Jeep 4.0 no start troubleshooting puzzle

Weird trouble with first start of my new engine

When I put in my new engine I expected it to start up right away. I had carefully aligned the distributor, checked all the electrical connections and bled the air out of the fuel system.

But when I hit the key it simply spun over and never fired.

I put a light in one of the spark plug wires and it flashed indicating I had spark. That meant that the Crank Position Sensor was working.

I put and LED in one of the injector wires and it flashed indicating that I also had the cam sync signal. I verified the fuel pressure was correct.

I checked compression and added a squirt of oil into each cylinder in case the rings were not seated.  I got a pop or two but no start.

I tried spraying starting fluid in the throttle body in case I had some bad gas.  I got a backfire or two but still no start.

I pulled the number one spark plug again and looked inside with a bore scope.  I saw the mark on the piston showing .040 over bore and everything looked fine.  I verified the mark on the harmonic balancer was correct at top dead center.

After several hours of various checks, I tested a different Crank Position Sensor. Same result.  I even tried turning the distributor 180 degrees even though I knew it was correct. No improvement.  I verified the firing order again and the arrangement of the spark plug wires on the distributor.

I had fuel and spark and compression but no run. The engine sounded like it was out of time.

I read and re read the shop manual on how the ignition timing is determined.  The holes in the flex plate signal the computer to fire the coil based on the input from the crank position sensor.  There is no other input.

I even took down the transmission and replaced the new flex plate assuming something was made wrong about it.  No difference.  I mounted an old bellhousing, so I could mount the CPS and see how it lined up with the marks.  It was on the fourth notch at 4 degrees before TDC just like the manual said it should be.

I was also having a problem that none of the timing lights I had on the shelf worked. So, I had to order a new one.

One that arrived, I was able to confirm that yes, the ignition timing was off. WAY off.  So far off I am surprised the rotor still made enough contact with the terminal in the distributor to give me a flash.

According to the manual, the cam sensor plays no role in the ignition timing. It only tells the fuel injectors which back of three to start on. After that, the computer no longer looks at it.

However, the ECU must see a pulse from the Cam Sync Sensor before it will allow the start of the engine.  So, in this sense it does control the ignition.

I disconnected the distributor and I got one single pulse from the ignition, but it was right on the mark!  I was on to something.

I got my old distributor out of the discard pile and plugged it in to the harness. The new one still hooked to the plug wires and bolted in.  I found I had to spin the distributor by hand to give it a pulse while cranking the engine but as soon as I did, the engine roared to life!

The new distributor I purchased was bad and somehow causing havoc with the ECU.  My factory manual did not offer a way to test the sender without an oscilloscope, so I just cleaned up my old distributor and put in back in and returned the new one to Amazon.

I had replaced the distributor because I was getting a cam sensor fault before the engine swap. But I had traced the problem to a skinned wire on the engine harness while the engine was out.

I have learned way more than I ever wanted to know about how the ECU works but I still don’t understand how a bad cam sensor cause the timing to be way way off.

The new engine runs great now. I just hope I did not take too much life out of the starter with all the hours of cranking I did while trouble shooting.