After a ridiculous amount of troubleshooting, replacing parts, and countless troubled days I've come to the conclussion that my idle issues and engine stalls at normal operating temperature are happening due to a faulty ECU
I HAVE A 96'JAGUAR VANDEN PLAS 4.0

It runs insanely smooth and nice when you first start it up and for the first 20 mins of driving. After that it stalls out. Ive done the fuel pump replacement and tank cleaning and fuel filter. Ive replaced my IACV , Cam sensor , 4x O2 sensors,Egr valve , all my vacuum lines, cleaned my throttle body ,coils and plugs ect.

STILL RUNNING GOOD AT START UP AND BAD AFTER WARMING UP. HELP!

 

Have you tried a set of known good coils? The coils are much more likely to crap out once the engine gets hot than the ECU. Ebay coils and Eurospare coils are junk. If you need coils you need to get the ones made by Diamond in Japan. SNG Barratt has them, I have never seen an alternate option that works well.

Do you get any codes at all? What are your fuel trims? What are the STFT (Short term aka "right now") fuel trims when it runs nice and when it stalls? The fuel trims will tell you whether the ECU thinks that the engine is running lean or rich. When the car stalls, does every warning light on the dash illuminate? Do you get any OBDII codes?

It's not very common for the ECU to fail intermittently, though not impossible. They are relatively inexpensive, so ruling out is not hard. You will need to use IDS or another diagnostic tool to get the ECU to learn which oxygen sensor is plugged into which harness. If they were plugged in differently on the donor car, your car will run poorly after ECU swap for a while. My money is on either coils or a leak opening up once the engine is fully heated up. Could be an exhaust leak (more likely) or an intake leak (less likely).

 

So, the new ECU came today and I installed it with Zero issues. Car fired up as normal ,ran for a bout 25 mins no issues and then threw me a new code the other ECU wasnt giving me. P1777 ....car stalled. I towed home.
I then removed the new ECU and put my old one back, and tried starting the car and it stalled just after starting.
So i took your advide and put in another ,more exspensive and well known brand set of coil packs and Voila!!!! Running strong!

Thank you my good man!

 

The coils as they heat up and expand can short out to the wells of the coil insert

The valve cover is electrical isolated by the plastic shoulderd bushings but nothing is perfect

You ECU runs on pure DC power or the best your rectifier and noise suppressor ( a capacitor ) can filter

But the coil arcing can induce a noise or dirty signal into the ECU , this is known as dirty electricity

Before coil changing the cheaper try is to ensure all of your engine grounds are maintained and cleaned

This include the large main engine block ground strap on the starter mount bolt

Best from the bottom side with a 15 mm and 3 inch extension for the ratchet and a 8 or 10 mm on the car frame bolt

If you turn your stereo to AM you my hear it change as a background noise with RPM

 

So you're not going to share the brand of coils?

It is something we keep track of, here on the forums. It draws lots of interest.

 

Your story is very similar to mine. My 1996 VDP started "bucking" whereby the engine would stop running entirely for a couple of seconds and then start up again. The mechanic, after a good bit of time, found that the fuel pump was loosing it's signal to run from the ECU. He actually saw it happen in real time. He sent the ECU off to be repaired and was told that the ECU was fine. Then we sent the ECU to ECU Depot in Texas. They have a way to hook the ECU up to a monitor of some sort to check for intermittent faults. After three days the fault showed up for ECM Depot. But it turned out to be a non-repairable problem with the processor. My car ran just fine for the first thirty minutes or so. Then started acting up. The worst of the worst of electronic problems. So I went to Ebay and bought a replacement. If the numbers match the ECU is plug and play. That solved my problem.

Hope this helps.

Bill

 

The ECU has 2 power sources

First power from the RH engine fuse box king relay ( ignition positive relay ) which can be swapped with the LH engine fuse box king relay ( which in reality only controls your car horn )

The big ECU controlled relay is the second power to the ECU as it is controlled by the ECU to bring power back into itself ( this relay can be swapped with the Headlights , fog , cabin heater water pump

This is on the basis that a relay is intermittent and floating

The king relays are controlled by the ignition switch and the command ground is though pin 5 to car frame

The RH engine fuse box king relay is pin 3

See page 32 at 7 stopsign double hash and page 40 noticing the wire runs through the crash switch that gives problems

The crash switch can be bypassed with a paperclip jumpering the 2 white wire sockets

http://www.jagrepair.com/images/Auto.../jagxj1996.pdf

You can take this wire to ground to test drive the ignition switch if this is your issue

32 circle double hash in the hand pic is your first power coming into the ECU though the RH engine fuse box king relay

Notice it goes through the troublesome Papa Indy 1 connector that corrodes as the White / Pink wire in position 4

Have you noticed if the ECU connector is missing any socket 2 halves as mine was missing 4

My mom says I have to go to bed now