Jaguar Repair: fuel injection, fuel pressure gauge, fuel pressure regulator

Question
Hi Howard, I have a 1985 jaguar XJ6 4.2. Some months back the engine started to run a little rough on idle and would cut out if the accelarotor was not pressed down. Now it will hardly start at all, and if it does my mechanic says there is too much fuel being delivered. There is black smoke from exhaust and the plugs have gone black. My mechanic says he thinks there is a problem with the fuel injection system but cannot locate the exact problem. He changed most parts on the fuel injection manifold (injectors, sensors etc. but did not yet try the fuel pressure regulator yet. Have you any ideas on what might be the cause? Thanks for your time. Also, how do I leave feedback?

Answer
Hi Martin,
You should not replace parts until you know what part has failed. The test for a pressure regulator is to put a "T" in the fuel line just as it meets the fuel rail and connect a fuel pressure gauge. (Injection pressure gauges are a little expensive so you can just get any industrial gauge of 0 to 100 PSI)Be sure to put hose clamps on every joint.
Remove the air filter and turn the Ignition to just "ON" position. reach into the air flow meter and open the flap and read the gauge. (with the engine not running like this you should see about 42 PSI and not much more). If you run this test with the engine running you should see about 32 PSI and not much more. Remove the vacuum hose from the regulator and shake it to see if there is any signs of fuel in it. Even if the gauge readings are correct you need to replace the regulator if any fuel is found in the vacuum hose just after the engine is run.

This is a Bosch "L" Jetronic injection system and it has several items that can cause a excessive rich mixture (black smoke is a sure sign of a rich mixture)(You should avoid running the car like this because if you have the Two CATs that it comes with here in the US, you will destroy the CATS and stop up the exhaust.

There are major and minor influences on mixture. The major ones are the coolant temp sensor, the air flow meter and the fuel pressure. The minor ones are the 0-2 sensor and the air temp sensor.

You said "the PLUGS have gone black". If all the plugs are flat black then that tells you it is not a stuck open injector. However it could still be a stuck open cold start valve or a short in the wire that has the cold start valve held open all the time.

The throttle position switch only works at close to full throttle but if something is wrong there and the switch was telling the ECU that you had full throttle all the time it could cause a rich mixture. To test just unplug the wire and use an ohm meter and operate the throttle to see that the switch is only working at full throttle. (there are three pins but only two are used)

The ECU can be the cause, but that is extremely rare even though it is the most complicated part of the car.

Howard