Recently my '03 4.2 has been hesitating when running at a constant speed - almost like a mini-misfire. I had the codes read (my OBD had stopped working) and it showed P0191 fuel rail pressure sensor circuit range/performance. The error code was deleted. It has had a new fuel filter and also the pipe from the tank to the filter which had a bad connection since the problem started and neither has helped this specific problem (although it did go better generally after the filter swap).

I bought a bluetooth OBD reader and got the Torque app for my phone and have set it to show Fuel Pressure - whilst driving at steady pace, say at 50mph the pressure is constantly fluctuating, possibly inline with the hesitations I can feel so obviously I wonder if it is connected. Does anyone know if this is normal? Having not looked at it before the problem started I don't have a direct comparison to make so not sure if this indicates the problem or is unrelated. Anyone able to confirm?

On a slightly different point it is interesting to see that I can only get just over 80% throttle showing even when flooring it with kickdown so wonder if there is some adjustment possible to throttle position through the cable or similar - will probably start another thread on that though.

 

I have the RealGauge oil pressure mod installed and get a constant readout of oil pressure via an electronic gauge installed in my rearview mirror. At 50 mph, the oil pressure should be about 60 to 65 psi and it will fluctuate +/- 3 or 4 psi.


As for the throttle, this is an old problem with our cars and an easy fix. Just take up the slack in your throttle cable. There are lots of threads on this forum regarding this issue.


Mark

 

Cruising at constant speed the fuel pressure usually stays within 1 or 2 psi of 55.

There are things that cause it to change though, like getting out of the throttle quickly and coasting at highway speeds.

I think the pump shuts down, or nearly so, because the pressure will drop to less than 20. Comes right back as soon as you touch the throttle.

 

It is probably moving more than that, the real issue being that it is moving quite suddenly. Interestingly I am getting almost the opposite to what you say when I come off the throttle, the pressure then increases to about 60psi until I reapply it - not had chance to try it for long periods but that is certainly the case in a short lift.

 

At constant speeds, say with the cruise on, it should be pretty stable. If you see big, sudden swings I would suspect the sensor before the pump.

I do see it go up to 60psi sometimes, usually in town. The only way to reduce the pressure is to use the fuel in the rail, so at idle it can take a bit to come back down.

The pressure drop on coasting I have only noticed at highway speeds when you get completely out of the throttle for say an exit ramp.

 

I see a different response for my oil pressure. At highway speeds, say 70 mph, I see 72 to 73 psi, but it does fluctuate +/- 3 to 4 psi. I should note that I have a digital gauge, not an analog gauge. While idling I see about 25 psi and it linearly (roughly) increases as my speed increases. I do not see a noticeable on throttle/off throttle response. Also, the fuel pressure is higher when the engine is cold, until it warms up. See the photo below, voltage is on the left and oil pressure is on the right (the engine is cold here and idling in my driveway.)


Mark

 

Thread hijack, sorry. Where did you get the mirror with read outs and how does it work?

 

Not sure why people,are responding with their oil pressures, it is fuel, not oil. Anyhow, your fluctuations, and ccfulton's info is on par with my experience. Interestingly, the ecu will not throw a code if fuel pressure is too low, I know that from experience, as my pump was bad and at half to full throttle, pressure would drop down to 10psi, never a code, ever. The sensor circuit code is simply a self check by the ecu to be sure the circuit isn't open or shorted. If that code is gone now for whatever reason, I wouldn't be concerned about it being the root of your problem.

I also had what I characterize as a slight, very slight inititially, surging, mostly only noticinle intermittently on the highway. No codes, monitored all kinds of settings including fuel pressure. Over about 1000 miles it gradually got worse and more frequent to the point that I could make it surge, or not, by manipulating the throttle. Mostly if the throttle was barely enough to maintain speed, it would surge. Finally it started throwing po1226. I resolved the problem by replacing the throttle body. I have not heard of many 2003 and up TB's needing replacement, but it resolved my issue. My 2004 had 100,000 miles when this happened.

 

First, sorry for the off topic oil pressure comments. I miss-read the topic.


As for where I got the gauges in the mirror, it was from a place called RADAR-Mirror (Radar-Mirror). These guys put displays for high end RADAR detectors into rear view mirrors but they also put gauges into mirrors. I had considered doing the 3 gauge conversion because I really like the gauges (and they are important when it comes to Jags) but I also have no natural sense of direction and rely on the GPS (which I updated with a Garmin GVN-53 on the stock Nav screen). This gives me the best of both worlds and they work very well. However, it only makes sense if you also do the RealGauge mod.


Mark

 

Thanks for that, interesting. Rather than a surge mine is more of a hesitation, almost the opposite of a surge like a mis-fire. I've used plenty of fuel injector cleaner but may try cleaning the throttle body as a next step and see if that makes any difference.

No problem about the off topic oil pressure - interesting to see some issues there too, just they don't solve my current issue!

 

Sorted the throttle now thanks, really easy to do but I hadn't realised there was an issue until I saw the reading - certainly responding better now.

Back to the hesitation/uneven running........