Here is the story; I bought a 1997 xj with a "blown head gasket", well it turned out to have cracks in the head and one of the pistons had a fragment leave itself in the cylinder wall. So I went to craigslist and found a 91 HO 4.0 that "ran good". I install it and hook everything back up and it starts up; idles a little rough then I go to give it gas and it pops and backfires out the intake. it will not rev past 1200 rpm or so.

I have checked; distributor position is correct; I rotated the engine clockwise until compression was felt and stopped then found TDC then used that as indexing the #1 cylinder. I then confirmed with a timing light that my timing is about 10 degrees BTDC. it jumps around a bit but it is ball park.

Fuel pressure is good and stable at 49 psi or so.

The catalytic converter is not clogged; I hooked up a pressure gauge to where the O2 sensor was and it never got above 1 psi as much as I could rev it.

I even swapped out the entire throttle body assembly with another xj's and it did the same thing!
weird observation; just now in the dark I noticed that the exhaust manifold was starting to glow when I let it idle!

I am thinking that it is either; 1) the timing has jumped or 2) crank position sensor?

Help!

 

Personally I swapped in an engine and had the same running issues. Even though your gut says the timing is right I would still try a tooth forward and a tooth back and see if it helps at all.

 

So I pulled the valve cover off to check the valve timing and everything looked good; the exhaust valve opens near BDC and the intake valve opens just past TDC. After some jeep meditation I am thinking that since I reused the distributor that maybe it was damaged in the overheating and that timing wheel has moved? I think this is my problem.

 

ok, so I am mature enough to admit when I made a mistake. I installed the distributor incorrectly. Once I installed the pin to hold down the rotor plate and re-installed it, it runs great. A few teeth go a long way on the distributor!

 

It happens to the best of us. I've done it which is why I recommended checking it. The main thing is it runs great now and you have your jeep back!