2003 Chevy S10 Running Problem

[Todd68Chevy]

I will start out with-- If this is in the wrong area feel free to move it to the correct one but when I did a search the ones I found were all in the message board area.

I have an independent auto repair shop and a buddy with a running problem. He has got advice from several of his coworkers (he works at a prison) and now finally brought it in to have us look at it. We are also drawing a blank. Here is the story.

It is a 2003 Chevy S10 2wd 4.3L vin X. He has been fighting a low idle issue and an interm. misfire at higher speed (70-80mph). He has done some stuff himself before bringing it to the shop for us to check. I know he has replaced the dist. cap, rotor, spark plugs, plug wires, fuel filter, O2 sensors, cleaned the idle air control valve and removed the cat. convertors (someone told him it would help). He says his only problem now is that it is idling to low and acts like it wants to die. We brought it into the shop yesterday and found the idle speed is ok but it is misfiring on cylinders 2, 4, 6 and bank one is reading lean all the time. Codes P0154 heated oxygen sensor circuit insufficient activity bank 2 sensor 1, P0155 heated oxygen sensor heater performance bank 2 sensor 1 , P0300 engine misfire detected. Scanned data - found the pcm is staying in open loop and if it goes to closed loop it is only for a few seconds and then switches back. Bank one is showing it is running lean short term fuel trim is 10.2. Checked the fuel pressure and volume 56psi @.5 gpm- good. Checked for a vacuum leak internally and externally- shows good. I know the tech. used propane but not sure if he smoke tested it yet. Swapped both front O2 sensors to verify the problem stayed with bank 1 and it did. Checked sensor ground and pcm connectors for corrosion- good. Performed an injector balance test (1- 12PSI, 2- 12PSI, 3- 14PSI, 4- 14PSI, 5- 12PSI, 6- 14PSI) and wave form test- shows good. Checked for tech. service bulletins and tech. line cases- only one found showed an O2 sensor failure- performed test as per tech. line case and could not prove failure of sensor, wiring or pcm. He has also mentioned that he has had O2 codes for a while and has replaced all the sensors at different times and finally left the light on.

It sure acts like a vacuum leak but just can not prove it. Is there anywhere specific to look or test? Is there a gasket or hose issue that someone has had trouble with? Thanks for any and all suggestions and advise. Have a great Holiday.