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.
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.
?
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
3
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.
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.
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 āDā.
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.
According to the figures below your values look correct.
The tables are for the Lucas engine though, I donāt know if they would be different for the Marelli cars.
Iām inclined to agree. When my V12 is idling, even on a āgood idle dayā, 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. Iāve done this one several cars Iāve owned, including various Jags, and have always been pleased with the improvement. I always use this service:
Note, though, that I said āimprovementā and not ācureā. Every Jag Iāve owned over the last 25 years has had some sort of idle-quality issue. None have ever idled perfectly 24/7/365. Itās always been more like 20/5/300, if you get what I mean If you go back to decades-old road tests of these cars youāll find that inconsistent idle quality is mentionedā¦and thatās when these cars were new.
Over the years some have reported a consistently perfect idle but I honestly think thatās 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 Iāve 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.
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 wonāt be perfect after a few days of city driving. Then Iāll 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.
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 safeā¦or when the cops are watching
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 Iāve owned had a similar problem: both Oldsmobiles with the early short-stroke high-comp 400 engine. They would carbon-up and youād get a lot of ignition pinging. This was apparently an anomaly related to the shape of the combustion chambersā¦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 carbonā¦and then install water injection.
I never get black smoke when opening it up on the highway, and from what Iāve 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 youāre 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 isnāt 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 ā¦ canāt compare it to modern technology, even cars from 20 years ago!