Coil Test 4.0 93 VP
 

Hey guys/gals.
Our 93 VP XJ6 will not start after sitting a couple of winter months in the garage. I have downloaded the 4.0 Engine Management Manual and started the diagnostics. We have voltage at the coil in the correct amounts. I have checked the coil with test #3 and found the voltage does indeed rise when cranking to over 13volts, but it's not a steady reading, more like on/off/on/off etc.
Diagnostic tree says this indicates "defective coil, replace and retest"
We have yellow/orange spark on coil lead, not fat blue.
Question is, does this indeed indicate bad coil?
fwiw, fuel pressure seems adequate, spark plugs are brand new. Old ones were fuel fouled and wet. When replaced, engine started and ran for about 2 seconds then died. will not restart at all.
Thanks ahead for any opinions.:icon_toast:

got the new coil, cap and rotor today.
guess it's not those causing the no start.
So what's next, crank sensor or ign module?
Started on first crank, died.
won't restart.

Firewrench,

Did you do any work around the throttle body or directly under the throttle body? Did you disrupt the TPS (throttle position sensor)?

When cranking the engine, what is the tach registering? It should have at least a two hundred while cranking. If not, crank position sensor.

Since you're following the engine management manual, I assume all air delivery components have been checked...?

I did remove the throttle body and perform the cleaning process drilling two holes and running electronic cleaner both directions till it came out clear. Did NOT loosen the screws or disrupt the setting.
Tach is showing cranking rpm.
Upon checking the ign module, i'm getting readings that are not in the manual.Voltage test v4 should read "near 0" than rise to "approx 0.3 volts" while cranking. I'm getting .3 volts then rising to .5 to .75 volts. First test it rose to over 1 volt and the engine started, then died.
Seems that the start and die should be a dtc setting event however, there are no dtcs when i press the vcm button. Battery voltage is 13.5+ and all other voltage tests at the module are spot on (not less than 1 volt under battery voltage).
Last fall it ran pretty well with the exception of dtc for egr temp sensor. i did remove the egr temp sensor and found a wire that was crimped on the insulation with no actual wire attached to the connecting pin. soldered that up and cleaned the housing.
I have not checked the air flow meter as it is getting fuel but has weak spark. Orange yellow instead of blue.
checked and tightened all ground connections under the hood too.
Thanks for the reply and your time and expertise.

Hi Firewrench,

Just to throw out some things to check off the top of my head:

I assume that the battery voltage _while cranking_ is in excess of 10V or the ECM would not trigger the ignition at all and you would have no spark. However, if the battery voltage is sagging while cranking to, say, 11V, that could lead to weak spark.

Since the spark plugs depend on a clean ground of the engine block for full current flow, it would be worth cleaning both ends of the engine ground strap that connects the block at the starter to a point on the left inner fender.

The ECM relies on three grounds in the engine compartment for proper operation. Two are at studs on the intake manifold - between cylinder 1 and 3 (on our '93 they are at cylinders 1 and 2 but the electrical manual says 1 and 3). There is an additional ground on the firewall behind the cylinder head. Corrosion of those grounds can cause difficult-to-diagnose gremlins, so it would be worth giving them a good clean.

Since I don't recall you mentioning that you've replaced the plug wires, to rule out a failing king lead it might be worth disconnecting one of the spark plug wires and using it to temporarily replace the king lead between the coil and distributor. If the car starts and runs (on 5 cylinders), suspect the king lead and replace all the plug wires.

The ignition modules (amplifiers) are not reported to fail often, but the harness wires do harden and crack due to their proximity to the exhaust, so carefully check all the wires and the electrical connector at the amplifier (mounted directly below the coil).

A failing ignition switch can cause starting problems. On your '93 the switch as six wires, four of which are used. Different combinations of wire connections turn the ignition on, power the auxiliary devices, and power the starter. Sometimes the starter position works but the connection to other ignition circuitry is intermittent.

Below are links to photos of the engine compartment grounds and cleaning procedures, and the ignition switch replacement procedure.

Please keep us informed.

Cheers,

Don

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It lives!
 

My son and I spent some quality time on the Jag yesterday. We pulled the injectors and placed plastic cups under them and energized the fuel pump. I was checking for a stuck open injector as it was acting like it was flooding out. No flow when not cranking.
Re-installed injectors and clamped off fuel return line. Cranked and no start again, rather, start and die in .5 seconds.
removed large vacuum hose from intake boot and sprayed in ether while the boy cranked the engine. Once I opened throttle all the way up, she fired and ran for a few seconds. Repeating this process 3 or 4 times, we managed to get her running and to remain running. I then removed the clamp on the return line and I'll be darned, she fired up and ran decent although we now have a large vacuum leak at the #6 injector. Will be installing all new injector seals today as they were in stock at Napa and less than $13 for all 12.
Thanks for all the ideas and support.:icon_beerchug: