my 84 xj6 has a really rough idle, it hardly stays running as rpms go under 7k at times. It is kind of chugging like a hot rod i guess. Any clues as to what i can do or where to start?

Thank you in advance, i am new to jags and trying to take care of any of the minor stuff myself so i can afford this project, I just love this car.

 

rough idle:
1) dirty throttle intake, or butterfly out of adjustment, ( .002 )

2) coolant temp sensor gone bad...disconnect green wire on top row of sensors, passenger's (intake) side of engine and see what happens, engine should cough and lose its breath if good,

3) oxygen sensor needs additional grounding, (typical of 1983 thru 1985 XJ), add a test ground wire to body of oxygen sensor (where you would put the wrench), and touch the other side of wire to a clean chassis bolt, see what happens, {careful the O2 Sensor and manifolds get dangerously HOT}.

4) clogged fuel filter, when was the last time it was replaced...
5) clogged air filter, open up canister and shake it to clean it if too dirty,

other possibilities I'm just not familiar with: for example checking the AFM is something I've never had to do in mine.

there are other reasons, but I like the easy steps.

 

Thank you so much! At least I have somewhere to start. Is the fuel filter in the boot with the pump?

Thanks again, that is a great punchlist for me to get started with!

 

Are you getting dark exhaust fumes with this rough idle?

Fuel filter is in the spare tire well, right front corner

Cheers
DD

 

you welcome! Doug will give you a list of the difficult steps :-))

yes, fuel filter is in the boot, a silver-grey canister made by Bosch.

if you're keeping the car, order the Jaguar Shop Manual CD, best investment you'll make.

 

^ This is the best place to begin. ^

After cleaning the throttle body adjust the idle speed to give about 900 rpm in "P" with the climate control on. This setting is a bit on the high side but my experience is that some Ser III 4.2 just won't tolerate a lower idle speed....for various reasons.

Idle speed is adjusted at the air distribution block

Idle Speed Adjustment







The coolant temp sensor often causes problems but let's clarify this a bit.

The device on the water rail with a single green wire is the sending unit for the dashboard temp gauge.

The coolant temp sensor for the fuel injection has two wires. It is just aft of the temp gauge sending unit or, in other words, third from the front on the water rail.

Unplugging the sensor should result in immediate engine stall-out but this doesn't prove the the sensor is good. However, bridging the contacts (use a paper clip) inside the connector will take the sensor out of the picture and trick the computer ("ECU") into thinking the engine is warm. If the rough idle goes away, you're on the right track.

The sensor can be tested if you have an ohm meter:
Testing the Coolant Temp Sensor

Cheers
DD

 

Doug is correct, I was thinking of my '65 S type which has a somewhat similar CTS and the way to test is to disconnect the green wire. Things can get confused.