I have a 1993 jeep grand cherokee with a 4.0. Here's the situation...It seems to start fine, but when it idles it will climb up to about 2000 RPM and then go back down to a normal idle. Sometimes when it goes back down to a normal idle it will almost die. When I give it a quick burst on the gas it has a split second hesitation. Also, when I drive it it has NO power at all in fact it will almost die the gas has to feathered to keep it going. Here is what I have done to it. First I replaced the fuel filter, plugs, wires, distributor cap and rotor. That didn't help. Next I replaced the fuel pressure regulator and the throttle position sensor. No help there either. Finally I dropped the gas tank cleaned it out and removed the fuel pump, cleaned the small internal filter on the pump (it was really dirty, I thought that was the problem but it wasn't). That didn't help. I bought a pressure gage to check the pressure in the fuel system, haven't done that yet, but my plan is to do it Saturday morning, in fact my goal is to get it running right Saturday. Some other things I considered was the fuel pump, crankshaft position sensor, camshaft position sensor, and testing the injectors, or adjusting the timing. If I adjust the timing, how should I go about doing it? I have been told to adjust it by rotating the distributor cap is that possible? Could I need new sprockets and a timing chain? Also maybe a vaccum line? Without a computer to check it I am left at trial and error. I have been replacing the less expensive things and working my way up. The jeep isn't making unusual noises when it runs. Just a inconsistent idle with no power. Any help would be greatly appreciated!!
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UPDATE: I just checked the fuel pressure at the fuel rail. I got a reading of 19 psi. I blew out the lines and checked again still 19 psi. I was told that it should be 31 psi at idle and 39 psi with the vacuum line removed at the pressure regulator (can you confirm these settings?) When I pulled the vacuum line there was no change in the psi, still 19. Bad fuel pump??

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have u tried checking to see if the iac is dirty or maybe damaged? i had idling problems and power problems , i took out the iac and it was very dirty, cleaned it up and the car reacted alot smoother and better idling, dunno farshot but if u havent checked give it a shot... <img src="/forums/images/graemlins/patriot.gif" alt="" />

check the mass sensor & check your vac lines. if you have an autozone near you they usually will do the handheld computer for you. & if your grand is like my 93 cherokee then you can do the self test: you run the vehicle for a few minutes in park then run it up to about 2000rpms & kill it..then you turn the key from off to run 3 times without starting it..on the third turn to run leave it there & watch the check engine light. it will flash codes. you can look them up that may help too.

What and were is the IAC?

The IAC is the idle air control and it is at the throttle body. There is also at the throttle body the TPS (throttle position sensor. I think of the tps when reading your posts.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

I put in a new fuel pump check the psi and it went right up to 31 psi. Better instead of the 19 psi I had before. Now it works fine. No more lose of power or rough idle. I beat it!!

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Glad you got it fixed. Good feeling, isn't it?

Well on my jeep with similar symptoms, I replaced the O2 sensor, which fixed the idle problem, but I still have the weak acceleration. How do I check the fuel tank PSI?

TOG what gears are you running with them 33's?
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

the stock 3.55. ya i know they need to be changed to something with a lower ratio, but the jeep has been running 33"s since August, but this problem has only been around since February, and it only lags about once each time I run the engine.