XJ6 & XJ12 Series I, II & III 1968-1992

87 vanden plas losses spark

 
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Old 09-01-2014, 01:47 AM
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Default 87 vanden plas losses spark

I have an 87 vanden plas, 87,000 miles 8,000 miles on new lower end original head. Previous owner stated "started car, backed out of garage, got a cup of coffee, went out and car had stopped running, that was the last time it ran" hasn't run for Eight years, previous owner spent several thousand dollars ( yes that is correct) having the car towed to assorted repair shops around Michigan. I have the Jag repair manual and have tested every part if the fuel delivery system, all checked out good, got it started drove it to gas station put in good gas, I can drive it!
I tested all the ignition parts, per the manual, all the resistance and voltages are good.
Now the fun part.... car starts and will idle about 400 RPM has almost no manifold vacuum, car will ..hiccup....and rpm's come up pulls good vacuum, I have to keep idle up around 1050 rpm,I can put it in drive if I keep the rpm's up,if it drops below 1000 rpm spark disappears as proved by timing light, the coil has been replaced, distributor pickup has been replaced, new cap and rotor, new plugs, mass air meter,assorted CEI modules(I own an auto salvage yard) compression around 145psi. Car dies weather it's first started or warmed up and driven.
Does the ECU control the ignition? I know it gets a Tach signal to run the injectors.
A "no start" would be sooo much easier.

Thanks for any help, if anyone has had the same symptoms, please let me know.
 
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Old 09-01-2014, 08:53 AM
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Originally Posted by fauxoscar View Post
I tested all the ignition parts, per the manual, all the resistance and voltages are good.



You have to test everything immediately after the engine dies. Is that what you've been doing?




if it drops below 1000 rpm spark disappears as proved by timing light, the coil has been replaced, distributor pickup has been replaced, new cap and rotor, new plugs, mass air meter,assorted CEI modules(I own an auto salvage yard)


Assorted modules? Please clarify. There is an ignition module inside the amplifier, the AC Delco D1906 type. Have you changed that?




Does the ECU control the ignition?

No



I know it gets a Tach signal to run the injectors.


Right.


A "no start" would be sooo much easier.

Indeed!

Thanks for any help, if anyone has had the same symptoms, please let me know.


Have you *very carefully* examined all the wiring relating to coil, amplifier, and distributor?

If yours has a ballast resistor on the coil have you bypassed it (or thrown it away) yet, just for the heck of it?

Cheers
DD
 
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Old 09-01-2014, 09:02 AM
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I have a guess for you. It is in the distributor. Swap it out with another. Hopefully from a car that ran when it came to you!!! or get one from David Boger at Everydayxj.


For whatever reason, the pickup isn't functioning at low rpm!! It suggests a gap issue.


Why? Distributor shaft wobble. At RPM, a gyroscopic effect stabilizes it and it sends the coil the needed make and break to get spark to the plug.


Way back when, I had a tough looking 57 Ford short bed pickup. many miles, but with a Tbird long block 312. It ran like stink. However, it was the original distributor that was wobbly as a top. Oddly, if I set the point gap correctly, no start. But, if set wide and with a good starter spin, it would fire and run great, except for a raggedy idle. I reached into the past and recalled pal roy's fix of the distributor in his slick running A. A better distributor and my Tbird ran great at any speed and fired at a touch. And, wow, when I wiggled the shaft in the old one...


Never did get it's stove 4 barrel carb right. Later, learned the name came from their propensity to catch fire!!!


Oh, how id the button n the center of the distributor cap??
And the contact on the rotor. One I had that was new was so lousy I installed the old one.




;
 
 
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