Just bought the car with a rough idle that I believed was a simple vacuum leak. But it doesn't seem to be. The car is a 4.2 with 120k miles and seems well cared for.

Symptoms: starts at once but then idles rough. Idles in neutral about 1000 rpm, in gear about 750. If I hold revs at 2000 rpm in neutral when cold it continues to misfire a little. If I start driving it runs extremely well and smooth, good acceleration, no misfires or power loss, but when stopped it goes back to a very rough idle, a little better when hot.

At a light with a cold engine on one occasion, the car stalled and was difficult to restart, seemed to be flooded. It hasn't stalled when hot.

Checks so far, self and trusted mechanic: one poor injector seal, replaced, ether spray found no other vacuum leaks, no improvement to the idle. Valve settings good, Compression good. Plugs all look OK. Inlet vacuum measures poor.

My mechanic (experienced with sporty English cars) suggests that the timing chain is noisy and therefore worn and is the cause of the poor inlet vacuum. A test to confirm this is the same price as replacing the chain, so a simple choice. But in all the lists and contributions on this forum I don't see any that identifies a worn timing chain as a possible cause, and I've never run across it before.

I'd greatly appreciate any comments.

 

The "could be" list could get very long.

Easy starting point: check that the flexible duct between the throttle body and the air flow meter is secure and 100% leak free

More later

Cheers
DD

 

suspects of stalling at idle: Like Doug said, the list is long:

1) Coolant Temp Sensor ( replace / cheap part ). First thing I would replace with a new one regardless.

2) Fuel Regulator needs tested

3) Vacuum Leak. Use a smoke detector or cigarrette smoke to find a leak. Old hoses develop cracks.

4) If engine stalls when applying Brakes to stop at red light, then becomes hard to start, Check Spark plugs and spark plug wires to ignition.

5) Thermo Time Switch ?? another sensor.

6) failing Ignition Module ( Delco GM Part).

7) Vacuum Leak again.

8) clean shiny the Grounds grouping bolted to rear top corner of cylinder head, intake side of head near aluminum ( aluminium ?) intake elbow.

9) failing Ignition DASH SWITCH that shuts engine off. They do that when worn.



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I agree that CTS replacement has fixed several problems and that is so easy to replace.
The warm engine restart problem is a recurring theme and can be really frustrating and it makes it sound like the engine is flooded. (if you have gas leaking through the hose that runs from the fuel pressure regulator to a mysterious little plastic piece on your fuel rail, your have a hole in your regulator and you need to replace the it before you have a car-b-q).
I tried replacing the delco ignition module (taking it apart and putting in a new module) and still experienced the issue. I do believe the distributor generally excellent and changing it can cause any of a number of new problems, so I ended up putting in a MSD 6A ignition control module and the corresponding coil which is a relatively easy way (though maybe $400) to bypass the existing module as it runs from the coil to the distributor directly. Don't remove the existing module since you will create a large vacuum leak. I mounted the 6A under the air filter (or to the front of that space). That ended up preventing the hot start issue for me. Then I did the O2 sensor fix with a ground wire and heating wire -- and the idle immediately became so smooth my wife though the car had stalled when she drove it. I'm not sure why it made such a big difference -- I realize the O2 sensor is really modulating the mixture at low rpm, but it was significant. I also grounded the O2 sensor to the big ground that connects to the negative battery terminal and that might also have made the difference. I think bad grounds in my car were a source of poor idle as well. I never used a meter to check for grounding issues, but i wish I had.

 

correct me if I'm wrong but the 1986 XJ-6 O2 Sensor does not need a separate Ground wire, they came already grounded with a separate Ground wire.

I use a Ground strap in my 1984 and it resolved and smoothed the idling, but the O2 Sensor is the old 1-wire Bosch type used up to 1985.
.

 

Hi Jose
Don't know the details of when they switched from the single wire, but that's the one I had on my 1986. I also have seen someone state that the 1986's have the ground wire. But the day they put my sensor, they were still using a single wire.

ADDED a bit after the above statement

Just found the JagBits answer: "LAMBDA OXYGEN SENSOR, 1-WIRE, FOR 1980-1987 XJ6 AND XJS UP TO VIN 134285 W/1-WIRE SENSORS. SAME AS DAC2549 (AGU1108)"

Jagbits knows...

 

ok DK65, I understood the single wire sensor was until 1985, then Jaguar changed to 2 or 3 wires, (with the third wire being a pre-heater ?)
Very confusing.

But yes, grounding the O2 sensor does the job but does not explain the stalling.

 

Just found the JagBits answer: "LAMBDA OXYGEN SENSOR, 1-WIRE, FOR 1980-1987 XJ6 AND XJS UP TO VIN 134285 W/1-WIRE SENSORS. SAME AS DAC2549 (AGU1108)"

And I don't why I dove into this but this is what my research indicates. Series 3 cars left Coventry with a single wire and no ground wire. The XJ40 left the factory with the 3 wire (ground and heater + signal). It appears that Jaguars never left the factory with the 2 wire. BUT, California and other markets may have started requiring a modification to include a ground wire which was added to all cars in that market by the dealer as required. But some markets never touched the single wire. Now I think this makes sense for the seemingly inconsistency and i don't even know for sure if this is all true, but it seems to have a lot of truthiness to it.
David

 

For the USA market all the Series III 4.2 cars had a single wire sensor EXCEPT the 1987 model year. For 1987 model year the heated sensor was used on all USA market cars. Three wires. At the same time the oxygen sensor replacement warning lamp was removed from the dashboard.

As part of an emissions recall some 1986 model year cars were retrofitted with the 3 wire sensor. I think these were California sold cars only, as mentioned

I refer here to USA-market model year designations....which were sometimes different than other markets

Cheers
DD

 

Interesting to me. Beyond the smog issue, having the 3 wire improves the idle considerably. I wonder if the cars without the US required equipment were just fine without the 3 wire? The US jags have the air pump and so many other things that can impact idle. I would love to know what the car felt like without the smog stuff and either a manual transmission or a better 4 speed transmission. One can dream