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.

[68C15]

what kind of scope do you have? can you use a vacuum transducer and synch it to #1 while also scoping inj. waveform and primary ignition? these should give you a good idea of the mechanical integrity of the engine.

if the symptom is predictable I would unplug all O2's and retest.

the closed loop PID is misleading, it merely means it is capable of entering CL.

a vacuum leak would cause high idle and/or high IAC counts.

by any chance is there wetness in the ECM connections? (gasoline)

are the CKP and CMP waveforms in time and clean at all RPMs?

if all testing is a bust then you have 2 options. a used ECM (same year and engine) is cheap to just try or get a subscription to www.iatn.net

[Todd68Chevy]

I know we have a Snap-on Vantage that the tech. uses but I am not sure if he uses anything else. I will check with him on that.

I agree that a vacuum leak ussually causes a high idle but we have had several Ford engines with intake gaskets leaking and no high idle just lean codes. I am not sure what the IAC counts are. I will need to check with the tech. on that.

I know he said he checked the connector at the pcm and said it was clean with no corrosion but I will recheck with him about wetness.

I will also need to check with him on the CKP and CMP waveforms.

We were thinking possible PCM issue but just could not come up with anything that confirmed it. We are members of Identifix but it has not been any help. I will need to check out www.iatn.net I have not been on there before.

THANKS 68C15 for your time and all the great information and suggestions.

[Todd68Chevy]

Tried a used PCM and it did not change anything. Ended up finding a wire going to the oxygen sensor bad, repaired and it has been 4 days and so far so good. Hopefully it is all solved.