Hello folks,
After five years of welding,rebuilding,painting and finally suffering a dropped valve seat my XJS is finally back in, well almost, one single piece. But it seems the old dear has one last sucker punch, unfortunately, this could be the final straw for me. I'm loosing the will to continue battling the machinations of the big cat, so with that I need a bit of help. The engine has been fully rebuilt, nut and bolt, by me. Meticulously put back together, with most of the ancillaries cleaned or replaced. The injectors are the originals as is the wiring loom, both are good nick. The problem I have is this, the car refuses to start! There is spark,fuel,static timing seems ok. Both CPS sensors are new, I have gone over the looms and can't find anything? The dizzy is set as per the instructions for the marelli set up, the list goes on... all that happens on attempted start ups is occasional backfiring through the exhaust until the battery goes dead?

I know this subject has been covered in the past in various threads, I'm just throwing this out there because I really am at my wits end!!

any help, as always, is received with immense gratitude.

kind regards,

Neil.

 

This *could* be quite a long, painfully long list of things to re-check. Maybe it wouldnt hurt to take a series of hi-res photos of the engine to see if anyone can spot an out of place wire, or some other red flag.

 

Are you 100% sure the ignition wires and or ignition rotor are correct and in the correct firing order? A backfire through the exhaust is usually a timing issue. I'm not familiar with the V12 jackshaft, but on the XK engine it is possible to assemble the distributor drive 180 degrees out.

If it is backfiring, you know you have fuel! It has to be a spark or timing issue. Can you connect a timing light and see what the light illuminates when cranking? It should show correct timing, if it doesn't you know something is wrong. If the timing light does flash, then you know at least you have spark.

 

To clarify....

You have correct fuel pressure and the injectors are pulsing?

Cheers
DD

 

Never mind, I missed the backfiring part.

Cheers
DD

 

(my 2 cents).
Backfiring through the exhaust on a COLD engine is caused by one of two reasons. Ignition timing incorrect? or valve timing incorrect.
Backfiring through the exhaust on a hot engine is a totally different issue. (Overfueling).
Normally valve timing is correct, when the ignition timing is incorrect the engine will backfire through the intake.
Given that you stated you rebuilt the engine from top to bottom, I would verify that the valve timing is spot on or
a.you don't have the exhaust valves too tight leaving one or more of them slightly open
b. a leaky exhaust valve
c. an exhaust valve hanging up.
I would go back and do the easiest first which is ignition timing, then valve timing.
Backfiring through the exhaust on cold engine means the spark is igniting the fuel when an exhaust valve is open since the engine is not hot.

 

1. Ok, are you saying you have injection? If no, we need to check that the white wire on pin 18 of the ECU is getting the 5v square wave from the ignition (Marelli) that passes from the ecu in the A side kick panel to, i believe, the engine bay, where it meets up with the old Lucas white wire going back to the ECU.

2. The temperature sensor on the B bank will absolutly F' your day up if it's not providing the ECU with the proper resistance measurement due to being corroded, bad, wire-off, etc. Very few cars will start, and most will over-fuel to the point of not cold-starting if this has gone wrong. The pins to test at the ECU on that are different depending on facelift/non-facelift. On non-facelift the pins to check the resistance are across #5 and #19. A way to test the wiring is to put a resistor across the harness pins and double check at the ECU plug: Sean's Jaguar Tech Pages - A Coolant Temperature Sensor Emergency Switch

Paul K.

 

Broken rotor arm

Bank B


DOH!
Firstly thanks a million for the replies! Really appreciate it. It would appear I have been a right d@#k here. I went back into the garage today, in a spanner frenzy, ripped off the manifolds and rocker covers to check cam timing I'm embarrassed to admit that I found the dizzy timing 180 degrees out!! A few weeks back I replaced my snapped rotor arm (thats another story) I removed the dizzy to sort a stripped bolt. I have my own timing mark on the breast plate of the engine, this aligns with one of the triggers on the crank pully, It can be seen just by looking down the front of the engine. Obviously it has been a full turn out
I've been totally paranoid about timing with this engine, its sods law that it would be that where I'd dropped a bollock!
I've attached some photos, to illustrate my dumbness.
just got to put the bugger back together now!
best wishes,
Neil.

 

We have all done such things! Well found.