I need some advice from fellow XJers. NAXJA has several posts on Renix problems that never really seem to get fixed so now that Pirate has an XJ form I figure I'd throw it out and see what you guys come back with.
I have a 1990 4.0 5 speed. New top end on the engine. It runs great when cold. Here are the problems and it had these problems prior to me changing the head. So I'm sure I didn't screw that up.

1.I am getting back firing when the engine is warm from the 1500-2100 rpm range when I'm accelerating. Over 3k plenty of power runs great.

2. If I idle for more than 10-15 min after its warm it starts to drop cylinders and is very difficult to rev past 2k rpm.

what I've done

1. cleaned ground on the firewall and engine
2. changed the ICM (different problem, engine would shut off like turning the key off and the tach would drop to 0 while the engine was still turning. ICM fixed it)
3. checked and fixed any vacuum leaks
4. new plugs, wires, cap and rotor
5. removed injectors cleaned, new o-rings
6. Fixed cracked header
7. New intake gasket
8. Unpluged coolant temp sensor, didn't fix or really change anything
9. Unpluged inlet temp sensor, changed rpms but problem still existed
10. Unpluged anti-knock sensor, no change

I know the Renix seem to a lot of these kinds of problems and it might be a mix of screwed up components but I don't really want to shotgun the fix. I still need to check fuel pump pressure.

anyway, thanks for any suggestions. If anyone cares I will post progress and a fix when I find the problem(s).

ever had a shop plug it into a diagnostic machine?

See, this forum is gonna be the next NAXJA...

If this is gonna be a 'hardcore' XJ forum, there should probably be a disclaimer somewhere...

Ask in the newbie forum. But if I were to put a finger on the problem, it sounds like a bad muffler bearing. When was the last time you replaced yours?

All the bull shit post aside... check your fuel pump

fuel pressure at the rail should be about 31 PSI with the vacuum connected to the regulator, and should jump to 39 psi if you pull the vac line.

also, check the fusible link going to the pump, mine did some funny things and the link was half burnt, only sending 8V to the pump.

Also, there's a fuel pump ballast resistor on the drivers side fender, it's a small white block. Bypass it with the appropriate sized wire.

Buy all those parts, do all that work, ask around on the Internet.......have you taken it to a shop? That's the type of problem a shop usually finds fairly easily. I ask because you didn't list that in the things you've already done.

My last two examples of weird problems that got fixed when I took it to the shop, for less than $200 for both.

Engine was running really rich, changed MAP sensor and TPS and cleaned IAC, was on vacation so took it to a shop. Found the plastic fitting for the line to the MAP sensor at the throttle body had melted closed, and I would have NEVER found it.

Had a draw when everything was off that drained the battery in a couple of days. Pulled fuses etc, nothing obvious. My onboard welder was malfunctioning and had an internal current draw. Another one I would have never found.

All the bull shit post aside... check your fuel pump

Just got a day off work and had a chance to work on my pile. I picked up a fuel pressure gauge at NAPA and checked the fuel pump pressure. 26 psi with the reg hooked up or unhooked. put in another pump and pressure was 38 or so and it runs like new now. Besides time the only part I put on that wasnt needed was a MAP sensor and it was a cheap one from the salvage.

try a throttle position sensor. i did a motor swap in my 89, i changed everything (throttle body included) when i went to break in the new cam i could not get the motor to run smoothly below 3k, it would back fire,pop, etc... unplug the tps and motor runs great and idles right down normally. i ended swapping the sensor back from the original. all ok now

i have had the same problem and replace all sensor . last thing i replace was the injectors. running bad problem was fixed. i used injectors from and wrecked 95 xj.

I hope its been fixed sometime in the last 6 months:D