1998 XJ; Slow/Underpowered Acceleration, Shift Problems, Error Codes P0700 & P0720

Hello!

Thanks for reading, new here, acquired my XJ about this time last year. When I first bought it, I was told I would need new shift solenoids. Against better judgement, I bought a cheaper pack on eBay for about $121 for all 3. Mostly, the shifting wonkyness was from 2nd to 3rd gear, I'd guess. It was also throwing a P0700 code, transmission control system. Anyway, I replaced the solenoids, and it seemed to be okay for a couple days, code was gone, but it shortly returned to its symptoms of late shifting. I never bring it above 3000 RPM, it's always seemed a little underpowered since I bought it, and I feel like I'd blow a gasket to even try it.

That was about 4 or 5 months ago but, lately I've been noticing that it'll slip out of gear around the same shift area (2nd, 3rd). It'll pop right back in, and it's never really a rough slip, but I've got some error codes again. I first got the P0720 OSS Circuit, which would come & go, but today I've got the 'ol P0700 too! I'm hoping it isn't the solenoids, and from reading the forums, would these symptoms be in alignment with a dirty/gummed NSS?

My next mission, before I buy a new OSS, would be to remove, disassemble, clean, then reinstall the NSS to see if that helps. Then I was going to clean the throttle body, & IAC valve.

Any other simple tips/fixes you guys would recommend before biting the bullet on this OSS?

Thanks in advance!

You might want to take a look at the throttle position sensor. If it is a little dodgy it can cause late shifting

I had had a TPS code also, when I had bought it, forgot to mention it. I had replaced it around the same time as my shift solenoids.

So that got rid of all the codes and CEL?

No, I still had the transmission problems after I bought and installed a new TPS. The TPS code was an earlier number, was told to fix the lower number codes first. Replacing the TPS, if I recall correctly, did help with this hunting idle issue I had had in traffic.

The solenoids I replaced after the TPS, since the TPS was a cheaper and quicker fix. The codes disappeared for a while, only recently came back, though the sluggish shift/acceleration has basically always been there. Was only moderately improved after the solenoid/TPS replacements.

When was the last time you changed the fluid and how many miles on it?

Changed the tranny fluid same time as the solenoids, still at level, that was about 5 or 6 thousand miles ago. Bought it with 146k miles, it's at 153k now. You know what i have noticed? The catalytic converter is rusted to shit, could that cause tranny codes to be thrown?

No but could have something to do with being underpowered due to the honeycomb coming loose and plugging the outlet. Also slow shifting could be caused by the throttle being depressed more than usual