Hello Everyone,

Here's the skinny;

Car was running rough on idle(more like hunting and surging) One fine spring day last week, it died at the light and i could not restart. Had it towed home. Been cleaning connectors, hoses, distributor, checked FI harness, coil. etc

Have spark fat and blue at coil and plugs. I do notice a drop in volts at crank from 12 to 10v.

i can hear injectors clicking. replaced fuel filter but not pump. new vacuum advance in dist.

I'm at a loss. replaced ECT sensor ohms out fine.

Any suggestions???

Thanks in advance Charlie B

 

I see that you have not had any replies so I thought a shot in the dark is better than nothing, have you checked vacuum hoses , the reason I think of this is that at one stage ,in my car , a vacuum hose touched the exhaust manifold or something close and burnt a hole . I would also look at the fuel pump , perhaps disconnecting a fuel line and seeing if fuel is getting through ok .

 

If you have a good spark and clicking injectors, and it will not start, you almost certainly have no fuel getting to the cylinders. If you cannot smell fuel after trying to start the car for several goes, then that is confirmation.

If you have never cleaned out the fuel system, then that is a good idea; but first, as a quick check, disconnect the A bank fuel pressure regulator at the point just before the regulator, and ask a friend to turn the key to ignition on, and see if there is a decent really strong blast of fuel. It should last 2 seconds or so before the ECU stops the pump. Try this several times.
If the supply is OK, then I suspect the B bank fuel pressure regulator (US driver's side, the one that actually is important) needs changing. If the supply is feeble, then you have a pump/filters/fuel pipe blockage problem in the system. If you look at a recent thread this is discussed in detail and what to do about it;
https://www.jaguarforums.com/forum/x...5-xj-s-163410/ Good luck
Greg

 

Thanks guys. I think it could be the spark plug leads (HT), getting fuel pressure. I pulled the wire from the coil and got a great cracking spark. pulled one of the leads and the spark wasn't so great. i'm gonna try new plugs and wires and go from there.

 

Good move.

Also, remove the cap, carefully, turn it upside down, observe the carbon brush in the centre. They wear away, fall out, just disappear, and that will give very weak spark AT the plugs.

 

Although access is not good it's always better to check spark at spark plugs.

A weak spark with plug out could equal no spark with plug in cylinder.
The voltage needed to jump spark plug gap depends primarily on two factors
1) spark plug gap (0.025" on V12)
2) the air pressure the plug exposed to.

Simply put with spark plugs out at atmospheric pressure less voltage needed to get spark to 'jump the gap' than when in cylinder and exposed to compression pressure (200 to 240PS at normal operating temperature)

 

Here's the latest,

New battery, plugs and coil and cap new coil. losing voltage at coil on crank, spark a little better but no start.

still at a loss??

ugh

 

Losing volts at the coil on crank are classic ignition switch electrical section dying from old age. Very common now. It can be removed, dismantled (carefully), cleaned, and returned to working life.

Also, the numerous battery live terminals under the engine bay are also probably in need of dismantling, cleaning, retightening. There is one on the LH side firewall, where the main cable from the battery arrives, then a mirror of that on the RH side. They are both under a rubber cover, but fairly easy to get at. Disconnect the battery first please.

As a try, run a dedicated power wire from one of these posts direct to the +ve of the coil and see if that spark improves. This basically by-passes the ignition switch, etc.

Next, inside that Ign amp (on the LH inlet manifold) is the module, AND a small cylindrical condensor. Remove that condensor, throw it away, reassemble the amp. Those condensors leak to earth, and since they are connected to a +ve terminal of that module, cause spark issues. Usually NO spark, but 1 had was weak like you have.

 

Thanks Grant. I discovered on the LH post that there is a ground somewhere. I disconnected all the leads to the post and tested it for continuity to ground. the rh post doesn't but the LH does. i'm thinking of making my own junction at this point in a sealed box.

any thoughts??

 

CB
The two firewall posts are LIVE with a direct thick cable connection to the battery positive. If one is going to ground, I suspect that the positive (assuming it is disconnected) is touching the body in the boot. Also, as the post is a though-firewall terminal and is connected to powered looms on the inside, maybe somehow something connected to these is leaking to ground. If the post itself was grounding then your battery would fry and possibly catch fire in seconds!
Greg

 

Assuming that you have a V12 then I had the same thing which turned out to be 2 things failing at the same time.
The AAV (auxiliary air valve) went down and was causing hunting until the car warmed up.
The coolant temperature sensor failed which then meant that the car wouldn't start at all until it had cooled down.
Changed both and no further problems (fingers and toes crossed!).

 

Greg has explained it well.

If you have an earth path on the LH with the cables removed on the engine side, means that the items on the cabin side of that terminal may be going to earth. I am not 100% what is powered off the LH post, but ALL the cabin internals are fed from those 2 posts.

Being a LHD car, the Brown wire TO the ignition switch would come from that post for starters, and if that switch is aged and hissy (as they all get eventually), that could be your earth leakage, and the reason for the volt drop at crank.

I reckon you got some very time consuming diagnosing ahead.

 

Thanks Greg and Grant,

Update;

using my my meter i began investigating. Blue and white wire at headlight relay has continuity to ground, also the red and slate(grey) wire that goes to the bulb out gizmo also continuity to ground. As well as the brown and blue wire on the passenger side relay (yellow) grounds to continuity. I noticed the headlights dimming on crank. new battery.

Love jaguars. keeps you on your toes.

Any thoughts??