

Although the cold-start valve seems like a potential candidate for a hippy fix, it's coil is not designed to take a 12V load for more than 10s. If you override the thermo time switch (TTS) if may burn up on you before you get home.


Parts:

Rather than limp home, it's much nicer to simply swap parts and continue on your journey. Carry some junkyard spares with you:

If you don't have them or happen to be helping a stranded bus owner remember these limp home tips.

My advice is to carry a new TS2 on board with you at all times. They only cost $15-20 and lasts 15-30 years. Order 0 280 130 012 from any Bosch vendor.


References:

History:

03/16/05 - Created
09/07/11 - Fixed broken photos, added translate button, updated footer