Rough Fluctuating Idle

I replaced the fuel pump on my 1986 XJ6 and couldn’t get the car to run afterwards, I cleaned and replaced all the seals on all the injectors. The injectors were in very rough shape, the car will start up now but idles rough, engine shakes and the rpm fluctuates. My question is if the suspected problem is with the injectors or if there are other possible problems. I have read a bad coolant temp sensor can throw things off, I have one to put in but want to know thoughts of what the problem can be. The car will randomly stall if idling long enough.


are you sure the pump is correct and working properly?

Good luck


75 XJ6L 4.2 auto (UK spec)

Did the engine run as it should with the old pump…?

Three preliminary tests; place gear lever in ‘D’ and turn key to crank - you should hear the pump running. Remove air filter and prop open the AFM flap - pump should run with ign ‘on’. With the pump running; open filler lids and check that fuel is returning - confirming adequate pump pressure…

If pump is not running - find out ‘why’. It’s a quirk that sometimes the engine ‘may’, when started, run on gravity feed without the pump - but veeery poorly…:slight_smile:

The only way to verify proper pump function, if running, is to run fuel pressure tests. If correct, the injectors themselves are not byond suspicion…

The CTS is easily tested, but if faulty - there will be some difference with a hot and a cold engine.

The injector seals are there to prevent external leaks; they do not interfere with actual injection - which may be the problem.

xj6 85 Sov Europe (UK/NZ)

As I have recently learned, do not overlook the Spark and Timing part!
I permanently installed a fuel pressure gauge at the fuel rail. It helps rule out any fuel issue with a simple look. The book calls for about 35PSI.

While you are at it make sure you don’t have an air leak somewhere.

Good luck with it. Getting the idle right with the XK engine is not all that simple.