The magnetic pickup unit

 

I have a 1994 XJ 12 great car when running...1 problem is you need to find a sweet spot coming out of park in order to engage starter/engine. Now it seems as if the engine is getting no spark the engine fan turns but no spark. Some days it will start and run other days no??? When running it rev's up and down on its own....does anyone know if it could be vacuum leak, crankshaft sensor, etc. What would be the best thing to try first based on this info?

sometimes will start and idle up and down when in park/driving.

Need help on the solution any help from the jaguar pro's would be greatly appreciated.

thanks,

Mark

 

Hi everyone I am new to this forum and was looking for similar answers on a rough idle and stall issue as well. I recently bought a 1987 XJ-SC 5.3 with the Lucas distributor and have gone over it with a fine tooth comb. I have never had it running right, as it was a purchase with intent to fix. I have replaced plugs, wires, distributor cap and button, fuel pressure regulators,injector o rings,relay switches, and now even the ECU with no gain. My buddy and I were able to trick the ECU to think it was getting 20 hg of vacuum to it and it ran like a purring kitten, so I thought the ECU was bad. I replaced it and still same problem, so I started tracing down vacuum lines to see if that would fix the problem, and the engine only seems to produce 5-8 hg in vacuum. The long vacuum line from the engine compartment to the trunk holds vacuum from one side but not from the trunk side so I think it has a regulator along the way. so after more frustration, we checked the distributor vacuum advance ( capsule to you Brits) with a hand pump and it won't hold vacuum. Perhaps this was the problem all along but does this sound like I may have isolated the cause? I m getting frustrated, and could use some help from you all, the V12 is a monster with its many complex systems, and is a step up from my old XK 6 cylinder!

 

The vac line from the engine cross pipe to the ECU should be completely vac-tight from either end. It should be a completely clear pipe with nothing in it in nthe way of valves etc etc. Blow it out with some high pressure compressed air. I seem to recall Grant Francis had a mystery problem that this solved. If having blown it out there is still the same symptom from the pipe, it must be fixed it as this has a very good chance of being your problem. The vac leak could be:

• In the two short flexibles that connect each end of the cross pipe to the manifolds,
• in the flexible between the cross pipe centre spigot and the solid pipe running under the car,
• or the flexible at the other end.

The vac capsule MUST be changed for decent fuel economy, but a failed one will not stop the engine in the way you describe.


Ggreg

 

I don't think that is a valid test.Vacuum applied to the line at the ecu end will never hold a vacuum as the engine end is open ended hose/pipe. It should hold in the other direction as it is not an open ended hose at the ecu.
I also thought some of the advance/retard capsules incorporated a bleed hole but I am not certain of that.

 

100% correct. I should have mentioned that I was assuming the vac-holding check had been done with the line blocked at the other end. There will always be some air leaking into the manifolds though the AEV and the throttles, naturally.

Apologies
Greg

 

What, exactly did you do to get this result?

Greg

 

Thanks again everyone for the quick replies, I can see you will all be a wealth of experience. To expand on my description of what has been done, while checking the long vacuum line between the engine compartment and the trunk I did have it plugged on the receiving end both times. Later last evening I ran a vacuum line (new) from engine to trunk ECU without improvement, still only generating 5 hg. The way I "tricked the ECU was to use a manual pump w/guage connected to the MAP tube pressurized to 20 hg, and the car starts and idles smoothly without smoke or even a stumble. Once I remove the fake 20hg it stalls. That is why I bought a rebuilt ECU thinking that would solve everything but when installed it changed nothing. I have inspected all the vacuum lines and traced them/replaced sections that needed it, without any change as well. the Vacuum capsule was checked by applying a hand pump to the nipple and we couldn't get it to build pressure. I really don't want to keep buying part after part as diagnosis, and am tired of failure!

 

Yep,

Remove that vac line at the ECU, blow some serious compressed air through that line from the engine end. Have your buddie hold that hose in the boot inside a tin can or similar, as sometimes the black gunk will stain the carpet, and that is simply not good.

I used a large syringe (guy next door uses them for whatever), and simply applied my own vacuam to the ECU in my XJ-S and the wife hit the starter and away we went, and I could "play" with the fueling quite easily. This led to the restricted hose being found and sorted.

The vac you are reporting is obviously waaaaaay too low.

The list is long, but some prime i have had over the years are.

The inlet gaskets, they are well documented as a leaker, and sometimes simply tightening the inlet nuts sorts it, sometimes replacement is required.

The throttle discs are full of GUNK, so clean them WELL, and I mean WELL. Reset the disc clearance at idle to 0.002". This is critical.

Ensure the engine is idling at 650RPM, or better. Too low will kill the vac, and the engine.

The AAV has a 13mm hex head bolt sticking out the side. This is the HOT engine idle adjustment, just in case you knew not of its existance. DO NOT mess with it when the engine is cold.

Some markets have "over-run" valves on the front of each inlet manifold. MONGREL things on a good day, so I remove them, and make a plate to blank off the manifold triangle hole. The VAC will rise significantly with them removed.

Ignition timing. If its out, advanced or retarded, the vac will suffer. More on the retard scale than the advance.

In the VERY extreme cases the timing chain "stretch" has the camshafts running out of synch, so a redial is needed, but that is the extreme as I said.

Systematic diagnosis is the ONLY way on these very simple engines. Simply throwing things at it solves nothing, apart from decreasing your bank balance.

 

OK, if you fooled the ECU by (if I understand you correctly) connecting a vac pump to the vac spigot on the ECU and pulling the vac required that way, and having once pumped the vac required the ECU/spigot it held it, and under these circs the engine ran perfectly, then the ECU you did this with seems to be OK. So a few ideas to try:

So the question is why isn't the engine pulling the vac to the ECU it should ? You have tested the tube from the cross pipe spigot to the actual end of the flexible pipe that goes onto the ECU and all is OK with no leaks, and blown it through thoroughly? If yes, then there is too much of a leak into the cross pipe.

Why not first, repeat the vac pump procedure to fool the ECU, block the cross pipe spigot AT THE SPIGOT and see what happens. If all OK; UNBLOCK the cross pipe spigot and see what changes, and report back.

Also, I suggest you temporarily block off the vac pipe to the vac capsule, so no extra air is entering the manifold from the failed capsule, and carefully go round all the many tubes that take off from the manifolds and block them too. If all runs OK like with with the fooled ECU, then one by one release the pipes and see which one makes the thing stop.

Just an idea, I have never done this, but why not push a bit of compressed air into the ECU-end of the vac pipe and see if you can hear it hissing out somewhere in the engine bay? What puzzles me is that normally a vac leak means the engine runns too fast at idle, rather than refuses to run.

Greg

 

Remeber, the more vac, the leaner the fuel, and teh less vac the richer the fuel, BASICALLY.

Other trimming factors kick in, but that process firmly in the brain makes it easier.

That ECU is fine, so leave it there and get into that under car pipe/hose, then the engine bay.

It you got smoke, is it BLACK/GREY fuely smoke, or White, or White/Blue?????

I would also unplug and cap the vac hose going TO the brake booster. You may have a ruptured diaphragm inside and that will create a huge vac loss in the engine.

You mentioned "failure", NAH, the V12 is an adventure like no other on this planet, and if you are upset about "set backs" then move on from the V12. I know it sounds harsh, but it is reality. This car will seriously "test" you until its sorted, and then its a marvel like no other.

These engines, and their systems. are as simple as they come, but the previous persons messing under the bonnet and elsewhere, really do mess things up a lot. Then the car sits for years, still with issues, then you, or another eager person, gets the car, and attempts to start it without knowledge of the dead cars issues, and the maze goes on from there. I have ressurected many over the years, but starting at A, and ending at Z, with NO deviations is the only way.

Also I took up drinking some 50 years ago, and I recon that helped a hell of a lot, especially when our fleet of V12's grew to 4, with 5 6cylinder Jags just for giggles.

 

Just a thought. has your vac line to the ECU got a cylindrical black plastic object in it, about 3 inches long? This is some sort of vac 'smoother' I believe. Grant will know, that's for sure. Anyway, mine had one and the thing ran like a dog until I removed it. Greg

 

Good thought Greg.

Yep, all the HE cars had that for the shortest time available. I gave them to next doors dog in exchange for large syringes for my workshop drawer.

Be interested in what reports we get back in due course.

 

Still working away at these ideas, and no luck after sending straight vac line to ECU without the little vac buffer. I don't want to buy a vac advance capsule yet, so I am asking one of the guys in my club if I can borrow parts to test. I am really appreciating your input fellas, and hope we can tackle this issue together. and BTW the line from engine compartment to trunk is clogged ! good catch

 

Have you unclogged it? This is 100% surely a major part of the problem!

Greg