2003 Daihatsu Hijet TE-S210P-TMRK Rough Idle Revs up and Down

Hey Everyone,

I am new to the site and have a 2003 Daihatsu Hijet EF-SE 659cc that has a problem staying at a steady idle. When on first crank it kinda acts like it is going to die but doesn't. Then it will rev up and stay at kind of a high idle for a minute or two. Then when it is warm it steadily climbs up and down in rpm. Like its revving up and down by itself. I tried all day to check for vacuum leaks, disconnected the TPS sensor and all the other sensors associated with idle and vacuum by the throttle body. Brand new spark plugs, coil packs work just fine. At one time today after it was warmed up I actually pulled a vacuum line off the throttle body and the engine started running like it was perfect. But it was also hissing bad from the vacuum leak i created when I pulled the line off. So I am kind of at a stand still right now. I have a code 23 which is a oxygen sensor code but the person that had it before me cut the catalytic convertor out so there is only the front oxygen sensor now. Also I have a code 51. I am not sure why i am getting this code. i read somewhere that it had to do with the throttle position sensor. Also i read it had something to do with the a/c button being either on or off. I just don't know what else to do. Any help would be greatly appreciated.

Thanks,
frankdatank

My first thought is that mine rises and falls in revs once warm and that is caused by the water level being low. Top the bottle up and the problem goes away until the level drops again. However, I am a little concerned by the possibility of an oxygen sensor having apparently been removed. I don't know whether that would affect it but it would certainly be easy enough to check the water level in the expansion bottle, if only to rule that out.