xj hard 2 start and runs ruff

i have a 1990 xj with a EFI 4.0 that has had a few years but few miles since its last rebuild.It was runnig great then it started a hesitation in throttle response. Then after a weekend off mild trail wheeling, turned to a very ruff idle that fluctuates to the point of almost shutting down to running away with itself. When i get it going it has no power and back-fires through the trottle body. It was starting fine before but now i must mash the throttle and crank it till it fires over or runs the battery dead trying. i have replaced plugs, wires, cap, rotor, fuel pump, and filter. Nothing has helped at all. I checked the timming as well and its correct. I thought it might be a head gasket but it is fine also. Any ideas? oh yea, it also has a strong smell of gas. I'm sure thats from having to hold the trottle to the floor but even long after that at idle, if it stays idling. and the cat. converter is burt up inside(rattles and viberates) so this makes me think O2 sensor but would it cause the other before mentioned problems?

Backfiring through the intake can indicate a lean condition. I'd start testing as with a RENIX, you don't have any OBD codes to give you clues. Troubleshooting is a process of elimination and you need to adopt a testing strategy; simple stuff to more complex.

First, be sure you don't have any vacuum or exhaust leaks, both of which can lead to symptoms. Other things to check:

1. Fuel pressure at the schrader valve. Must use a gauge. Know you have replaced pump but you need to establish that your fuel pressure numbers are in spec. Your target is 31 psi at idle and 39 psi with fuel pressure regulator disconnected

2. Test your map and coolant temp sensor. Both are involved with determining your fuel/air ratio and both can be tested. Be sure the vacuum hose to the map sensor is in good condition.

3. Test AND adjust your throttle position sensor. Your vintage is adjustable.

Here is a link to an article that might be helpful in testing many things on your RENIX.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

I agree with birchlakeXJ, and the link he provided should be bookmarked on your computer--that whole website has tons of XJ information, much aimed at the Renix XJs.

One thing I would question is your having to hold the accelerator to the floor while cranking to get it started--sometimes. That is the "clear flood mode", and the only way that would make sense is if you are flooding the engine trying to get it restarted while something else is the actual underlying issue.

Rough idle--basic tuneup, when was that done last? Old fuel/contaminated fuel or did you clean it out and refill? When was the fuel filter last changed? Idle high/low issues--intake leaks, TPS, IAC.

Post back with what you find.