All,
Last year I started down various rabbit holes chasing an misfire that would not generate any codes other than P300 and a P305, which the P305 disappeared when the coil was moved to another position, but the misfire symptom - two cylinders not firing did not, and too lean codes prevailed.

I followed the advice of many for the obvious: battery, grounds, voltage levels, Fuel Pressure, valve timing, crank timing, Part Load/Full Load/Injector air wash hoses and intake plenums, smoke tests, all before trying MAF sensor, TB rebuild, gold pins upgrade, TPS, plugs, coils, O2 sensors, cleaning the fuel injectors, then pulling the intake.

The failing of all this was not to pay attention to the mechanical obvious things - the essential elements of fuel, spark and of course compression.

Having eliminated the first two of the equation, I pulled out the Leak Down / compression tester, and if anyone remembers, the result was air blowing by all 32 valves and low compression on A and B Bank not due to valve timing, but valve and seat pitting.

The source of the valve pitting remained a mystery, these AJ26/27 motors appear to suffer from that symptom.

So a subsequent teardown of the top end (cylinder heads) resulted when it appeared the primary and secondary tensioners needed upgrading with chains and guides, valve seals, and a full extensive lapping of each valve and seat to remove all pitting for a proper seal and resultant valve lash adjustment with shim spreadsheet tools from the internet and shims from this Forum's own Bob Roy and my local JLR dealer maintaining my shop account for parts, all uniform compression of over 175 Psi on each cylinder was the reward for my time and effort on a 108k mile AJ27 NA motor.

However, the original symptom - lack of throttle response - drivability still remained.
All sensors renewed, all harness points pinpoint checked, grounds perfect

Had I a full IDS/SDD setup or the list of full PID's to customize Torque Pro, I would have resolved the issue.

Any guesses? Write down you answer first, then read more.

I finally had a chat with my local JLR service advisor when I was in for some parts for my 2000 Xj8 and he reminded me they would perform a full diagnostic and re-flash as necessary for $148 complete, included a full in/out car wash and vacuum..
Surprised I was, as I did not think there was anything that could be done in JLR Service for less than a cool grand, excepting an oil change.

So I limped the car to the dealer, had a long chat reviewwing three printed pages of work I had performed on the vehicle with all symptoms discussed with the shop foreman over about 45 minutes, expressing the appropriate caveats since he did not have experience on these old girls.

My suspicion all along was the ECM but I had no means to verify that self diagnosis, which I expressed to the Shop Forman, recommending he reflash the ECM.

He asked if he could have it for 3-4 days to do the diags and get a feel for the work I performed, I agreed if it was all within $148.

Three days later the shop foreman emailed me with some IDS/SDD printouts and screenshots, he had performed and verified all my diagnostics, and confirmed ...

Bad ECM.

He explained there were some "02" codes only accessible if you have the IDS/SDD for a deep dive diagnostic into the various computers, which he could not reset those codes that were characteristic of a failed ECM processor not delivering any real DTC's. He cautiously warned that if he were to attempt a reflash, he might brick the ECM netting it not able to function at all, and he did not feel comfortable trying to save the vehicle configuration and security before reflashing due to the displayed codes..

Then he asked me if I wanted a line mechanic job … he was quite impressed with the quality of my engine rebuild, where I explained I was already trained on British Leyland stuff and preferred computers I built and programmed myself …, and as a consultant I might make a little more than a line mechanic on flat rate.

Enter - my google search for Foreign Auto Computer Repair in Holly Lake Texas. After a short series of texts and a phone call, they would turn my ECM around with a reconditioned replacement - transferring the security codes with a 5 year warranted plug-n-play Genuine ECM for $275, included express return shipping.

Further discussion shows at least three different ECMs for the years 1998-2003, some have electrolytic capacitors that leak and eat the circuits, others had a rash of bad CPU chips that fail in different ways.

They tested the ECM, confirmed the chipset was not functioning properly, transferred the security chips to a reconditioned ECM and in 3 days I was standing beside my 2001 XJ8 with ECM in hand, battery disconnected, ready to plug in.

Absolute success. Battery reconnected, twist the key to ON, system test completed, twist the key to Start and varoom! an odd series of DTC's came and then went over 3-4 drives.

Now when the throttle is pushed forward, the corpse in the driver seat sinks back far deeper than ever before under the acceleration up through the revs and gears to electronic limit MPH. The motor responds probably like new performance or better.

On to the 2000 XJ8 AJ27 to take care of the suspension issues and reseal the cam covers.

Cheers!

 

That’s really good news to hear of another successful revive of these old cars. Most people would simply throw them away seeing that there could’ve been a truckload of money thrown at it thru various Indy/dealers. The AJ26/27 ECM’s are not physically interchangeable so that would narrow down the number of ECM’s to look for. If it were me personally, I would’ve opted for another used ECM. I’m not saying that the individual that rebuilt yours did a bad job, but the likelihood of error resoldering on an ECM is much greater by human vs a machine soldering on it when they were new. There’s lots of used ones on eBay for cheap and I’d get one as a spare. The VCATS 3 digit prefix would be the only limiting factor. You’d have to get the used ECM to shake hands via software is the only thing. Anyways, congratulations on getting the car running properly and I feel your pain on the 32 valve lap job and shim adjustment. I’ve been there and done that and if I ever have to do that again, I’ll get the machine shop to just grind the valves and seats. Waaaay too much time involved in that.

 

I think my total parts was about $3800. Since I have two of these I am still considering an IDS/SDD setup if I can locate one for a decent price,
$750 for just the genuine VCM alone I can't justify unless I add Jag's to StagByTriumph Garage list of vehicles.

I had considered a FleaBay used ECM for $50-$100, but I'd had to take it back to the dealer to get the Security and key chips reprogrammed adding another $150 - $200 and no ECM warranty. So the Texas ECM repair was a better deal, plug-n-play with 5 year warranty.