Series 3 4.2 rough idle / warm start problem

I searched in the old posts but didn’t find a definitive answer.

My XJ6, after a major overhauling, rides very well and smoothly. Perfect cold start (under 2 secs).

Two issues, maybe related: when hot, the idle is not smooth. Not so rough, but not so smooth as I believed. Furthermore, on warm start the engine requires always over 5 seconds of cranking.

The mechanic believes that the injectors have to be cleaned, because of the rough idle and the warm start due to the injectors leaking. But I suspect that the fuel accumulator could play a role at least for the warm start issue, despite the fact I read about a modification in 1983 that should prevent warm start issues about definitely.

What do you think? How smooth has to be a 1986 4.2 idle? And about warm start?

Not sure what you mean by ‘fuel accumulator’.

Injectors often DO need cleaning. In my experience, having them professionally cleaned will very often improve general performance, idle quality, starting, etc. Worth a try.

The Series III 4.2 engines never seem to have a glass-smooth idle. A slight ‘tremble’ seems to be typical. Others before us have gone through great efforts in this area with mixed results. One fellow composed a comprehensive list of the various corrective measures. I’ll try to find it.


Here you go !



with all that work done - don’t get lost in preparation; chances are idle will get worse with extended idling. If you get the car roadworthy and take it out for some careful, yet committed driving, you stand a good chance that some gremlins are exorcised, the engine is decoked and all moving parts get used to being used again …

Enjoy that car


75 XJ6L 4.2 auto (UK spec)

Check out groovemans post in the XJ40 list, he has just posted about cleaning his cars injectors very cheaply and satisfactorily.

What is the cold and hot idle, Patrizio…?

The higher the rpms the smoother the idle; do not fall into the trap of thinking that engine temp itself is responsible for idle quality…

Idle is set with the engine fully warm - nominally at 800+ rpms in neutral. Starting cold, the idle should be some 200/400 rpms higher - caused by the AAV. Dropping gradually as engine warms up.

Obviously the higher cold rpms gives the smoother idle - at high rpms the xk engine is dead smooth, even with major issues…

Further; in gear, and AC compressor load, the rpms will drop further - and the idle invariably less smooth…

As for hot start delay; have you checked that the AAV functions as it should? If the AAV malfunction, and particularly if idle is set with an AAV fault - starting may be impeded.

And I trust you are not using the gas pedal while cranking…

xj6 85 Sov Europe (UK/NZ)

The idle is set to about 750-850 rpm when hot, more when cold (this evening I will check but I think it starts from about 1200 rpm).

Surely it becomes rougher as the engine gets hot, but not always: in certain conditions is very smooth, in others is a little rough, not extremely rough but it seems to me it has some misfires, I clearly notice some single events that make the car a little rattle. Let’s say, one every second, not more.

About the hot start: sometimes a little gas helps, but it starts also just only cranking, in about 5 (extremely long) seconds.

Which roughly matches the experiences of others (including me) over the years.

An actual misfire should be detectable by listening at the tail pipes…you’ll hear a muffled pop or thump type of noise. For this the easiest go-to possibilities would be injectors or secondary ignition system: plugs, plug wires, distributor cap, etc.

Fuel injector wiring also merits a close look. The wires often suffer heat-bake. A gentle jiggle of the wires might produce a response from the engine…but under the circumstances at lack of response doesn’t confirm the wires are truly fault-free. You’d have to open up the harness for a good inspection.

In situations like this be prepared to simply eliminate possibilities. You may have a stack-up of minor faults. And, even if the problem is due to a single fault, it might not be obvious and glaring.

Just out of curiosity…

My first Series III (and some others I’ve worked on) had a warm-start oddity. None were fast starters but, if you cranked for 2-3 seconds, released the key, and then turned the key again, the engine would fire instantly. By “instantly” I mean one revolution of the starter or less. This oddness was consistently repeatable. I never knew why it occurred. Some speculated a fuel pressure issue but pre-priming the fuel rail never made a difference.

I’d be curious of yours behaves the same way even though I have no explanation for the anomaly.


And, watch the tacho. Dead steady or a just perceptible drop at times. On board miss detector.

I wonder if it still works. Way back when transistor radios were popular. Little devices. very sensitive tot any frequency. AKA “static”.

Fire the engine. tune the radio off any broadcast. listen to the “buzz” aka 'static". Steady r wavy??? former = ignition good. Latter, not so much, missing fire at times!!!

I start my two cars “off pedal” Each cold idles at about 700. Warm at 500. The Lt1 lopes at times on first fire. Clears rapidly. I think i’ll exercise it today on an errand.

Jeep got it’s outting yesterday…


I think you may have misread my post. In the weird situation I described, pre-priming did not make a difference.

On the Ser III XJ6 4.2 this is easily done by putting the gearshift into “D” or “R” (or anything except P or N, actually) and turning the key to start. This operates the fuel pump and pressurizes the rail…but the starter doesn’t engage


It’s almost the same also for me. I had a similar experience with my Porsche 924 and it was the fuel pressure accumulator, over the years the membrane gets broken and it can’t mantain the pressure so it’s difficult to start the car when warm. Of course we are talking about a completely different fuel injection system (k-Jetronic).

Not dead steady, for sure.

This is likely the elusive ‘xk uneven idle’, Patrizio - and in earlier post Doug stated it admirably; ‘great efforts in this are with mixed results’ trying to eliminate it…

You are welcome to try - but ‘not dead steady’ is about as good as it gets. It is sort of a personal perception; when does idle get bad enough to induce action - and caused by an identifiable fault…

As for the starting; in this phase the xk is very sensitive to air and fuel, and getting it right involves setting the throttle gap, idle screw and a correct functioning AAV. Using the gas pedal upsets the balance, and leads to engine baulking and prolonged cranking. Hitting a sweet spot with the gas pedal is hit and miss, usually the latter…:slight_smile:

Setting the idle is with engine hot, ‘750-850 when hot, more when cold’ is simply wrong procedure. The ‘idle set’ and the ‘actual idle’ at different engine temps are two entirely different beasts.

Fuel mixture and air ingress during starting is varied deliberately with engine temps, the first by coolant temp sensor inputs - the latter by the AAV during warm-up. With hot idle properly set with the engine fully hot; the engine management system automatically varies the mixture and air to suit the engine needs at actual temps…

If idle is set at other temps; the engine needs will not be met, as is the case if the gas pedal is used - and the engine will protest. With hot start; the AAV is closed, only throttle gap and idle screw providing air - which regulates engine rpms…

That said; apart from the temp information from the CTS, which, together with air flow information from the AFM, tells the engine the amount of fuel to deliver; a bypass screw on the AFM allows for mixture setting.

Which is a critical factor in starting (or running for that matter) - with fuel pressure as an additional factor. The latter is fixed by the fuel pressure regulator - but as there is no feedback; faulty fuel pressure means incorrect mixture…

As the mixture is gradually leaned out with increasing temp; a very hot engine may be to lean for its liking. Cranking enrichment is built into the ECU, but is again temp dependent - and again require correct hot idle setting for proper function…

While a 5 second starting delay is no great shakes, it is principally too long - and something is wrong. ‘What’ is more uncertain - but verifying idle set is a first step. But other factors mentioned above may contribute - to e pursued in due turn…

As an aside; a one-way valve is fitted between the pump and fuel filter. It maintains fuel rail pressure whenever the pump is turned off, like when stopping the engine. Fuel rail is then ready to deliver as cranking starts. However with a failed valve; pump has to pressurize the rail - as is the case with any leaks. Including leaking injectors - which in itself may cause problems by excessive fuel delivery to one or more cylinders…

In short; engine behaviour during starting, idling and driving will usually give some indications for lines of inquiry…

xj6 85 Sov Europe (UK/NZ)

As a first soft measure I bought two cans of professional injection cleaners and I will give them a try.

I own classic cars from 2003 and I always been told that these cleaners can do more damage than benefit, but the mechanic that follows my cars is not of the same advice and states that the right professional product can do a very good job. Before dismounting and bath-cleaning, I will try (also in my Carrera, despite it’s fucking smooth at every start, no matter how many months you leave it parked, a really different world).

I have to plan a good trip to make the cleaner do his job, what a pity… :grin:

P.S.: according to my nose the car is everything but lean.

The point is that injector cleaners are not engine ‘candy’, Patrizio - excessive use may harm the engine…

Some oil companies routinely add ‘Tecron’ or equivalent, in suitable quantities to their petrol. Further addition is likely counterproductive. ‘More’ is not ‘better’…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


yes, a Porsche flat six with anything from K jetronic on is smooth as silk from the turn of the key. That’s why so many owners ruined their engines …

As your car is no cat no o2 sensor I’d guess that Browns Lane tuned them pretty much like their carbed engines. The fact that Euro spec engines hardly ever required stakedown kits at the exhaust side supports this. So I wouldn’t be surprised if your car were running rich.

I’d say the tell tale test is pulling the HT wires on a hot running engine. For every wire idle should drop clearly and get lumpy - and for every wire in the same way.

Everything else is more for a beauty contest or some personal quest.

Good luck at any rate


75 XJ6L 4.2 auto (UK spec)

My apologies, but HT wires stand for? :smile:

I don’t know if it’s related, but I’m afraid I discovered an issue I had on the 911… look at the picture:

This is the throttle micro switch, right? The one that detects the throttle closed? If so, it’s disconnected (and a pin is missing). Looking at the diagram on the Haynes

And according to the cable colours, it should be the no. 349. The manual is not clear to me, it refers to the no. 310 as throttle switch but is it an alternate part for the no. 326 + 349? Or there is another throttle switch, in addition to the 326 (full throttle) and 349 (closed throttle)? My car is an euro late XJ6 with no emission control, what is the correct configuration?

Going further: if the mechanic has left the switch disconnected, the car never detects the closed throttle condition. The fact is quite absurd because a Porsche specialist made the same mistake during the last maintenance of the 911 and had to enrich the car so much to keep it idling and my nowadays mechanic blamed him so much for this… I think that the functioning is indeed not the same because the 911 without that switch operating idled at 1100+ rpm but the Jag is idling at the correct rpm - surely is quite rich, according to who follows.

`The only difference this switch makes in running is that it cuts fuel above some 1400rpm when the throttle is shut.
But you can connect it either way (the two transparent spades are correct) and make sure it just clicks when the throttle closes. On European XJs this is the only switch.

HT wires are the ignition (high tension) wires, at the spark plugs. You can do the same test by removing the connectors at the injectors; I am scared of sparks so that‘s what I do.
They do run lumpy if not 100% perfect, and most are not perfect, even many that are well cared for.

326 must be the full load enrichment switch (Only if you are correct - I have no idea what that switch in the three-way box is supposed to be). Which is hooked up to a vacuum line at the rear of the manifold. That one‘s important.
Mine stinks too, but leaner makes it worse, and so does richer.

349 is the microswitch you photographed according to my legend and 310 a nonexistant throttle switch I believe. Either 310 or 349+326 as a system, possibly.

As already mentioned, that throttle switch is not on the USA cars but on the European and Middle East cars.

I looked it up in my Jaguar Series III Parts Catalogue and the part number is EAC2516. I checked on eBay and there was one listed by a seller in Australia. You can find it by searching “Jaguar EAC2516” if it turns out that your switch is not usable and you want to install a good switch.

I have no affiliation with the seller other than as a satisfied customer for a few hard to find OEM bits and pieces that I purchased from them for my Jaguars.


So far, so good, Paul, but that switch isn‘t responsible for the roughness. Also, it is a standard roller microswitch, normally open. A decent Omron won‘t cost much.
I wisch it would do something useful at idle!

As David says, Patrizio - the ‘European’ and ‘US’ set-up differs. On the former the tthottle swich does not influence idle or mixture.

Another difference is that the ‘European’ has a ‘full throttle switch’ enriching the mixture under high load. This is operated by vacuum - and if faulty may run the engine constantly rich.

While the mixture is adjustable by the AFM bypass - but it has a limited range. And CO measurements must be used for reading and correct setting of mixture…

As an aside; the long, straight 6 is prone to harmonic cycling set off by minor anomalies. The 911’s boxer engine is inherently more stable - and is a more modern construction as well…

xj6 85 Sov Europe (UK/NZ)