Roger Bywater finished my ECU, I should have it in about a week or so. Here is his diagnosis of the fault:
The main problem with your ECU was that the ADC (Analogue to Digital
Converter) chip had developed a fault, which is quite unusual. I have
replaced it and checked for any issues that might have provoked it but more
likely it was just a random failure. I have seen it happen about two or
three times over the last 20 or 30 years so it is quite rare.
Itâs awesome when a cause is positively identified.
When all is said and done, and your car is running as it should, perhaps you could write up a summary that includes this description from Bywater along with a description of the symptoms your car was exhibiting and what diagnostic steps were taken. You know, for someone else who may run into the same issue. Bywater may have only seen it a couple of times, but for all we know that may be because a few dozen cars were sold for scrap because they couldnât find the problem!
Yes, thatâs a good idea. I will do that once I have it running again. Fingers crossed!
Measuring resistanceâŚ
Going thru this thread, wanted to correct a bit of misinformation. This specific injector has 2.4 ohms resistanceâŚnot 2.8 ohms as was posted. When using a meter to measure ohmsâŚthe meter must be calibrated or zeroed prior to making the readings. And it is done differently depending on the type meterâŚdigital or analog. For an analog meter, the two lead tips are touched together, and a zero adjustment dial on the meter is turned to put the dial pointer onto zero ohms. The meter internally adjusts the pointer and the resistance to compensate for the ohms resistance of the wire leads. On my analog meter, that resistance of the leads is 0.4 ohms. If you donât compensate for the lead wires and you measure this specific injectorâs resistanceâŚyou get 2.8 ohmsâŚa combination of the injectorâs coil wiring resistance of 2.4 AND the meterâs leads of 0.4. Zeroing the analog meter removes the resistance of the leads (0.4) so the reading on the injector is 2.4.
With a digital meterâŚ(on mine anyway)âŚthere is no adjustment capability within the meter for ohms. So, the method is to touch the two leads together and note the resistance on the display. On my diggy meter, it is 0.4 ohms. And you mentally remember that number or write it down. Now you read the ohms on this specific injector and get 2.8 ohms. You now subtract the 0.4 ohms resistance of the leads and you end up with 2.4 ohmsâŚthe proper number.
Does that make a difference ? MaybeâŚmaybe not. Probably not for the guy just wanting his car to run properly. Would it make a difference to someone like Roger B when working on ECUâsâŚmaybe. Would it make a difference to a Formula 1 engine tunerâŚprobably. Did the difference in the cold weather temperature make a difference on the seals of the Challenger ? Maybe thatâs reachingâŚbut you get my drift.
Years ago an old boss of mine had a land survey done. The surveyor never could get the survey to close. It was off 2 inches. My boss didnât accept the survey. He asked the surveyorâŚHow would you like two inches extra on the end of your nose?
And I hope the ECU gets this car up and running. This may be the longest thread everâŚsaid with no malice.
SD Faircloth
Note that single-digit ohm readings are a challenge for a low-cost VOM. Itâs pretty much what separates a bargain basement VOM from a first-rate model. In fact, I think some of the really cheap models donât even have an ohm scale below 200 or some such. Also note that the battery in the VOM needs to be in pretty good shape to get a good single-digit ohm reading.
The fancy digital meters use a 4 wire method to measure resistance, starting price probably around US$500. That is more accurate for low resistance, under 10ohms say.
If you need to measure a low resistance more accurately use 2 cheap DMMs ( digital multimeter )
One measures current, one measures voltage. Just choose your voltage source, maybe with series resistance to limit current, so you donât cook anything.
Then its ohmâs law, R = V/I. Much more accurate even with cheap DMMs.
Guess what? Got my SuperEnhanced ECU back from AJ6Engineering. So, here goesâŚquite a lot to digest and still figure out, but I think Iâm close!
Engine started right up first crank, first time ever. But here are the vitals and the issues:
Issue: Cold idle is 1500RPM, warm idle is 1750RPM. I assume some throttle adjustments and looking at the AAV will fix this?
Good: Warmed up at idle (1750RPM), vacuum is now a healthy 18inHg. Oil pressure is 50psi. And I canât believe how smooth these V12âs idle - I think I could have balanced a glass of wine on it.
Major Issue: As soon as I put it into R or D, the idle drops down to about 300rpm, gags and stalls. If I catch it before it stalls and try to put it back into P or N, idle is still terrible. It will eventually stall, or continues to idle terribly in P or N. Once I let it idle poorly, and within 30 secs it corrected itself and shot back up to high smooth idle at 1750rpm.
When I shut it down or it stalls, I canât get it restarted. It cranks, but sounds funny. Doesnât sound like no fuel or no spark, just sounds like itâs cranking and kinda running at the same time? If I put it in R or D, no crank at all, which is what I expect. I usually have to wait a while to restart successfully. It will idle high like before, but no issues. Until I put it into R or D, and then the terrible idle/no start again.
Could this be an issue with some PNP sensor? Or an issue with my torque converter? Does putting it into D or R use the O2 sensors, which could mess up the ecu mixture for a while if O2 sensors are bad?
I understand some adjustments need to be made for idle. But how could this affect start up?
Hi Greg,
According your symptoms first thing I would try is switching your ignition pick-up wire polarity.
Reg. AAV take it out and do a benchtest with hot water. Should fully close at 60 to 70c. If it doesnât - Thermal bulb is tired or/and valve stuck. I can offer you a rebuild service or I can sell you necessary parts (new thermal bulb with adapter and new stainless steel piston) and Iâm sure youâll be able to rebuild it yourself.
Now that the ECU is fixed, it should be relatively easy to sort out the running issue. First check the AAV. You can take off the air filter cover on the B bank side. With the engine cold, do a start up and feel by hand or the dollar bill test, the suction from the hole at the rear of the filter housing. You want to feel if there is a difference in suction as the engine warms. If there is none, likely the AAV needs workâŚmost likwly problem the valve is stuck. There is also a bolt on the AAV, to do minor adjustments of idle rpm. It may not remedy a 1750 rpm, but give it a try. Get that fixed if itâs bogus. Then if still no joy, set the throttle butterflys.to .002. You need to do this anyway as a general matter for the engine to idle correctly. You also need to check the rubber/nylon bushings on all the throttle levers, etcâŚmake sure they are not missing or wornâŚno sloppy playâŚboth sides of the engine. Forget the torque converter. The O2 sensors are not functioning at idle speed or at WOTâŚwide open throttle. In those two engine modesâŚthey are in what is called âopen loopâ operation. Do 10 minutes of read up on closed loop and open loop O2 sensor operation with a Google search. SD Faircloth
A few more tests before I went to work today:
I let car sit overnight. This morning, engine started right up, cold idle at 1500rpm. Within 10 secs, I shut it down, and now canât restart, or if it catches, can only get near 200rpm and stalls Similar to issue once I put it in D or R.
I was able to restart 5 minutes later. Nice smooth idle at 1500RPM. I then tested removing vacuum to ECU or wire to CTS, instantly causes engine to almost die. (thatâs good!) I plug it back in, and the engine can only maintain a 700rpm very rough idle. I give it a bit of throttle, and it bogs down, almost stalls. I keep doing this, and eventually it works and it jumps right back up to 1500RPM smooth idle.
I tested TPS a couple months ago, but will now retest with engine running and symptoms. I still have the original black Bournes TPS.
I will definitely check my AAV tomorrow.
Just read this in Kirbyâs book: "In Park or Neutral (starter permitted), the EFI operates in an âopen-loopâ control mode, where the oxygen sensors are ignored and the EFI controls to a fixed fueling map. When in Drive or any other moving gear (starter inhibited), the EFI operates in âclosed-loopâ mode, adjusting the fuel mixture to obtain the correct oxygen sensor feedback. "
Iâm not quite sure which cars operate like this. Does mine? 1988 XJS V12, VIN:141,xxx
That would explain a lot, if my O2 Sensors are faulty. If I disconnect them, what should happen?
Roger Bywater also said he did this to my SE ECU: âI have also updated the circuit board to run the later programs which have better control of Lambda feedback.â
Reading through a lot of archives, I am now looking at a combination of a stuck AAV and bad O2 sensors as my likely culprits. They are original. Will report back tomorrow if I can.
A couple more tests today, if anybody is readingâŚ
Started cold, idle 1500rpm.
Once warmed up to 190F, idle creeped up to 1750rpm.
I put my hand over port for AAV and could feel it sucking. I put a rag over it, and got it down to 1600rpm. I then pinched off vacuum line to distributor, which brought it down to 1400rpm. I then totally plugged the port to AAV and the engine died.
I will remove AAV and test it tonight. Piston is moving. It opened up when cold. When I put the bulb in 80C water, piston moved almost closed, but left a gap of 2.5mm. The adjusting screw is also far out, at least 5.5mm left to screw bolt in.
AAV piston is not stuck so that is a good thing 
although thermal bulb is âtiredâ and needs replacing or you can try squeezing it a bit in a vice so the valve closes at ~70c. I think with age they leak a bit and the pin doesnât move far enough to close the valve.
Iâm actually rebuilding one today that was sent to me for a rebuild service. In this case the valve was stuck completely open:
So itâs not getting sufficient air to idle from the throttles. Fully warmed up, corking the AAV inlet should drop RPMs to 300-400 but it should still idle. Check the throttle gaps.
If your throttle gaps are correct and it will not idle ( but will when adding aav/screw air ) then itâs too rich.
What is the ECU trim set at? Where was your old ECU trim?
Info from Roger Bywater on this subject:
âto run out of feedback it is only necessary to ground the black/green Lambda disable lead running from pin 25. The ECU then settles at the middle of the correction range.â
Hi Greg,
If you disconnect the O² sensors the ECU will go to open loop automatically.
Next thing to check is the ECUâs Base Fuel Map, i.e. how much fuel it feeds the engine by default (in open loop). See details in Kirbyâs book.
When the Fuel Base Map is set correctly the car should run and drive fine even with the O² sensors disconnected.
You need O² Sensors in good working order though to set the ECU.
I suggest you change the O² Sensors, they are long due if they are still original, and put 3 wire ones, it helps a lot.
As mentioned before, the AAV is important for maintaining a good idle and should be checked, I also do suspect that is not working properly.
TPS, CTS and throttle butterflies must also be in specs for the engine to run.
An other item I would replace regardless is the Ignition Amp. Module (itâs been a long time and I donât remember if you already did) an extra assurance that costs only $10âŚ
Aristides
Thanks for the replies, I really appreciate it! 
Regarding AAV, I am going to use vice trick to put it more in spec. The actual AAV innards were quite clean, just a bit of grime in there, so piston worked fine. Fully closed had a little 2.5mm gap, but I couldnât believe how much sucking that could produce. I assume it should be fully closed at 80C.
One thing I noticed, there was a ring of black sealant around thermostat. Was this OEM? Or could the PO have tried to seal it because of a coolant leak? Easy way to test before re-assembling? The 90 elbow hose also had some cracks in it. I will have to wait for the hose and a new gasket before I reassemble and test engine again.
I may need to ask Roger Bywater, but do I need to set my throttle gaps differently with the SuperEnhanced ECU? I believe he sets them based on his much larger throttle body upgrades?
I have ordered new O2 sensors. Even if it doesnât help idle, it will surly help mpg.
I never set old ECU trim, and donât want to mess with current ECU trim which Roger Bywater set with superenhanced ECU. Unless I have to?
CTS is new and works fine. TPS was checked a while ago with ignition on, but not while engine was running. I will do that. Entire Ignition Amp was replaced a couple months ago.
I should probably check for other vacuum leaks too once I get the AAV sorted. Could major vacuum leaks make the engine idle high in P and N, and then stall in D or R?
You saidâŚ
Started cold, idle 1500rpm.
Once warmed up to 190F, idle creeped up to 1750rpm
I am sure this has been covered, but it still needs attention. High idles are MANY times the result of a vacuum leak. On a cold engineâŚso you donât burn your fingersâŚvisually look AND use your hands and run them around the front and rear of the intake manifolds. Both sides of the engine. The front and rear of these V12 intake manifolds have several barbed ports for connecting hoses from various locations. I think there may even be a port under each of the throttle butterflies. Make sure all of them are either capped off or have a good pliable hose connected. On those with a hose connectedâŚfollow the hose to itsâ terminus, and make sure it is connected to somethingâŚAND that the hose is pliable and not cracked.
SD Faircloth
Thanks I replaced all vacuum hoses with silicone, and did a check two months ago. But you are right, worth double checking.
When I closed off the AAV port, I should have got 300rpm, but the engine instantly died. A vacuum leak âshouldâ have let it keep idling? I am in the middle of fixing my AAV (getting it to close at 80C). I will also address the throttle gaps, double check vacuum leaks, check timing, and check TPS while idling, and report back later today.
According Roger Bywater this is not true. This is his explanation what happens if o2 sensors are disconnected:
âThe absence of any input to the Lambda sensor pins
therefore results in the ECU making continual correction
jumps from weak to rich and back again. The injector pulses
are therefore unstable as they jitter about around the
mid-point. The reason for this strategy is that it is
impossible to provide a mid-range bias voltage that would
produce stability because a change of just 0.020 volt can be
sufficient to switch the direction of correction.â
