First thanks to all for responding to my new member intro. I want to describe an odd problem encountered with my newly acquired 89 XJS 5.3 with Marelli Ignition. The car has 18K original miles on it and looks in every way like it came out of the new dealership showroom. It started and ran perfectly until I decided to replace the inoperative cassette radio with a high end Alpine. Out for a ride one night, I cranked up the volume a bit, blew that insufficient 2 amp fuse and the engine shut down and would not restart. After being flat bedded home, it started reluctantly the following day only to stall out and get flat bedded again. I have since made the following checks, replaced general maintenance parts, and all seem to be within normal limits according to the ROM:
1. Installed 10 amp fuse in place of 2 amp fuse
2. Checked power distribution from Ignition to relays
3. Tested Main and Pump relays with Snap-On relay tester
4. Installed new fuel filter (old one not gunned up)
5. Tested fuel pressure and installed new regulators
6. Checked spark at plug wires with smart tack and Kal spark tester
7. Checked and replaced coolant temp sensor
8. Checked air temp sensor
9. Changed oil and oil filter, both air filters
10 Drained and replaced antifreeze
New parts for the above were installed for maintenance purposes only, but none of the above seemed to cause the shut down based on my tests. At present the car will start from cold only if the accelerator is to the floor, and even then somewhat reluctantly. It will run perfectly and start immediately, time and time again, once the engine reaches operating temperature. I'm baffled by the source of the reluctant cold start problem, and whether to chance driving it on the street without further checks. Any advice would be greatly appreciated.

 

Sounds like you know your way around these cars. I honestly feel that the radio has little to do, if anything, with its problem. Owning a Marelli car, I became pretty understanding of its operation, as I bought a non-running project that I wound up building a entirely new ignition harness for. IF, and I really mean IF the new radio has anything to do with it, my first thought would be to disconnect it entirely, and go see what happens. Its not completely out of the question, as for some reason starting in the 90 model year, Jaguar/Marelli added coax to the ignition harness going to the amps. It already has coax going from the front and rear crank "sensors", which are really not sensors at all. They are magnetic generators that imput a AC signal to the Marelli ECU. Yet, starting in 90, the coax had to be added for some reason. Enough homework in TSBs might find out why, and it very well could be something to do with signal interference from the AC system, radio?, etc...... On the front and rear crank "sensors" it has something to do with the hertz frequency. The AC voltage could somehow effect other signals, or even its own signal? who knows. I just know it wouldn't work with a twisted shielded pair.

Anyway, to make a long story longer, check your temp sensor output with wire unplugged at the sensor, then plug it back it, and go unplug the ECU harness in the trunk, and check it there. I chased that gremlin too. Hard starting came down to a bad connection in the harness where it gets REALLY hot on the water rail at the left front. The wire felt "baked"? Corrected that, and my hard starting went away. Last, but certainly NOT LEAST, is to check the clearance of the front and rear "sensors". They lose their magnetism over time and heat, and begin giving a signal to the Marelli ECU that isn't in line with its built in tables. This is along the same reason aircraft magnetos have to be replaced as part of normal service. On my car, I filed the mounting bosses to achieve as close as I could to .020, and this made the car run great! Feel free to PM me your phone number, and will be glad to call and discuss.

 

Thanks so much SuperC for your advice. I'll start with checking that temp sensor wire and trace it to the ECU, then the crank sensors if necessary. I'll post back. Really appreciate the advice!

Bud