stumbles and hesitates

[Ex-Jay]

My 1994 XJ Sport 4.0 hesitates and stumbles at 1500 RPM's until 2000 RPM's. Also just read that timing can't be adjusted. Other than doing a tune up, wires, cap, plugs, clean the throttle body, is there anything else I can try.

Thanks

[Jeep-King]

I would check to make sure that the fuel pump and fuel injectors are working proporley. in my 1989 cherokee my car would hesitate at low rpm's but after fixing/cleaning these components the hesitation stopped.

[wwwildman]

It could also be the MAP sensor or vacuum line going to it, TPS, or IAC. My son's 89 did the same thing; it was a pin hole in the vacuum line going to the MAP sensor.

[Ex-Jay]

I have a new MAP sensor and new tubing to the sensor. Also, the connection at the manifold is clear and has good suction. One thing I also noticed was a rich exhaust. Very gassy.

I have a new O2 sensor, new MAP, new PCV's (front and back with tubing) new plugs, wires, cap and rotor, flushed the throttle body, and injectors. the only thing I haven;t done is pull the plug on each injector to test them, but then again my exhaust is rich, and also haven't replaced the coil.

[Ex-Jay]

Oh, I also forgot to mention that I did a pressure test on the fuel rail and it was good ... 31 PSI with vaccum line on and 39 PSI with the line off. Perfect accoding to the book. So, that leaves to think that the fuel pump is good. Prior to that I changed the fuel filter.

[Ex-Jay]

Oh, I also forgot to mention that I did a pressure test on the fuel rail and it was good ... 31 PSI with vaccum line on and 39 PSI with the line off. Perfect according to the book. So, that leaves to think that the fuel pump is good. Prior to that I changed the fuel filter.

[sneakybert]

The fuel pressure spec for a 94 xj with a 4.0 l is 49 psi plus / minus 5 psi

[Ex-Jay]

49 +/- 5 PSI ..... hummm. I double checked my book and did notice a broad range earlier in the chapter, they mentioned 39 to 50 PSI. But if it's suppose to be 49 then I'm off, and since I'm off I want to look at my pump. Is this easy to access?

[Ex-Jay]

Ok, I reread the book. Specifically in the MPI section with a fuel regulator on the fuel rail it say 31 PSI with vaccum tubing on and 39 PSI with the tubing off.

Can anyone else verify this and let me know if its 31 PSI or 49 PSI. BTW ... I've looked at the whole removing the fuel pump thing ... so don't reply with a how to on that. It's simple. My main concern is the 80 bucks for a pump, and whether or not I need one since I'm confused on the PSI specs.

[wwwildman]

Originally Posted by Ex-Jay

Ok, I reread the book. Specifically in the MPI section with a fuel regulator on the fuel rail it say 31 PSI with vaccum tubing on and 39 PSI with the tubing off.

Can anyone else verify this and let me know if its 31 PSI or 49 PSI. BTW ... I've looked at the whole removing the fuel pump thing ... so don't reply with a how to on that. It's simple. My main concern is the 80 bucks for a pump, and whether or not I need one since I'm confused on the PSI specs.

You are correct with the pressure reading, 95 & earlier is 39 with the line disconnected and 31 with it connected. Sounds like your pump & fuel filter is fine.

[sneakybert]

49 psi plus or minus 5 psi is the manufacturer published specification. This is checked with the fuel pump relay powered and the engine not running .

[Ex-Jay]

I've already replaced the fuel pump and regulator and did a test. it was 31 with the vaccum line on and 39 with it off. no change from the first test (with the original equipment). 94' sport 4.0 HO.

[Ex-Jay]

btw .... my problem was 2 bad injectors. they were replaced (after the fuel pump and regulator, since they did not fix the problem) and all is good now.

[89ltd]

how do did you know the injectors were bad? I'd like to test mine as well.