'87 4.2l has spark but isn't starting, assuming fuel injectors aren't firing.

Can anyone confirm the coolant temp. sensor used in the EFI is the second from the firewall? (Connections are badly rusted) Can anyone describe behavior with this out? Does it crank and not start due to no fuel or should it disable more than that?


Also, what does the thermotime switch do? Is it part of the cold start system?


Lastly if anyone has a link to a faq or list that summarizes the 4.2 fuel system or common non-spark related no start issues, that would be super helpful. I know my way around the 5.3 but am quickly finding out the systems differ more than I expected so the I6 is all new to me.

 

Click and scroll
Quick Reference: Top Twenty XJ Repair Issues

The coolant temp sensor for the fuel injection is third from the front



With the sensor disconnected or flat-out inoperative the engine will not run, period. With the engine running, disconnecting the sensor will kill the engine instantly.

Some have suggested that an unplugged CTS will cause massive over-fueling. Others say it outright stops injector function. I'm not sure which is correct


Yes, part of the cold start system. It provides a ground for the cold start injector....which itself operates entirely independently of the regular fuel injection system. The cold start system operates only when the starter is engaged and only for a few seconds. Typically an inoperative cold start system will not cause a 'no start' situation

The Lucas CEI ignition on the Series III 4.2 is virtually identical to the Lucas CEI used on the V12s of the 80s.

Good article on the F.I.

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

Cheers
DD

 

Here's a diag list for the CEI system.....basically a paraphrasing of what's in the ROM

The test assumes a fully charged battery

1) Measure voltage at coil "+" terminal with key "on". It should be within
one volt of battery voltage. If not suspect a problem with the wiring to the
ignition switch, or the switch itself.

2) Measure voltage at the coil "-" terminal. Result should be the same as at
the "+" terminal. If Ok, go to step 3. If not....
Disconnect the wire from the amplifier from the "-" post of the coil and
measure voltage again. Less than 2 volts means the coil is faulty. More than
2 volts means the amplifier is faulty.

3) Disconnect distributor pickup coil from the amplifier (this is the
harness from the distributor that plugs into the amp). Measure resistance
across the terminals. It should be 2.2k to 4.8k ohms. If Ok, go to step 4.
If not, replace the pickup.

4) Reconnect the pickup to the amplifier. Measure voltage at coil "-" post
while cranking engine. The voltage should drop. If OK, go to step 5. If
not, the amplifier is faulty.

5) Check distributor cap and wires, distributor rotor arm, spark plugs, coil
wire


Cheers
DD

 

Thanks for all the info Doug! I guess I assumed the fuel system was very different when I saw the airflow meter, but I noticed (and cleaned) the resistor pack and saw a familiar box nearby that looked like the ignition amp.

The connector for the coolant sensor is rusted as well so I can't just stick a paperclip in to check. Very obviously mucked up so I guess this is the first place to check-off.

 

Lots of similar components but the fundamental operating basis is different: one based on air flow (your airflow meter, as mentioned), the other based on pressure sensing (the manifold pressure sensor inside the ECU on the V12s).

The ignition amp is almost identical to the V12. I think it lacks the zener diode, though. Bit of brain fade at the moment.

Cheers
DD