1995/6 XJS swapping over ECM check engine light

Check engine light problem. The OBD II reader shows Codes 0300, 1313 &1314 misfire on banks 1 & 2. Car starts on the button every time and when you don’t experience the misfire the AJ16 is a beautifully smooth unit. It never stalls but you think it might when you pull up at signals etc. When I located existing ECM (footwell passenger side front) the unit was not even secured to the bracket just resting on a couple of the threaded studs. Someone in the past clearly has had the unit off and not bothered to correctly reinstall it. I really can’t stand this…a botched attempt at trying to resolve the problem!!! I don’t know if it’s working or not. The previous owner gave up with trying to deal with this issue. I am determined to get to the bottom of it.
I’m about to replace the coils and spark plugs and have another ECM which I’ve acquired (same part number and in untampered with condition with no corrosion).
Grateful for any advice comments. She is a beauty and I just want her running consistently smoothly.

There is history in the Archives of water leaking into the area of the ECU
plug and causing problems with engine misfires and such…

Just do one thing at a time so that you know which was the fix at the end of the journey, those AJ16 engines are bullet proof, my brothers went up to 333,000 klms before he sold it and he saw it a year later and it was still going.

OK, I’m not sure what your problem is, so I’m going to start with the codes you have shown us.

Obviously there is a misfire being detected, but it usually points out a specific cylinder if it were a specific coil. Here is the path I would take.

  1. Are the two catalytic converters in good shape? Are the blocked? You show fault codes for both banks.
  2. Assuming you don’t want to check the catalytic converters, I would replace all four oxygen sensors. If you want to be cheap, then start at the two downstream sensors. They do all the heavy lifting, the upstream ones are simply there to make sure the downstream ones are working. Go to Rockauto and buy the NTK or Denso brands.
  3. Replace all coils. Amazon will sell you a full set of QYL branded coils. Only these or the more costly Diamond “Made in Japan” have been proven to last. Don’t waste money on any other brand. While here replace the spark plugs. Cheap Champion 71 or RC12YCC is what you want. That or the NGK BKR5e, no need for iridium, platinum, etc.
  4. Replace the intake manifold gasket. Leaks here cause strange occurrences, like what you’re experiencing. While you’re there make sure you replace the often forgotten, but critically important bastard hose. (EAC3454)
  5. Remove and clean out the throttle body. It’s not hard.
  6. Replace the throttle potentiometer sensor (TPS). It causes mystery problems, and while I don’t believe that’s the issue here, it could be.
  7. Replace the MAF. Also, I don’t believe it’s the issue, but it could be.
  8. Replace the CTS, or the coolant temperature sensor. Could be the issue, and for a $20 part, would be worth the gamble.

Thank you for the informative response. I have just bought the QYL coils. The champion CC plugs are scarce & can’t seem to locate any. I will settle for alternatives. I will start with new plugs, coils and try the other ECM I’ve acquired and hope this solves issue. The Cat was replaced last year so was the MAF. Oxygen sensors maybe outside of my ability. I can do most jobs but it depends what’s involved. Are these accessible? Again intake gasket and bastard hose I could do if the above don’t solve problem.
Thank you again for taking the time to help.
Temperature gauge moves to the red pretty quickly after about half an hours driving (I live in South Florida). I don’t hear any cooling fan come on. Does it have one fitted? My car is a 1996 MY. If so I presume it’s thermostatically controlled? Need to investigate this.

Your radiator fan is mechanical, not electrical, so you may have a bad fan clutch, if its not working. That fan is made to spin when the car is at idle.

You do have a small electrical auxiliary fan. You may want to make sure that’s still working.

Champion 71 should be easy to find. If not, then I recommend NGK BKR5e.

Oxygen sensors are not outside of your ability. Just need to put the car on ramps, or jack stands and get to them. They’re pretty accessible once you’re under the car.

Thank you. I’ll report back!