Computer voltage problem
 

1997 XJ6 non turbo automatic tranny. Runs really good when engine is cold. After 5 or 6 min. it starts running bad, as if someone was turning the ignition off and back on very quickly. I have tested all the pin voltages coming out of and into the Engine Control Computer. There are 2 voltages that correspond with the problem. The ground that controls the Air Injection pump relay, and the ground that controls the fuel pump relay. Every time the car acts up both of those grounds are dropping to 0 volts and snaps back to full ground (-) in ms (I am using a vantage pro dual trace graphing meter from snap on tools) I have also exchanged both the relays with no change. I thought it was the Computer, so I replaced it. still no change.
I am a Master technician that is lost. I have tested everything possible that my info and years in this field allow me to test. If anyone could help I would be most appreciative to them. Thanks : Autotec40

Firstly WELCOME.

The 97 XJ6 you have is known as an X300, and waaaaaaaaay too modern for the beasts in here.

Secondly Jag Petrol engines have a SuperCharger, NOT a Turbo. I know I am picky, but I am OLD, and Biased a little.

The fact its fine when COLD indicates all and sundry are doing what they do. My best guess on a Sunday morning is the fuel tank venting is kaput, soooooo, remove the filler cap, go for a drive, see if anything changes. Cars with in tank pumps generally do not have venting issues, but its free to test it out.

If NO, then blocked/OLD fuel filter taps on the brain.

Fuel pump getting tired is next, as for some odd reason they go legs up at "about" 100K miles, as does the CAS (Crank Angle Sensor).

Then I would look at the actual plug at the front RH edge of the timing cover where the CAS connects to the rest of the car. It is mounted vertical, and the vacant holes in that plug allow gunk to get inside and that kills the signal with conviction, especially once things heat up. Mine did that, and it took some finding, and there is a write up on it I THINK in the X300 section "stickies"??.

Im not sure where to post this. If there is a better place please let me know and I will repost it there.

I have retested the crank signal, it has a pristine signal when the problem occurs. I also did a complete fuel pressure and flow test. and drained the tank and installed new fuel. Thanks

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