Gents,

I’ve got a miss and any diagnostic advice would be most welcome. The chief complaint is that it causes shakes at idle (stopped at a stoplight, for instance), and I lack that V-12 smoothness. A few notes:

1. The miss doesn’t seem to be there on the first 2-3 minutes of operation, regardless of engine temp. For instance, if the car is stopped for a few minutes and then re-started warm, the miss is absent and then returns after a short distance.

2. I replaced all the plugs and wires a year ago. I pulled plug #3A to take a look and the porcelain looks nice and toasty, with no excessive carbon or the like.

3. I’ve done many Italian tune-ups, run STP injector cleaner through several tanks, and driven the car 1,500 miles on the highway in the past two weeks.

4. She averages 15.7 mpg on the hwy.

5. I don’t smell unburned fuel in the exhaust

6. I did have a coolant leak this summer—I don’t think the engine ever overheated, but I did cuss the Jaaaag for five straight days as I replaced the dozen coolant hoses and two temp sensors.

Any other thoughts? What other basic diagnostics should I do to narrow this down?

Many thanks in advance.

Morgan

 

fuel consumption could be a little better but we don't see your style of driving
as for the "miss" don't bog yourself down with 100 & one suggestion what it "might" be, spend a few $'s and take it to a dyno workshop & get a positive answer.

 

Plugs and leads, GOOD.

Language in #6, BETTER.

I sugest, just due to unknown"

Cap and Rotor.

After that, IF its still on 11, leave it running, carefully and quietly unplug ONE Injector at a time, and note the happenong. This will noarrow down the Bank, and the Cylinder.

Then its a matter, of"

Dud spark plug on that cylinder, it happens.
Injector sticking, it happens, and I have 100% got that fixed with a SMALL Tack Hammer, and TAP, and I mean TAP that Injector metal section with the engine running. NO BASHING ETC, just a light tap, tap, tap, and as I said, 100% success for me.
The EFI loom, NAH, maybe, but I doubt it, as these things fire the Injectors in 4 batches of 3, so you would have 3 out I would think, and that would be a shocker to drive.

As said, dont throw parts at it except for required things, plugs/leads/cap/rotor is always the #1 for any V12 with issues.

ALSO

You are not saying what year this thing is. Your profile has US, thats it. I suggest adding the Year etc to your Sig via the CP tab, saves a lot of back and forth.

 

Oh yes, sorry, it is a 1990. Headed today to the shop to try to do a bit of sorting on her, so thanks for the tapping advice. I’ll give that a go, then start pulling leads and busting knuckles.

 

If it is just at idle, I would first check to make sure your throttle valves are properly set to the correct clearances. If you have an old inductive timing light, you can check each plug lead to see if all cylinders are firing.

 

It appears a Washington, DC rat has made a feast of one of the ignition wires. Surprisingly, however, that doesn’t seem to have solved the issue.

I tried unplugging each wire one by one, and that was too subtle of a difference for my powers to detect the miss.

 

get a dyno tune

 

in north america, this would be a difficult task.

i dont know of any shop that uses a dyno to tune Jags.

 

any dyno can tune and make or model of car, jags are no exception...........so what state is Murphy in?.........

 

THANK YOU.

That makes it what is known as a Marelli Car, and that Marelli system does have its own issues.

Most of them are Bank related, not cylinder, so I stay where I was.

I know near zero about the Marelli system, other than what I have read here and elsewhere, which you can also do.

Bugga about the rat.

As for the Dyno, same in downtown Adelaide, none of the very few operators want anything to do with the V12, and that is 1st hand from many years ago, and I would suggest even worse these days.

 

Check that the inlet manifold nuts, securing the manifolds to the head, are tight. 7/16ths A/F socket required.

 

Is it actually a miss or maybe just rough idle?

Check that idle switch is correctly adjusted. The idle switch adjusts ignition timing for closed throttle running in conjunction with engine speed and coolant temperature. Might explain what you are reporting if it isn't working as intended.

Doug and Warrjon helped diagnose this when I had a similar issue. Make sure your throttle butterflies are properly adjusted, and then adjust the switch at the switch not the rods.

Attached is a copy of the Marelli supplement you might fine helpful.

 

Thank you for that Marelli supplement. She’s definitely a miss. The car idles fine until it warms up, then there’s a noticeable shake. Could even be two cylinders misfiring, although I would think I’d have a bigger drop in gas mileage or smell fuel in the exhaust. .