L-jetronic Troubleshooting, Runs Rough and Runs Rich Options

[BENBRO02]

It starts okay when cold and only has a slight miss but as it warms up it starts to miss more frequently, bucks, and if hot and idling may cut off completely. If I'm on the highway it may lose 20 mph before it starts running well enough to maintain speed or accelerate. I have recently had the following replaced:

fuel tank
fuel pump
fuel lines and hoses
fuel sending unit
vacuum hoses
large and small injector seals

The shop also checked the vacuum advance and cleaned the injectors.

I emptied my bank account with the repairs so now it's time for me to find the problem. Any Ideas? The car has a Pertronics ignition.

[ClayPerrine]

Get your stethoscope and touch each injector when it is misfiring. You should hear each of them click. If you don't hear one clicking, that is the bad cylinder.

Another way to check them is to pull one injector plug at a time and see if the engine speed drops. The dead hole will not cause an RPM drop. Hard to do on the #1 cylinder under the airbox, but you can remove the airbox and just leave the air flow meter hooked up to make more room.

Once you identify the bad cylinder, swap the injector connector with the one next to it and see if the problem moves. If it doesn't, you have a bad injector. If it moves, then start checking the harness. As stated previously, check the resistors. They are notorious for breaking one wire and causing a dead hole.

If you have a bad injector, go to FLAPS and get a set of 4 that fit a 1980 Datsun 280ZX. Nippondenso license built the L-Jet, an Datsun/Nissan used them on their cars way up into the 90s. The injectors on the Z car are the same as the ones used in the 912E, and it used L-Jet. Lots easier to find than the 914 L-Jet injectors, and they flow a little more fuel too.

As damp Dave said, also check the ignition. The L-Jet is more sensitive to ignition issues because it uses the negative side of the coil pulse as a reference to trigger the injectors. If the ground strap on the breaker plate is frayed or broken, you will get all sorts of weird running issues. The vacuum advance will move the plate, and the ground will go intermittent. But it doesn't happen if you run a pertronix or other electronic ignition system, because they don't use the ground inside the distributor.