Hi all

I have a 1984 XJS HE and it has started to misfire after it has run for a bit I don't know why I have replaced the HT leads and all the spark plugs and it still misfires its got me completely stumped any help would be greatly appreciated.

Cheers Robert

 

The only thing I can think of is coil breaking down when hot.

Roger

 

Welcome to the forum. When time permits, please do an intro in the "New Members Area" when time permits.

As Roger has said.

Finding it on a HE is easy enough.

Run the engine, wait for the misfire to present itself, then systematically unplug ONE injector at a time, until the culprit is found.

Cracked insulation on spark plugs, especially the 2 front, and the 4 rear is common.

Plug gap MUST be at 0.025", or a misfire will occur.

 

Another thing to try if the above do not get it. Remove the dizzy cap and check it carefully for tiny cracks, carbon contact in the centre, general condition. Also the rotor. If you have not done so, renewing both is always a good idea.
Bit of a warning: if it does turn out to be the coils (you have two, one in front of the rad, one you can see by the dizzy, wired together in series + to +, - to -) you MUST replace them by genuine Jaguar low resistance coils, anything else will fry the system.
Greg

 

Hi everyone thanks for all the replies

I forgot to mention that if I start it up it runs fine until it warms up and Iv also removed the dizzy cap and found the vacuum advance diaphragm is broken as if you suck or blow on the hose that goes to it you can hear air inside the dizzy.

Cheers Robert

 

OK,

The mud gets thicker.

Take the vac line going TO that vac capsule, unplug it AT the capsule, and stick something in the end, thus plugging the vac supply, so you remove that vac leak as a culprit.

A vac leak that small WILL cause a misfire when temps come up, AND, that dumb 15 minute timer cuts out.

Once that is done, tell us the results, and we will keep thinning the mud, ONE step at a time.

Next will be that famous AAV at the rear of the LH cylinder head. If it fails to close 100%, there will be another vac leak, and a misfire could result.

 

Hi everyone

Today I spoke to a jag expert and he said that the coils never break down and that the misfire could be the IGN amplifier and the vacuum advance diaphragm will have no affect on how the engine runs at all that seems wrong to me

 

Have you changed the dizzie cap and rotor arm as suggested.
The vacuum diaphragm will have an effect under acceleration as it adjusts the timing but will not cause a misfire. It will just hold back the power.
Coils do break down, not often, but they do. If they didn't why would you be able to get new replacements. Is it a regular misfire or random. as if the coil was breaking down when hot it would be regular and all the time. Do you have a spark tester that you can connect between the HT and the Plug to see if you can isolate it to one cylinder. Have you checked the spark plug gaps. Do any of the plugs look different. They should be Tan colour. Have you checked compressions too.

Roger

 

HA.

Coils do wear out, it is NOT a Jag thing.

Vac capsules on V12's toast regularly, that IS a Jag thing.

The definition of a misfire needs clarifying, me thinks.

A misfire is a cylinder or 2 misfiring, easy. Like missing a beat.

Broken spark plug, HT lead issue, Injector being hissy, distributor cap issues, that sort of thing, are a cylinder component.

Some call an engine "drop and go" as a misfire. Now a "drop and go" is like a switch is turned OFF then instantly back ON. Now this can be:

Coil/s
Ign AMP.
Ign AMP wiring, into and out of.
Fuel pump relay.

Basically anything that effects the WHOLE engine, as versus a single cylinder or two.

The vac capsule will NOT cause a misfire as such. The small amount of vac that may leak, will be compensated for by the ECU fueling maps. Running quality as in timing trimming will be the fault here.

The AMP will shut the thing down, with conviction usually, and cannot give a cylinder misfire, as it controls the total ignition system, NOT a single cylinder.

Coil/s usually just go AWOL, and are usually very HOT to the touch when getting bad. Again, they are a system item on these cars.

 

Well if I start the car and drive it for about 3ks it will then start to misfire on 3 or 4 cylinders on the RH bank every time 2 of the plugs looked good while the other 4 on the RH side looked black and wet

 

OK, mud clearing a tad.

EFI loom in the bottom of the V is dying, age/heat are the culprit here.

Causing injectors to "stay open" and hence overfueling the cylinders.