dmoe69

O2 sensor locks up after about a minute, but not at idle in park. With the vehicle in drive and the e-brake on, the O2 sensor locks up at 4.80v and 16.0 AFR immediately when a little throttle is applied. Back in park it will return to fluctuating and closed loop. The REM is showing an error, which is on the "ST", short term fuel trim above 255. I found a post on this from 2018 88 4.0 Runs Rough After Engine Rebuild, tried everything, nothing helps - Jeep Cherokee Forum
This guy had the same ST error. He ran the problem down to an aftermarket flexplate made by ATP - part # 205. I have an ATP 205 on the back of my crankshaft. Apparently the timing windows are slightly out of alignment from the original equipment.

lawsoncl

So it starts reading back when you load up and torque the engine? I would inspect the wiring, particularly where it crosses between the engine and the firewall. Ensure the O2 sensor wiring isn't against the exhaust pipe, and nothing looks melted or flexed to the point of breaking. I doubt it's the flexplate, and the ST issue is due to the bad O2 sensor reading.

dmoe69

I unwrapped the wire bundle and flexed/pulled/pushed the whole length. No change. The O2 sensor test shows 13.5 volts to the heater, 4.8 volts from the ECM, and I previously ran a dedicated ground from the O2 sensor the the battery, which is still 0 ohms. I took a spare CPS and slid the sensor to the inboard bolt hole (advanced) and swapped it out for the installed CPS. I was able to drive the vehicle up to 60 MPH for the first time, although it would still go into open loop caused by the high ST, but not as easily. I think this lends itself to the bad flexplate theory. However, I also noticed that the high ST diminished when I pulled the vacuum hose off of the FPR (which is regulating fuel at 31 PSI/39 without vacuum) The ST would drop from around 230 at idle to 170. I purchased the installed fuel injectors back when I first got the XJ. I don't remember what quality they are, but they look like the ones that came off the old motor. I have purchased a set of remanufacture Bosch 746s, they should be here before the weekend. I am of the understanding that the high ST means the ECM believes the engine needs more fuel to resolve a lean issue. When the ST reaches >255 the ECM defaults to a preset open loop condition that causes the reduced performance. Perhaps I am not getting as much fuel through these stock looking injectors as is needed.

dmoe69

I should add, I have a new Bosch 12009 and NTK 23533 O2 sensors. I ran them both in the engine today with the same results

dmoe69

As of this evening my issues have been resolved. The original rev limiting resolved when I spaced the CPS off the bell housing with 2 washers. Looking in to the bellhousing with a mirror it appeared the sensor was over the forward edge of the timing windows. I had replaced the factory flexplate with an aftermarket ATP 205 flexplate. Moving it aft, towards the rear of the vehicle seemed to correct that issue.

The engine continued to have drivability issues though, after receiving the RENIX Engine Monitor (REM) from NickInTime I was able to see that the Short Term Fuel Trim (ST) was going off scale above 255. This is an indication of an extremely lean fuel condition, or at least the ECM believes so based on sensor input. I replaced the fuel injectors today with Bosch 746 injectors. With the new injectors the ST immediately corrected to just below 100 at idle and bouncing around 128 while driving.

Prior to swapping in the new injectors I replaced the fuel filter, as is recommended. The old filter (less than a year old, with less than a mile of drive time) drained both clear fuel and discolored fuel that it had separated. I think this is the root cause of my problem, and I would suspect many other who have engine issues. My fuel tank is encased in a factory skid pan and then further entombed by a trailer hitch. The tank appears nearly impossible to replace, so I decided to take other precautions to ensure the tank was drained and cleaned.

I pumped the tank down with the fuel pump (disconnected the hose from the fuel filter) until loss of suction. Then, with the fuel pump removed I inspected the tank internals with a mirror. It really did “look” clean.

I should have replaced the tank. I would recommend anyone who is working with an XJ that has sat unused for a while (mine was over 8 years) replace the tank. There is too much time and expense put into making the vehicle otherwise perfect to then have to endure the uncertainty and frustration of why your vehicle won’t run right.

lawsoncl

I think you meant NTK 23553. Don't want to confuse future readers.

dmoe69

That is correct NTK 23553 O2 sensor