zman32006

Got a 90 xj and just put in a 89 4.7 Inline 6. theyre both renix so everything hooked right back up. it was running fine for a day or so but I changed fuel rails, injectors, and did a tune up on the plugs wires and put a new battery in and now it will start and idle perfect with an occassional skip but as soon as any throttle is applied it wont rev at all and backfires. also when its idling the CAI hisses like its WOT but its idling at 500-1000 and once throttle is applied it stops hissing. any ideas? Ive checked the TPS and adjusted it both way and replaced the CPS as well as everything else I listed.

lolz_jeep

check your TPS

o wait just read it again. there might be a vacum leak some where cuasing it to choke out and get rich

zman32006

Any idea what kind of vacuum line would cause that? I put a new fuel filter on it as well.

lolz_jeep

really? did the little o ring get caught and could it be blocking the fuel line or something like that that would cuase it to idle fine but die when theres a higher furl demand

FL Denmark

I would also say a vacum leak.
try spray some breakcleaner on everywhere you think could leak.
you will notice a change in idle when you hit the spot.


good luck

Bustedback

Sounds like a fuel starvation problem.

zman32006

No it did it with the old filter and new filter. Ill check vacuum leaks today

zman32006

so I fixed one vacuum leak on the intake and its better but then I changed the throttle body and put in a new IAC and it idles fine and revs up to about 1200 rpms then backfires. Im about to check the fuel filter and fuel lines to see about a fuel starvation problem.

lolz_jeep

check your timing it sounds like it may have retarded

zman32006

that was my next guess. I replaced the fuel filter and the lines are solid. Im going to unbolt the distributor and turn it and see about that timing because the timing chain and everything else cant be adjusted, just break a gear or stretch the chain.

freegdr

timing is non adjustable .....

freegdr

iac sticking possibly

zman32006

Yeah I replaced the IAC when I noticed that. but the guy who sold me the engine came and trailered to his shop. He firgured out it was a combination of the TPS out of adjustment, the knock sensor plug came off slightly, and the plug for the o2 sensor was melted. He fixed all of that and it runs now.

Metlic53

my 90 just did this, it was the fuel pressure regulator, i changed injectors and somethin went wrong with the regulator, new regulator and cleaned the spark plugs and shes running fine

oops you already answered it, sorry for post.

zman32006

its all good.

I replaced injectors, plugs, wires, distributor cap, fuel rail, fuel filter, fuel pressure regulator. It sounded like a fuel lean problem which makes sense because of the o2 sensor.