Hi everyone. Any help appreciated. Still having problems starting my 1994 - XJ81. Will turn but not spark on occasion. Mechanic has replaced Crank Case Sensor, Battery, Starter Motor, Relays. There seems to be no rhyme or reason as car will start one day and not the next! Cheers - XJ81 Kiwi.

 

Hi Kiwi,

There are many possible causes of your intermittent cranks-but-won't-start condition. Here are a few I can think of:

1. Low voltage at the Engine Control Module. If the voltage seen by the ECM while cranking falls much below 11 volts, the ECM will not trigger the ignition to fire. Even with a strong battery, corrosion on battery power connections and ground points throughout the vehicle can reduce the voltage seen by the ECM, starter, ignition coil, fuel injectors, etc.

2. Intermittent failure of the fuel pump, often combined with a failed Fuel Pressure Regulator and fuel check valve. You can test the fuel pump and its wiring between the fuel pump relay socket and the pump. There is an electrical connector in the fuel pump harness behind the battery that can develop problems. One easy check for the fuel pump is that you should hear it run for a second or two when you first turn the key to Position II (ON) but before you crank the engine. If you don't hear the priming burst, suspect a problem with the pump or its power supply.

3. Since your mechanic is from the "replace parts until the problem is solved" school, you could replace the fuel pump relay preemptively. Just be sure to get the correct type of relay with a resistor and diode in parallel with the coil to protect the relay from premature failure. Also, if the V12 cars are wired like the straight 6 ones, the Oxygen Sensor Heater relay is in series with and upstream of the fuel pump relay coil, so that's another relay worth replacing preemptively, and it is also the type with a resistor and diode across the coil.

4. Check the distributor cap and rotor for worn contacts, condensation, etc., and check the distributor shaft for side-to-side play or signs of arcing to ground down the shaft.

5. If the Coolant Temperature Sensor fails at a "hot" reading, the ECM will not apply cold-start fuel enrichment.

6. Out-of-spec tuneup items like spark plugs & wires, air filter, fuel filter, etc. can collectively contribute to starting issues. Use only the correct original Champion spark plugs. Jaguar engines tend to dislike other brands or types of plugs.

7. Corrosion on ground points can cause intermittent electrical gremlins. Start by cleaning all grounds referenced by the Engine Management System (I'm not sure where they are on the XJ81; on the 6-cylinder cars there are two on the intake manifold and one on the firewall behind the cylinder head).

8. Check for oil contamination on the electrical connector for the Crankshaft Position Sensor and confirm that none of the pins in the electrical connector have partially backed out of the connector housing.

9. Check the wiring at the ignition coil and amplifier for signs of overheating or age deterioration.

10. Remove the air intake and crankcase breather plumbing and clean out all of the gunk. Clean the throttle bodies.

11. Low cylinder compression. Are you using the correct weight of engine oil? In most climates the correct weight for the 4.0L cars is 20W-50. I don't recall whether the same is true for the V12, but oil of too low viscosity can reduce compression in older engines with worn piston rings.

Cheers,

Don

 

Hi Don. Thank you so much for your reply. Very comprehensive summation and brilliant ideas. Will forward info on to my mechanic and see how we go. Cheers and thanks again. XJ12Kiwi