EMERGENCY - Terrible Idle/Backfire

I drove my 1988 XJS 5.3L V12 all day today and clocked a collective 20-30 miles. It ran great today and was averaging about 22MPG. Once I got home for the day, I parked the car and went inside. About half an hour later, I was going to go for a cruise. Once the car started, it idle incredibly rough (shaking the entire car). I tried accelerating to see if that would change anything and around 2,000 RPM’s it started to make a backfire popping noise from the right bank. I immediately shut the car off.
Could the timing chain have potentially skipped some teeth?
What should I check first?

could be a lot of things, but I doubt it’s catastrophic like a timing chain. Sounds like suddenly not enough power for spark? Coil? Or suddenly not enough fuel? Fuel pump?

Brand new fuel guage. Reads half a tank. Do you know what could cause a sudden change in the ignition system?

Only thing that will cause a back fire is a plug not firing and that raw gas gets pushed into the exhaust where it will get ignited. Voila, your back fire.

Update: I started the car after waiting about an hour. It was reving just fine now, but when I drove it, it started having issues instantly and wouldnt go much faster than 20 mph. It did back fire when the RPMS went higher than 2000.

Breakdown in the High Tension parts, rotor,leads etc

Marelli ignition or Lucas? If Marelli, please inspect cap and rotor and post pics if possible.

It is lucas ignition. I will inspect the rotor and under that cap real quick

When did you last change the coil? Do you still have the dual coil system?

When bad electric components, like the coil, get hot, they don’t work as well.

Inspection certainly won’t hurt, but I doubt if the Lucas cap/rotor is acting up.

Do you have one coil or two? If two, you might disconnect the remote one mounted forward of the radiator. It won’t rev over 4000 or so that way, but if it runs fine up to 4000 that’d tell us something.

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Was any work done to the car recently? If so, what and where?
It sounds like ignition, but I had exactly those symptoms, intermittantly, when my 6CU ECU went bad. Your car should have the 16CU, but it wouldn’t hurt to check. One or two bolts, and the ECU will come out far enough to check the markings on the side. The 6CU solder joint issues are infamous.
Barring that, beg or borrow an ignition scope and look at the secondary ignition waveforms, especially a parade of all the cylinders. You can rule in or out ignition issues quickly.

Yeah, unless the rotor suddenly broke, cap/rotor issues usually happen slowly. Worth checking still.

But I’m still leaning towards coil. If it’s original, it could have suddenly croaked.

I’m also thinking ignition amp, but that usually causes a no start or stall?

As the car was running fine before, and still runs ok when cold, i’m thinking electrical fault somewhere with the ignition system.

Sorry for the slow response time. I had a band concert last night and like any good American I partied the night away.
This morning, I inspected the distributor and all seems to be good. I started the car (cold) and when I revved the engine there is no backfire, but as soon as I drive, the engine proceeds to die. I’m going to try replacing the coil in front of the radiator with the one in my parts car. I will also try replacing the ignition amp with the parts one. Is the second coil located on the throttle tower? Or is this some kind of amplifier?

Colin, in the original Lucas system on your car, the main ignition coil is just ahead of the throttle turntable. That coil, by itself, should allow your engine to run fine up to around 3500 RPM. The second coil, placed horizontally in front of the radiator, allows the engine run at much higher RPM. However, if the second coil is failing, then your engine will not want to run. So, disconnect the second coil, and see whether your engine will run normally up to about 3500 RPM. Report the results. If you want to change several parts, change things one at a time, and drive the car before changing something else.

Just tested it with the front coil unplugged and it acted exactly the same

just replaced the ignition coil on the throttle and still the same. time to try the amplifier

And the same results from the amplifier. I have noticed that the car seems to hesitate when being put into drive and reverse. Is it possible the transmission is causing the engine to stall out?

Transmission failure usually results in the car not moving. So no. If you have a timing light, hook it up to any plug wire on the distributor, and set it on top of the engine facing you, so you can see it flashing. Start the engine and increase the RPM while you look at the flashes. Are they steady or do they become intermittent when the engine starts to misfire. That should give you a direction to follow.

I can try this, but the engine is no longer misfiring at all. I can rev it as high as I want with no issues now. But as soon as I put the car in reverse, it takes a second to engage and then drives 10 feet back. Then I put the car in drive and it will move a little but as soon as I try to increase RPM’s/Speed it will drop RPM’s and stall out


Back in my junk racing days, it was termed as “shooting ducks”. An ignition issue. In our flat head Ford V8’s, most of us had a spare complete with leads, ready to swap in…