I have bought this 1987 XJS V12 a few weeks ago and did a primary maintenance service to be on the safe side,
New ignition coils, new ignition amplifier (AB14), new plugs, new fuel pump and the rest.
But lately it has developed an akward situation which I cannot really understand.
These past few days I took it for short drives, not more than 10 miles. Engine starts beautifully, runs good but when I
slow down in traffic or coming into a roundabout or even stopping in front of my garage the engine shuts off unexpectedly and won't start again,
Seems no spark on the plugs.
I let it cool down for 15/20minutes and it starts no problem, It seems something is overheating and as soon as it is cool enough everything works again.
Any thoughts on this matter would be appreciated.

 

I'm suspicious of the white shielded wire. That was causing mine to cut out, and it can cause a completely dead engine. It runs from the ignition amp to the ecu and should only be grounded at the ecu end. Heat can cause the insulation to fail, then the signal wire can short to the shielding.

 

Thanks very much for your input Gareth. I was thinking along the same lines. As a matter of fact I have ordered a replacement lead. But what exactly do you mean when saying that it should ne earthed at the ECU end only. I have the ignition amp box earthed on one of the holding down bolts to the manifold.. Is that correct ?

 

Considering the same as permanent preventive maintenance.
Whom did you order from, and is it bulk shielded wire, or an actual part number lead?
For the Electrical engineers, would 16 gauge single shielded wire (7 or 17 strand, aircraft or marine spec) be sufficient in gauge for a new lead to ECU.
1. Ignition amp to ignition coil ? and
2. Ignition coil to ECU in the trunk ?
Rgds
David

 

I recently had a similar issue to you. Mine would run fine for 10 miles or 100 miles, but if I was stuck in traffic and going slowly the engine would cut out and only restart after cooling for 15mins. After consulting the fine folks here the most likely cause is ignition amp (you changed yours), coolant temp sensors (one on each bank) or air temp sensor. I ended up replacing all of these parts and now no issues.

 

Establishing loss of spark, or fuel, or Injector pulse, is paramount before it cools and all resets. NOT easy, but the systems are stand alone, apart from that signal wire, and thats a one way signal only. NO signal, NO injector pulse, spark or no spark, it matters not.

Engine earth strap fiasco around the LH engine mount. Make 100% sure its clean and well connected. It is well documented that they reek havoc. I run a dedicated earth cable, engine TO chassis.

I suggest opening that New AB14, and removing the Condensor looking thing in the corner. It was a noise suppressor back in the day. They dont like heat, and leak to earth, and NO spark is the result.

That shielded wire is standard TV antenna Coax. I dont use it in the engine bay these days. I use standard 3mm wire, and pick up that TINY core wire at the bulkhead connector in the aperture in front of the RH door hinges, under that tin closing panel.

The CTS in the B Bank water rail is the Prime signal to the ECU for fueling, NOTHING to do with spark.

ATS in the B Bank snout is a TRIMMER, and WILL NOT stall the engine in any way.

The SINGLE wire sensor on the A Bank water rail is purely for the dash gauge.

You are not mentioning HT leads, \but maybe you did??

The TPS being out of range can do weird things, particularly at idle and HOT. Age is against that sucker, and lousy adjustment/care by previous caretakers sits high on my list.

Throttle body Black GOO, and then blade clearance of 0.002" is another HOT idle issue,

All that should not take more than 12 beers, get into it.

 

I bought bulk. By my calculatuon it is either 18 or 20 awg. So I bought 10 metres of each gauge through Aliexpress. It was a down to earth cheap price. For the time being I fixed the slide end to the best of my ability. Used heat shrink to seal the joints.

 

My car had a black wire like that and the general consensus was that it shouldn't. If you ground both ends of the shielding it can cause a "loop" apparently, but I'm not an electrical engineer. I know you just redid it, but what I would recommend is to cut the black wire and see if the shielding is still grounded. I suspect it will be, at which point you'll want to remove the black wire. I cut it off by the firewall and taped the end. Its just ground so I didn't feel the need to chase it to the trunk. Then what I did was to cut the shielded wire a few inches, or maybe a foot, from the firewall. Carefully slice the shielding and fold it back over the insulation so that its well out of the way of the core. Then I spliced in new wire. I used silicone insulated wire for the heat protection, but its not shielded. The car runs beautifully now and the hot cut outs have been eliminated. I left enough slack in the wire that I can relocate the amp to in front of the radiator where its cooler, I just haven't gotten to moving it yet.

 

That is interesting Gareth. And I thought that the black wire was added later by a previous owner to bypass the damaged shielding somewhere along the lengrh up to the ecu. I haven't opened the ECU yet to check where the shielding earth is attached to but the way I am understanding you is that the shielding is earthed at the ECU end only and not at the slide end. And that does not make an earth loop.
Is it asking too much if we were to see a photo of the joint at the firewall. I suppose it is all taped up.
Regards, Joe.

 

Hi

JAG530

The White Shielding Wire, is Joined by the Left Hand Intake Manifold and goes to Pin 24 of the ECU

And is only Grounded at the ECU End ONLY (Not Both Ends) Follow Link Below to See Pics

Location of the White Coax Shielding Wire To Pin 24 of the ECU and only Earthed at the ECU End and NOT Earthed at Both Ends