i have an 88 xj rebuilt engine and with new map, egr, o2, tps, both coolant sensors, new distributor cap newer rotor wire plugs injectors 2 weeks ago it was running alright still had a rough idle and now it idles fine but when rev'd up it backfires and sounds like a helicopter im stumped on what the problem could be and am in dyer need of help i checked timing and timing is fine

 

Base timing is controlled by the computer. If it ran good at one time, it probably is not an indexing problem with the distributor, but still a possibility.

Backfiring in the intake can be related to a "lean" fuel air condition. Job #1 is to verify fuel pressure. For an 88, you should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to 39 psi. Be darn sure that the vacuum line leading to the FPR is pulling good vacuum and is not wet with fuel. If it's wet, you have a ruptured regulator.

Test your MAP sensor. New sensors fail more often than you would think; don't get tunnel vision and eliminate anything. And like the FPR, be sure that the vacuum line leading to the MAP is also pulling good vacuum.

Here is a link to an article on testing many things on the 87-90.

http://www.lunghd.com/Tech_Articles/...iagnostics.htm

 

For proper distributor indexing, search for Jeep TSB # 18-53-88.

 

the previous distributor had the tab cut but i thought the timing couldnt be adjusted by the distributor

 

The igniton timing isn't but the injector timing is. Anyway, you gotta be close and have that dizzy indexed properly.

 

that makes sense how do i set the indexing?

 

Check your fuel pressure first. I'm trying to find some decent instructions on Renix dizzy indexing.

 

i checked it right before all this happen and i was gettin about 33 idle and 39 with the vacuum unplugged

 

Okay. What is the fuel presure now that this started happening?

 

havent checked it yet i planned on doing it on monday but i wanted to get a few opinions on whats goin on ive been throwing money at the thing which i know was the wrong thing to do but i just want the thing fixed so i can take her out again

 

Yes I agree you should check the fuel pressure and to make sure you are getting good fuel pressure and if that is good you might want to check the timing again I have put a distributor in and the number one cylinder was a 180 degrees off of the compression stroke and it ran fine till you gave it a little gas then it would cut up and back fire

 

so i figured it out the distributor was off somehow i was 180 off and i knew that to begin with but didnt think it mattered so when i switched it back to how it should be i noticed it was a little too far advanced so i had to turn the oil pump some to get it to line up correctly and it fired up and runs like it used to it still has a rough idle but i think i narrowed that down to the egr crap thanks for all the input it was greatly appreciated