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Lacks power untill 3000RPM

serialphreak

serialphreak

NAXJA Forum User
Location
Estevan Sk Canada
Ok so im on the highway and i have very little power until about 3000RPM. It cant even maintain 100km/h unless im at like 2800rpm and even then it struggles. My XJ is an '89 4door with a 4.0L auto. my air filter is new and clean. i have proffesionaly purged my fuel system including changing the filter. my exhaust is simply straight pipe to a bottle. no catylitic converter or muffler. My jeep has been outfitted with 31x10.50's for over a year but the problem with power is recent. what am i missing why does it feel like it does nothing untill 3000rpm? im losing my mind.
 
I don't know about the TPS. You could try it, but the TPS is really only for sharp increases of throttle. Sounds more like a breathing (intake/exhaust), spark, or fuel problem to me.

The highish idle could just be a seperate problem caused by a vacuum leak.
 
Does it shift OK?
When the fuse to the TCU is blown (or TCU disconnected) it could be starting out in 4th gear instead of 1st.
While I was going thru the paces with mine, I found this out...it's sluggish starting out in 4th for sure.
 
Simple things first. Verify your fuel pressure at the fuel rail by installing a pressure gauge on the schrader valve. Compare to spec. Takes only a few minutes and if that isn't it, then there are lots of other things you can chase! (oh and X2 on tuneup stuff like cap, rotor, wires, plugs)
 
serialphreak said:
would poor fuel pressure cause lack of power till 3000rpm? if it was weak pressure wouldnt it lack power all the time?
Click to expand...

I had a fuel pump that was going out and not pushing enough pressure. Everything was fine untill i got in-between 2000, and 3000 RPM's Within this range i would have hesitation, and backfiring...
 
feuer895 said:
could it be that you have a straight pipe therefore no back pressure till about that???

On those 4.0s you need some back pressure for the low end torque.
Click to expand...

No no and no. I ran a straight pipe and it worked fine. He also stated that it was working fine until recently - much after he straight piped it.
 
Hey yours is an 89. I got thinking, perhaps your EGR is fully stuck open. Does it idle rough? hmm I guess it wouldn't have a high idle with an EGR valve open. It is a thought though.
 
ok i checked the fuel pressure while driving and it stays at about 30psi all the time even when its acting up. also i checked the tps with an ohm meter and there is no open spots at all. also made sure the cps wasnt open as some one suggested earlier. it had 240 ohms about. not sure if thats ok or not but its not open anyway. also i pulled a plug out it looked new cuz its only a year old so are the wires. i pulled the cap off it looked ok but i pollishied the tips up with emery cloth just to be sure. no changes, i still have the same problem. so you guys think that that fuel pressure is ok? also is there a chance that maybe the timing isnt advancing or somethin like that? and is there a way to check it?
 
Last edited:
I would check for vacuum leaks.
 
i've got the same problem with my piece of sh*t '88. power is sluggish, idle is irratic......

up to EXACTLY 3000rpm! it's like a light switch. at 3000rpm it pulls like a freight train and sounds perfect (as opposed to how it normally runs).

at what RPM does the Renix go into closed loop while cruising?
 
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