Fuel pressure numbers

Picked up a fuel pressure gauge to check the numbers on our 1998 Cherokee 4L and was wondering what a decent number (or range) should be? Any way to check the regulator with the gauge? I assume that can have a bearing on pressure numbers at the rail schrader valve? (sorry, new at testing this). Also, can I drive around with this connected to the fuel rail as I'm trying to diagnose an intermittent poor idle problem? Thanks in advance.

If memory serves me correctly, you should be approximately 49.2 psi at idle. If it falls below 44.2, you'll have troubles.

I suppose you could drive around with it connected but just make sure that it is secured properly, you don't want fuel spraying around in the engine bay if something goes wrong. Just how intermittent is this? I'd tend to just hook it up and leave it idle in the driveway until it acts up and check it then.

Why don't you explain your poor idle quality to us. What exactly are you seeing? The devil is in the details here. There can be a lot of causes of this and fuel pressure is only one of MANY. Cleaning of the idle air control and the throttle body is always a good first step for idle issues.

Thanks for the reply. I was confused about the numbers because another post on here said 50 PSI at the pump and 32 at the regulator. Not sure how that translates into checking it at the fuel rail with a gauge? I don't see a regulator on the fuel rail and read that it's on top of the fuel pump, so I'm not sure which number to look for?

The IAC motor and throttle body were the first things cleaned, both good suggestions. It idles fine 95% of the time, but what I've noticed is the IAC motor starts to close and it begins to run really rough, stays that way for a few minutes, then goes back to normal idling. Not positive but I think the IAC motor is OK but getting a signal to close? Hooked up a SCAN tool and the only sensor I could see was the TPS which is also strange. Figured fuel pressure is easy enough to eliminate one way or the other. Also, tool showed a trouble code for the front O2 sensor high voltage, but I'm not sure if that's really accurate given what's going on? Thanks again as this one is proving tricky...

The regulator is inside the tank as part of the fuel pump assembly so not sure what that other post meant. All pressure readings are taken at the fuel rail to my knowledge. What are your fuel pressure readings? There is a spec for running and not running (dynamic/static).

I wouldn't rule out that O2 sensor reading as a cause. If the symptoms started at the same time you set a code, there could be a correlation. Is your check engine light on now?? Perform a very close inspection of all wiring associated with that O2 sensor. A burnt/damaged wire is not uncommon and can set the code resulting in some pretty strange things happening. You can also test the oxygen sensor with a meter or even better, a good oscilloscope.

If the O2 circuit and sensor itself checks out, it's possible that your IAC is simply bad. Cleaning it doesn't eliminate it as a cause of the problem. I'd suspect that rather than something like the computer.

Stay on it, you'll eventually find the cause!! These gremlins can be kind of tricky to be sure. Keep us posted.

Thanks for the info and the encouragement, very much appreciated. Here's the thread I was referring to on the fuel pressure numbers: http://www.naxja.org/forum/showthread.php?t=77662

Not sure about the actual numbers as I haven't had a chance to hook up the gauge yet, am going to give it a go tomorrow as well as a closer inspection of the O2 circuit and general harness wiring. I think I'm also going to replace the battery cables too as I'd read poor connections can cause this type of issue and the current ones look a bit suspect.

If I don't see anything glaring, what do you think about actually disconnecting the O2 sensor and running it that way for a day or so. Would that rule out anything? I think it would put the engine into open loop mode, which would take the O2 out of the equation (maybe?) but I'm not sure what the effect would be on the IAC motor, which seems to be the trigger of all this.

i would think low fuel pressure would be more of a problem under load or wide open throttle than at idle. tps or bad o2 could be the problem.

Well the whole thing here is the chicken or the egg in terms of what's causing the IAC motor to close and induce the rough running, which is easily noticable both at idle and under load. I think if I fix one condition, I've fixed the other and am all set. Now the IAC does eventually (after a minute or a few) return to the open position and everything goes back to normal, which may indicate something? I don't know if a bad IAC valve would travel that much on it's own or on a small adjustment command from the computer?

So I'm wondering... if the fuel pressure drops due to a bad pump or filter, would that be sufficient to tell the computer to close the IAC? If so, that could be the root of the problem, but it's just a guess at this point and I figure it can't hurt to check it... Eliminate one variable.

And without being able to see any of the other sensor data on the scan tool, including the waveform of the O2 sensor (which would be very helpful), it's even more of a mistery. But when it runs normally, it runs strong and correct, which is somewhat good to know as I think that rules out anything major mechanically?

Got out to the Jeep today and hooked up the fuel pressure gauge. When I turned the ignition on, pump ran for the normal 2 seconds and the pressure came up to 43 PSI. Started the engine and it moved up to 48 PSI, the needle was vibrating between 46-50 really rapidly, but would even out when I brought the rpm's up. Eventually it seemed to steady at 48 PSI, which I'm assuming is normal pressure?

Had it idling for about 90 min's and in that whole time, only got one short period when the idle went bad (heard the IAC motor close again). Fuel PSI remained constant, so I think that system is OK?

Also noticed that no engine codes have been set since the last reset about 2 weeks ago, which seems strange?

Got my SCAN tool to finally view all the sensors too. Seems there's 2 modules, one is 24 and one is 209? My tool was reading from module 24 which could only see the TPS, but when I switched to 209, the other sensors became visible.

O2 waveform looked normal, varied between .2 and .8 Volts. Only thing I noticed is when I revved the engine and let it go back to idle, the output would go flat at .2 V for about 5 or so seconds, then start oscillating again.

Beyond that, I checked vac lines, sprayed gumout, wiggled wires, cleaned up main batt and other electrical connections, all to no avail. Intermittent problems are clearly the most difficult to fix!

Did you ever figure out the cause of these symptoms?