Starting suddenly one day, I noticed a near dead misfire on cylinder 6. I've been trouble shooting for days now to no avail. I know the coil packs are fine, brand new spark plugs that are properly gapped, and no other codes being sent. I just replaced the IAcv as I was having problems with it prior to the misfire. New part is functioning as it should. A sound test on the injectors plus how absolutely clean the 6th cylinder is, I'd assume it's getting fuel. Is it possible there is a vacuum leak? I can hear hissing on acceleration that sounds kindof odd, but needs more investigating. I know NOTHING of the vaccum lines on this car and have had it about a month. She's been down for a week. At this point I'm going to need help problem solving this.

 

How many kilometres/miles are on the vehicle?

What test(s) did you perform on the coil units?

 

Editing

Don't pull the fuel rail yet as it can get stuck and when broke free you can bend it out of service

I assume you swapped coil positions

There is the possibility that you swapped the coil connectors on the rear 2 coils, and you won't be the first one

Run your finger on the back edge of the valve cover gasket as a D shaped section may have moved out of position

The fuel pressure regulator will hiss but is not related the the engine RPM , tjere will be a slight drop in the bypassed fuel as more fuel is consumed by the injectors

See page 60

801S TITLE (jagrepair.com)

Note the cruise control vac lines are independent and has its own vacuum pump

What is not shown on the vac diagram is the brake booster and the line going into the intake manifold, it is a banjo fitting

This tester is in the back room at O'Reilly's Auto Part but not in every store so you would have to call first

Expect to run the test yourself and bring heavy leather gloves , These coils are 35 .000 volts and not the normal 30,000 because they're special

 

Alright guys, so ive came to the conclusion that it is a fuel problem. I believe my injector is dirty. My jag has 145k miles on the odo. the coil packs have never been replaced. i have swapped coil packs around and even bought three new ones. no matter how i swap, the misfire stays on cylinder 6. all the injectors sound the same. i just preformed a compression test by pulling fuse number 7 in the trunk(fuel pump) allowing it to run itself dry, and crank with tester connected and throttle wide open. i got exactly 150 psi. i added some lucas fuel system cleaner and octane boost and it seems to of actually made the misfire ever so slightly less pronounced. could a vacuum leak still be the cause of my problem, or are you all fairly certain its a fuel problem? and if so, is there a video anywhere of someone removing the fuel rail and injectors? i cannot find many repair videos on the x300 to my dismay.

 

The individual injectors have a basket filter that can be cleaned or replaced

There seems to be some rusting in the fuel rail based on the materials and water getting in there and sitting for a while

You have to go back together with fuel proof O - rings

After changing the basket filters swap the injector with a different position recommending 1 - 3 and see if problem follows warranting deep injector cleaning

The fuel rail removal from the installed injectors can be stuck so care must be taken that they pop off evenly and not bend the tube out of shape

But a tank injector cleaner may give you results

 

Alright so the fuel injectors have all been rebuilt, cleaned, and reinstalled without problem. I still switched injector 5 and 6 to see if problem followed. I also replaced the coil packs with working ones. added more lucas fuel system cleaner and octane booster. figured Id replace the fuel filter also before sending fuel to those clean injector baskets. once all put back together and ran at high RPMs for a bit(30 min total) it is apparent that i still have a misfire in cylinder 6. it is however, much much less apparent. im stumped, and once more am back to NEEDING to know if vacuum lines could be playing a hand in this at all. its difficult to find any information online. cruise control still works, and is the only definitive thing ive found so far that relies on vacuum for this car.

 

The cruise control is a separate and isolated vacuum source from a pump behind the left headlight

Both the fuel injectors and ignition coils all have the same 12 volts present at all times " sitting " on the devices

The ECU provides a timed ground to fire off the devices

The socket to look at on the ECU is Black 11 for the # 6 plug

And Black 25 for the # 6 injector

look for missing pinching fingers on the sockets

 

hey man thank you, super helpful. So I presume you suspect this is the issue? I've already checked the connections on the ECU and added some dialectic grease. I've never really worked with circuit boards before so I didn't know exactly what to look for apart from anything obvious, but then again the view is quite restricted.

 

Before tinkering with the ECU ensure you do not have a resistance in the wire loom between the engine and ECU at the black 11 and 25 ground paths

You can compare it with a known good cylinder or a couple

This would make the device to under perform

You can have some corrosion on the top of the sockets as the stranded wire goes into the sockets not seen from the bottom of the sockets

There are no splices in these runs



The sockets are available separate from the backshell body

 

so what youre saying is that i should remove the ECU again and look at the area on the socket where the wire is actually pinched into the connector for black 11 and 25? how would i go about removing the socket from the plug without damaging it...

 

When you think how the spark or injection timing works in the ECU the CKPS comes around to the target and gets a reference point

This reference point is electronicaly shifted to advance the spark point on a circle on high RPMS and retarded during the engine start start sequence

The firing order is X , Y , Z , ect

So the time between the 1st cylinder firing or injecting and the 2nd has to be calculated based on the present RPM , the # 2 does not have a mechanical reference and so on

Now we get to # 6

The CKPS signal can get corrupted "to start with or not good enough in the first place by that time and not be able to be used "correctly " on the # 6

The ECU was designed for a V8 so it may have 2 X 4 cylinders which would be 4 Darlington pair switches in this form

the pins black 11 and 25 would be the collector point



Bottom line is you want a good CKPS signal and is a shielded wire to not have this center conductor wire corrupted so it has a shield ground on the rear engine fire wall and the case of the ECU has a dedicated ground strap

The starter mounting bolt to the car frame heavy ground strap should also be cleaned

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