Trouble after Banks Header Install

Hi All,
I found out I had a cracked exhaust manifold after attempting to smog my '94 XJ (4.0L) so I could register it in CA. It ran decently for about 3 years (with a NM registration) with a small exhaust leak that never really gave me any trouble. No CELs, no reliability issues, just a slight hiccup in the idle every 20 seconds or so.

I bought a banks header after finding a pretty good price on one, and set about the job of installing the thing. As it always goes, I had put the whole thing back together and I was tightening the the stud closest to the firewall when it broke. After a look of disbelief, I took the whole thing apart and tried to remove the broken part. I failed and decided to pull the head.

I pulled the head, took it to a machine shop for a once over and to remove the broken stud. The head looked good, so I gathered all the parts I'd need and put it all back together. In the process I snapped off the temperature sending sensor, but I thought I could just replace it later. At this point I just needed my car to run. I replaced both studs and put the new manifold on, reconnected all the electrical and vacuum lines. I also replaced the upstream O2 sensor with a Bosch from Autozone.

I started it up and it seemed to run smoother, and a heck of a lot quietter without that pesky leak, but it keeps throwing code 51 every time I drive it. The CEL seems to come on when the engine is warmed up, no matter what load I put on the engine, or speed I drive, although it's hard to tell because of that broken temperature sender.

I checked for vacuum leaks, and rechecked the manifolds for an exhaust leak. I'm pretty stumped; any thoughts? I'm thinking about checking the fuel pressure, but I don't see how that could suddenly not work when it ran so well for so long.

Thank you,

Skyler

Scirocco84 said:

......broken temperature sender. .......The CEL seems to come on when the engine is warmed up ........

Click to expand...

Since the temp sender is not an optional part, you need to fix that. You might be staying in the open loop mode and running richer than the O2 sensor and ECM can compensate for. The CEL comes on as the ECM is trying to switch to closed loop mode and looking for sensor inputs. Also inspect the wires for the O2 sensors, it is easy for them to contact the hot exhaust and either melt through and break or just short out.

Recheck for vacuum leaks also.

Thanks Tim. Yeah I know I gotta fix the temperature sender... I tied up the wires for the O2 sensor pretty good and those are all fine, and I checked for vacuum leaks visually and with the carb cleaner trick. The strangest thing is that just today the CEL went away. I unplugged the broken temperature sending unit to replace it but I can't imagine why that would make a difference.

I suppose what I really don't understand is how the temperature sending unit that just connects to the gauge would have anything to do with the CEL or the open and closed loop modes. Maybe its just a coincidence.

In any case, I'll keep an eye on it and let you know what happens.

what's the brake cleaner trick for finding vacuum leaks... i have one and can't find it- let me know.

thanks-
andy

you spray brake cleaner around areas that could have a vacuum leak and if the engine RPM's rise at a certain place thats where the leak is

didn't know that- thanks!

i use a propane torch...not lit of course