96 XJ Fuel Pressure Sensor?

[East_tx_dad]

1996 XJ 5 speed 4.0. I've got the setup with the fuel pressure reg in the tank with the pump assembly so no return line.

My question is does the ECM have any way of detecting fuel pressure? I don't see any kind of fuel pressure sensor, but the fuel pump will run sometimes for a while before shutting off when the key is turned on instead of just the 2-3 second run time. At first it seemed to be related to fuel line pressure but I've seen it happen several times now with the fuel rail pressure reading upwards of 40 psi. When the pump is running after the key on, the jeep will crank but not start (almost as if the ECM isn't opening the fuel injectors). But as soon as the fuel pump goes through a "normal" cycle (key on, pump runs 2-3 seconds, pump shuts off) it fires right up.

Background, 1000mi on refurbished engine with 370k on jeep. New injectors, plugs, wires, distro with engine. Before engine swap, started having long starts occasionally when jeep would sit for several days or have cold/wet weather. Has gotten worse after engine swap. Fuel pressure reading around 48psi and pressure would slowly leak down after several hours so replaced fuel pump with new Bosch pump (assuming check valve leaking) but it hasn't really changed anything.

 

[RCP Phx]

They don't have a pressure sensor and it sounds like a pump going out on you but I would also suspect the injectors leaking.

 

[East_tx_dad]

Well, I just replaced the pump and the injectors where brand new 1000 miles ago. But what I don't understand is what determines when the pump shuts off? Is it just a timer in the computer? Fuel pressure sensor? Pump current sensor? the pump seems to run until pressure builds and then shuts off (but that's not always the case as mentioned in my post). If there is no way for the thing to sense fuel pressure, then what would make the pump run for 10 seconds sometimes but run for 3 seconds other times? Some kind of time delay relay? I just don't see anything in any of the wiring diagrams I've found that explains it.

 

[RCP Phx]

Anytime the ecu detects a running motor the pump is on. On initial startup there is a separate circuit that cuts out the pump until the engine starts.

 

[Anak]

I cannot think of any relay that would stick and cause that scenario.

The pump being "on" should not prevent the engine from starting.

My best guess is something wonky going on with your ECU.

 

[techno1154]

1996 XJ 5 speed 4.0. I've got the setup with the fuel pressure reg in the tank with the pump assembly so no return line.

My question is does the ECM have any way of detecting fuel pressure? I don't see any kind of fuel pressure sensor, but the fuel pump will run sometimes for a while before shutting off when the key is turned on instead of just the 2-3 second run time. At first it seemed to be related to fuel line pressure but I've seen it happen several times now with the fuel rail pressure reading upwards of 40 psi. When the pump is running after the key on, the jeep will crank but not start (almost as if the ECM isn't opening the fuel injectors). But as soon as the fuel pump goes through a "normal" cycle (key on, pump runs 2-3 seconds, pump shuts off) it fires right up.

Background, 1000mi on refurbished engine with 370k on jeep. New injectors, plugs, wires, distro with engine. Before engine swap, started having long starts occasionally when jeep would sit for several days or have cold/wet weather. Has gotten worse after engine swap. Fuel pressure reading around 48psi and pressure would slowly leak down after several hours so replaced fuel pump with new Bosch pump (assuming check valve leaking) but it hasn't really changed anything.

I have been going through the same thing for almost a year. Did a complete wiring overhaul with no improvement. https://www.naxja.org/forum/showthread.php?t=1163382

Recently I discovered online that this problem plagues the 1996-year model based on posters from England, Australia and here in the US. It bewildered me that that only seem to happen after the XJ was sitting for several hours. In my case, the fuel pump will run nonstop, and the engine will not start even though the fuel pressure remains near 50PSI. A few weekends ago I was home after a two-week absence and noticed when this happened, the normal lights in the dash will not come on i.e., battery, check engine etc. which should come on for a few seconds then go off whether the engine is running or not. This led me to assume that the fuel pump relay was getting ignition power the ASD relay was not getting power or it was simply finicky after an overnight shutdown. I replaced the relay and drove the XJ for 2 days and it started each time with no problem. That was three weeks ago. I will be home again at the end of the month at which time I will see if my fix is permanent or not.

The online posters claimed their fix was to replace the PCM. In my case, replacing the PCM did nothing to fix this issue. However, if I disconnect the power at either the battery or the PCM it will start just fine then the problem would return after a shutdown for several hours. I would admit though that when the engine is running, I could drive for several hours with never a hiccup. My XJ have almost 400,000 miles.

 

[East_tx_dad]

Yep, that's exactly the problem I'm having. I have to cycle the ignition switch several times until the CEL and GEN light come on and then she'll fire right on up. Once started, she runs just fine and I can turn it off for a few min and restart without any issues so it kind of seems to be heat related. I'm going to start checking grounds and wiring associated with the ECU and ASD relay.

 

[East_tx_dad]

FIXED!!!

OK, posting the fix for my issue for anybody else that might find this thread. Basically I had a bad computer (ECU). Replacing the ECU also fixed a couple of other things that I didn't realize were problems.
Basic timeline:

-For several years, the vehicle would be hard to start when very cold or if sat for several days. It would take several attempts to start and then it would run like crap for a few minutes. I would have to feather the throttle to keep it running then it would be fine.

-Fast forward to this past summer, with 370K on the engine, the #3 piston decided to quite being a piston so I swapped out the engine with a reman. No issues there and it fired up first try!!!

-Fall came around and it started getting cooler, I had issues with long crank times, I attributed this to the fuel pump check valve. Replaced the fuel pump (which was still original OEM) but it didn't fix my issue.

-Noticed that when the engine wouldn't start several things happened. First, the fuel pump would cut on and stay on until I turned the key off or tried to start the vehicle (it is supposed to turn on for a few seconds then turn off). Second, the check engine and gen light would not come on at all when I turned the key on. I found that if I cycled the key on and off several times eventually the check engine light and gen light would come on, the fuel pump would cycle on then off after several seconds and the engine would fire right up.

-This happened more and more frequently and also was much worse when the temps were colder. Finally one day it was 40 deg outside and it wouldn't start at all. Waited for the weather to warm up 2 days later and at 65 deg, it started with no issues.

-Thinking I had a wiring harness issue from the engine replacement, I tried cleaning grounds, checking voltages, and even replaced the ignition switch due to a slight voltage drop through the switch but nothing made any difference.

-Swapped out the ECU with one from Flagship One and no more issues. Even went through an unusually cold December (temps in the single digits) here in Texas and jeep started each time with no problems.

The things that I didn't know where broke, I use Torque on my phone with a Bluetooth OBDII reader and the inlet manifold vac sensor would never read below 20 in HG, also, the fuel flow rate would never read above 2.1GPH, and on occasion, I would have to cycle the key on/off to be able to clear engine codes. All of those issues are gone now.