There is a problem with my 1983 XJS. We rebuilt the distributer, put new plugs in, and checked ignition amplifier. When we first got it, it did not run and we suspected the ECU. We tore it apart and found a burned out solder bridge. We fixed it and after that it would start but it wouldn't run long. Then we ordered a used ECU but when we put that in it wouldn't start at all. We found that someone had broken the pickup in the distributer and after that was replaced it would run but still not well, and only with the old ECU. So now after plugs and fixing the signal wire going back to the ECU it is a lot better. Now for the problem, there is about a 50% chance it will start cold. If it doesn't and you go back in five minutes it might. It runs rough cold but once it warms up it usually smoothes out. Driving down the road it will be running perfect and then start to oscillate and quit. Sometimes it will start right back up sometimes it won't. Whatever is causing the problem is intermittent, sometimes it runs perfect and sometimes it misses and oscillates and you can barely keep it running.
Any help would be appreciated.

 

I chased a nearly identical set of symptoms for a long time. You never knew when it would decide to dump you off alongside the road. The only sure way to get it to start again was to call a tow truck. By the time he got there, it would start back up again.

Long story short, it was the ECU. Replaced it, end of problem.

From your report, it sounds like your used one was questionable in the first place to say the least.

My car is almost the same vintage as yours, manufactured in 10-83 it is a 1984 model.

Hope this helps.

Lou

 

That helps alot! We were hoping that someone had the same problem and could give us a good direction to go in.
Thanks!

 

Yours will have a 6CU ECU, and they are known for issues, mainly with the fuel pump control circuit.

Mine did this waaaaaay back, and after too much mucking around, I bridged the fuel pump terminals in the relay socket, that was it, bugga. Try that as another diagnosis, and you can either leave it like that (not the best due to fuel pump ALWAYS on when ignition ON), or get a 16CU and fix it properly. I think (?) 16CU came in at about 1987 in XJ-S, and S3 V12.

Found (stole actually) a 16CU plugged it in, been there ever since. They are a better ECU, in many ways, so it will be better in the long run.

Also, that module inside the ignition amp, is known to sometimes go intermitant (heat related), but they usually die totally, but I have a 3 in all my years that were acting strange. I use Echlin (USA made) modules, never had an issue, TP45 from memory.

 

I think the ECU is likely to be the culprate. There is someone local with a XJS that has a droped valve but it did run perfect. We are going to see if he'll let us borrow it and see it it fixes the problem. His is a 91 or 92 model. Will that still work and is it safe to switch them?
Thanks.

 

Be carefull with the '91,'92 ECU.

It will more than likely work, BUT, that year cars are Marelli cars, and the Marelli ignition ECU does communicate with the main ECU. That is where I reckon you will get into issues. These Marelli cars are pretty thin down here, and I have NO rexperience with them.

Have a look at the sticker on the casing, and if it is 16CU, go for it. If it is 26CU or higher, be carefull, you dont want to be buying him a new ECU due to some issue with mismatch damaging his unit mmm.

Others in your part of the world may know more about this compatability than me, and will hopefully chime in with the info.

 

We have not talked to him yet and I'm not sure what year it is, but if it is the 26cu we'll look somewhere else.
Thanks for all the help!

 

We bought a 16CU and it took away 99% of the running issues. Now it just needs some adjustments, but so far it has started and ran without any problems.
Thanks.