Rough Idle when 1992 XJS V12 Marelli is hot

I have been searching the Forum but have not found guidance on what to look for to address my problem.

I have replaced the Distributor Cap, Rotor, Spark Plugs, Spark Plug wires, and coils. Engine runs smooth as silk at idle when cold and starts easily.

When the engine gets up to normal running temperature, the engine idles rough. Also, it is sometimes more difficult to start the engine when at temperature (Idle is low and sometimes the car stalls.)

Please give me some guidance on what the check to address these two issues - rough idle and occasional hard start when engine is at temperature.

Thanks for the help.

Having just dealt with Coolant Temp Sensor on my car giving me starting issues, if your CTS is not sending a signal, the car will always be over fueling thinking engine is cold.


Oddly enough, this behavior can be described as normal. When cold the engine runs open loop, which is generally a tad rich which results in a creamy smooth idle. Once warm it switches to closed loop which is typically leaner and thus leads to a sketchy idle.

When warm, does it idle rough in park or neutral, or just in gear? In park and neutral, the fuel injection is always in open loop. In any other transmission selector position, it is in closed loop, once the O2 sensors become active after they warm up.


Marelli cars are closed loop all the time once warmed up, unless the special plug at the ECU is used to force open loop.

The CTS is a prime suspect.
If the CTS tests OK I would also check the base fuel map and the inputs from the O² sensors.

Uh, no. The loop is removed to force closed loop in park and neutral for setting the feedback voltage from the O2 sensors in the ECU. The plug is in place at all other times. All the Marelli cars did was change the circuitry in the 16cu ECU to accept the ignition pulse from the Marelli vs the Lucas ignition system. They go to open loop in park and neutral to get a more stable idle. There is information on how the loop works on the AJ6 Engineering website.

How rough is “rough”?

Obviously choppy, like you’d feel if a couple plug wires were removed? Or more like an inconsistent tremble?

If the latter, the problem might be as mundane as a few partially clogged injectors

IMO, a good place to begin here is cleaning the throttle bodies and making sure the throttles are gapped to the proper .002" clearance


And you can’t properly set the throttle plates if the throttle shaft bushings have disintegrated!

The Marelli cars are different. Study the wiring diagrams. I just went through this when I did my five speed conversion. Details are here:

I stand corrected. It would appear that some late Lucas ignition cars between 153560 and 156988 were the same way from looking at the bottom of the diagram. They must have done something to the ECU to stabilize idle in closed loop.

Incidently, on my 5 speed conversion, I added a microswitch to the clutch pedal to inhibit the starter unless the clutch is pushed. I had a bad experience with a car that did not have that lockout. I ganged that switch with another for the inhibit on the cruise control, so it kicks out when the clutch is depressed. I used the inhibit circuitry that was used to inhibit the cruise unless the transmission was in “D”.

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The engine idles rough in both drive and park when the engine is at temperature.

I would call it an inconsistent tremble. Nothing rhythmic about it.

I replaced the throttle shaft bushings about a month ago. No change in idle performance.

Today I checked the gap on the throttle plates. A side was at .002", B side was tight. So, I gapped B side at .002" and then made sure the throttle rods were the same length.

I checked the CTS and the Inlet air temperature sensor (IATS) resistance. at 79 degrees air temp with a cold engine the CTS read 1580 ohms. The IATS read 1510 ohms. Then I drove the car and got the engine warm. The CTS read 165 ohms and the IATS read 555 ohms.

Do these seem about right?

According to the figures below your values look correct.
The tables are for the Lucas engine though, I don’t know if they would be different for the Marelli cars.

Coolant Temperature Sensor - CTS

Temperature Resistance

0°C 5.9 kOhm
10°C 3.7 kOhm
20°C 2.5 kOhm
30°C 1.7 kOhm
40°C 1.18 kOhm
50°C 840 Ohm
60°C 600 Ohm
70°C 435 Ohm
80°C 325 Ohm
90°C 250 Ohm

AIR Temperature Sensor - XJ – XJS V12 HE

Temperature / Resistance

-10°C / 9.1 - 9.3 kOhm
0°C / 5.7 – 5.9 kOhm
10°C / 3.7 kOhm
20°C / 2.4 – 2.6 kOhm
30°C / 1.7 kOhm
40°C / 1.1 – 1.3 kOhm
50°C / 830 Ohm
60°C / 500 – 700 Ohm

I’m inclined to agree. When my V12 is idling, even on a ‘good idle day’, I can unplug the ATS (thus giving a slight mixture increase) and the engine feels and sounds smoother.

If no concrete fault is discovered I would consider removing the injectors for professional cleaning. I’ve done this one several cars I’ve owned, including various Jags, and have always been pleased with the improvement. I always use this service:

Note, though, that I said “improvement” and not “cure”. Every Jag I’ve owned over the last 25 years has had some sort of idle-quality issue. None have ever idled perfectly 24/7/365. It’s always been more like 20/5/300, if you get what I mean :slight_smile: If you go back to decades-old road tests of these cars you’ll find that inconsistent idle quality is mentioned…and that’s when these cars were new.

Over the years some have reported a consistently perfect idle but I honestly think that’s the anomaly. Others have pulled their hair out seeking, but never finding, the always-tremble-free idle.

Just my two cents, FWIW. Over the years I’ve become a bit nonchalant about some things.

It does grate, sometimes, that none of my Jags have ever idled as well as my old F250 truck, equally old Honda Accord, carbureted TR6, and other much more pedestrian vehicles. :slight_smile:


Totally agree. I was one who pulled hair out, although I DID finally achieve that 20/5/300 idle, but gave up on the 24/7/365.

I will point out as many here have also said, these V12s need to be driven. My idle won’t be perfect after a few days of city driving. Then I’ll take it on a 30-60 minute highway drive, and on the way home it idles so smooth at stoplights. They need some high rpm, they were originally designed as a racing engine.

City driving, I notice I am usually at 1200-1500 RPM. Lucky if i hit 2000rpm.

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Totally agree, Greg. My V12 is my daily driver and 98% of that is short-run, city driving. Whenever the engine starts misbehaving in any way the first thing I do is hot-rod the bejesus out of it until I no longer see black crud in my rearview mirror. Besides being lots of fun the engine always runs better. Even a 50 mile run at highway speeds helps a lot, when WOT throat-clearings are not practical or safe…or when the cops are watching :slight_smile:

I dunno, specifically, what it is about the design of the Jag V12 that creates this clogging-up/carboning-up situation. Only two other cars I’ve owned had a similar problem: both Oldsmobiles with the early short-stroke high-comp 400 engine. They would carbon-up and you’d get a lot of ignition pinging. This was apparently an anomaly related to the shape of the combustion chambers…although the exact technical explanation escapes my memory. The popular cure at the time, which I used, was to trickle water into the carburetor to break-up the carbon…and then install water injection.

I digress. It’s Sunday, after all :slight_smile:


I never get black smoke when opening it up on the highway, and from what I’ve seen when I peeked down a few intake ports with intake manifold off last year, I saw some very clean valves.

IMO, I agree minor (or major if you’re getting black smoke) carbon build up does it, and our early generation fuel injected engines just cannot be precise enough at delivering the perfect fuel mixture at idle if the engine isn’t at perfect spec. Especially the HE and Facelift that purposely run lean for better gas mileage. (I never hear about preHE V12s idling rough!)

Older carb cars simply enrichen idle mixture. Newer injected cars have a plethora of electric sensors and sophisticated ECUs so the mixture is near perfect at idle.

Part of owning an older car … can’t compare it to modern technology, even cars from 20 years ago!

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