Hi All,

I have a very slight stumble/misfire/vibraiton on my 1989 XJS V12 that has been there for a while at idle & low rpm, & I feel like has got very slightly worse recently so am trying to diagnose it.
To describe it more - standing behind the car when it's idling at maybe 4ft form the boot lid, I can hear an interruption to the smooth exhaust note and feel a slight gust of wind from the exhaust when it happens.
When driving the car I can't tell it's there. Sometimes I feel a slight vibration when stopped but that's the extent of it. power, economy all seem to be fine.
Holding the rpm above say 2000rpm & it also feels like it disappears, that could just be because it's a v12 though and is just being drowned out by the general smoothness.

It's not isolated to either bank - it's equally spread across both exhausts.

it's not a full dropped cylinder type misfire - disconnecting 1 injector is a very noticeable difference. I'd describe it as a 11.9/12 cylinder misfire if that makes sense?
It's felt a little bit worse recently after a big road trip.

The only diagnosis i have done so far is that when i disconnect the intake air temperature on the left hand side intake trumpet, the idle does feels like it does get a bit smoother & less bad, although is still detectable.
Servicing wise i've replaced lots of stuff/the usual suspects in the last 3k miles - cap, rotor, coils, leads, plugs. new crankshaft sensors. replaced intake gaskets, fuel hoses & injector loom, tested the injectors, throttle linkage setup, Oil etc all good, bit of an Italian tune-up and it accelerates well.

This is a UK car 1989 so no 02 sensors.

Anyone have any ideas?

 

The "could be" list might get a little long. Subtle faults can be so hard to find. Gah.

I'd revisit the injector situation.

"Tested the injectors" ....how? What was checked?

If you have one or two that are just slightly out-of-spec you may have a slight roughness....particularly at idle.

I've used Jaguar Fuel Injector Service many times. David measures the flow rate of each injector before and after cleaning and can provide replacements for any that won't come up to scratch.

https://www.jaguarfuelinjectorservice.com/

That's my Sunday morning, bleary-eyed, first-cuppa-coffee thought. Let's hear what others chime in with.

Cheers
DD

 

Did you have this problem before you replaced all of those parts? My car had a similar problem and I traced it to the coils. I had replaced the coils trying to correct the problem bit it turns out I had purchased incorrect coils (cheap ones) and the connector inside the output terminal was wrong. There was just a prong inside of the terminal instead of a sleeve and the spark had to jump between the prong and the lead to the distributor. It created a slight miss at idle but would rev just fine. I think the original coils I replaced must have been incorrect also. The incorrect coils were purchased on line and stated they were the correct coils for the 1995 6.0. Wrong.

 

Worth cleaning the resistor pack connector and terminals and the injector plugs on the injector loom and prongs on the injectors themselves.

 

Does it occur at idle in park and in drive? Mine is good at idle in drive but has a similar idle issue to yours in park, (HE 1983 UK, not a marelli).