erratic or random spark.

I have a 91 XJ 4.0L. I just installed a rebuilt engine and it wont start. erratic spark from the coil. Its like spark, spark, pause, spark, spark, ... plugs at 1 and 4 get spark but no body else. every once in a while, it seems to want to start, and the coil will put out rapid sparks but it never will fire or stay running. New crank sensor, ECM, cap and rotor and ignition coil. Grounds test good, Batt cable, 3 lugs at the dip stick, and the body ground at the back of the head to the firewall.. Weird output from alternator, and batt side of coil though. It seems to drop out. The engine still wont start even if I short the batt side of the coil to the batt+. Bad Alternator or what?

I saw a identical thread posted here from a few years ago, but it seems to gone dead with no resolution.

I had a friend come over last week with his OTC scope/scanner. After a few hours of daughtering around he throws his hands in the air, packs up his gear and left.

I'm getting tired of shotgunning this thing and I'm running out of money. I'm also into it too deep just to junk it. There's something simple or too stupid for me to see here, Its gotta be just one small thing that I'm missing.

Thanks for your ears, and your suggestions. Let me know if you need more info.

sounds like you may have the distributer in 180 degrees off. are you sure you installed it on the number 1 pistons compression stroke? it very common to make that mistake.

Link to indexing the distributor.

http://www.angelfire.com/my/fan/dist_index.html

Thank you for your replies. I've had the distributor in and out so many times I think Ive worn out the hole. I've followed the link you provided, and the FSM. I think its in there right. Of course it wont hurt to recheck!

What I was tracing down last night was the voltage from the Auto Shut Down Relay. It seems to blink with a test light at a constant rate when cranking. This supplies the injectors, coil, alternator and has a feed to the ECM. Although I've jumpered it to the Batt+ the engine still wont start. I'm thinking some thing is causing the drive to the relay to be flakey.

If I am understanding correctly it used to run, but now won't after replacing parts?
If so, you may want to throw your old CPS back in and see if it will run. Its not unheard of to get bad ones out of the box.

WHUMP . . . Did you hear that ??? that's me banging my head against the wall so hard it created an ultrasonic concussion to rival that of Krakatoa!

You guys nailed it! It was the distributor off by one tooth. It looked like 5-oclock to me, but it was more like 4:30.

Any way thanks again for the help! You guys are GOD's.

Easy fix; glad we could help. I always suspect distributor indexing for these types of symptoms after a rebuild.

Glad you got it figured

I did not realize how close the relationship of the sync pulse and the crank pulse had to be. I always thought the sync sensor was just to determine where to start the injector fire sequence. I guess if its off the ECM says 'Wait, I cant fire the injectors here ..." and gets confused where to put the spark as well as energizing the auto shutdown and fuel pump relays.

Great job guys, I just cant say thanks enough. You saved my a*s for my lady! Anyway have a great day, I will.