XJS Backfire

[BigBlueCat]

Hi All
Can anyone give me a pointer, took the XJS out today, stopped at the traffic lights all fine, accelerated to pull away and a huge back fire. Pulled over as the engine felt lumpy afterwards, not like a misfire more like a lumpy rumble. No smoke or anything at the exhaust on tickover, good oil pressure registering on the gauge. Car starts and runs but press the pedal and bang. Any thoughts before I take her to the mechanic. 1990 V12 5.3

[ng4xjs]

Hi Dave

I had something similar the other day (albeit mine is an AJ16), On examination turned out to be a number of leaves had managed to get lodged through the air intake and blocked the filter. Actually took quite a chunk out of the paper element.

Hope yours is easily sorted
Nick

[davidr]

Dave - being a 1990 car yours has the Marelli ignition. This is effectively 2 separate systems - one for each bank. They only come together at the the distributor but even then on 2 levels. The contacts and carbon brushes within the distributor cap can wear so the continuity is lost. Yours may be starting to display those symptoms. Typically this only affects one side of the engine.
Your car will also have catalytic converters and the danger will be that unburnt fuel is being pushed through the hot cat with the possibility of a fire.
If you suspect that your car is misfiring in this way or has even dropped a whole bank I recommend that you do not drive it.
It will still start and run with one side not firing and will even be quite smooth but will not have much power.
If what I've described sounds like what you're experiencing then try changing the distributor cap. Make sure you get a genuine one - the aftermarket ones are nowhere near as good.

[BigBlueCat]

Davidr, sounds about right, I'll pick up new cap and try it out, the quite smooth and low power seem a fair facsimile of what I have. Fingers crossed, I'll keep all informed.

[BigBlueCat]

OK, changed out the dis cap, the old one was a bit scored, but nothing dramatic, now she turns over, but not trying to fire at all. Thoughts.