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jbadenoch wrote:testing the 02 sensor for ground to the exhaust, use a test light and touch the body of 02 sensor with other end on 12v pos

Thanks for the idea. Will try that. Can't figure out why I didn't think of that way.

Just tried that and the ground is good. Light comes on bright.

Ok. I installed new AC Delco O2 Sensor - no change except that a code 45 (Rich Exhaust) now appears. SO, I went town the diagnostic sequence and got to a faulty ground circuit on the ignition module. I replaced the module and now I got a miss on #5 cylinder. Got spark and fuel and 160psi of compression but a miss.

Gonna back this one out and work on something else. This one just about got me beat. I am terribly embarressed to admit that.

OK. Now we know the O2 is out of the picture, we can concentrate on the code 45. I was thinking to myself earlier that I was surprised it hadn't thrown this code yet. Maybe the previous sensor was inhibiting that. So if it were me, I'd be looking for reasons there is extra fuel before chasing the misfire.

Case in point....... 2007 Chevy G2500 4.8
Dead miss on cylinder 6, flashing CEL, runs like crap, has codes P0050, P0059, P0300. Checked O2 heater...junk. Checked misfire graphic and confirm constant #6 but also had some on 8 and 2. Looked at 6 but fuel, spark all good. O2 sensor fixed it all.

Actually code 45 was in PCM memory when I first saw it.

And, wouldn't a miss create a rich exhaust mixture that the O2 sensor would pick up? What is interesting is that the PCM is at it's leaning ability limits and the O2 sensor is still reading rich. So, I guess you are right, I gotta figure out where all this extra fuel is coming from.

I guess it's time for me to start this diagnostic process all over from scratch.

Actually, a miss would produce unburned air/fuel mixture, but the O2 doesn't read unburned fuel. Because the burn is incomplete, there is MORE oxygen present.

Maybe a leaking fuel pressure regulator or higher than normal fuel pressure? An open purge valve? Bad injectors?

Here is an update: I cut off the exhaust system simply to prove to myself about the integrity of the exhaust. No restriction as the unhooked exhaust makes no difference. Also, the miss was caused by having 2 ignition wires crossed. So, now I am back to square one on this one. Extremely frustrating to say the least. I cannot for the life of me figure what I am missing on this one.

At least we're making improvements along the way.

ok im back were ya at. No more miss but still no power? The rich condition is gone now that the miss is fixed?

ok, now lets treat this like a new issue. when any chev comes into my shop lacking power the first thing i go to is fuel pressure and volume. a quick test on older vehicles i use is when it flat lines i spray cab cleaner into the air intake. if it juices then i look at the pump and filter. these pumps, at least in my experience had a high failure rate on high usage vehicles. i work on vehicles that do guard rail work and they never shut them off. i was doing a pump every year and 1/2 on these. the symptoms were always lack of power. wouldn't get out of their own way. if you decide to look into the fuel system and the filter is plugged or restricted there is a good chance the pump isn't far behind even if the filter resolves this issue. a mass air flow sensor may cause this issue. if you clean the reader in the sensor and it increases the power then replace it.

brianp87 wrote:ok im back were ya at. No more miss but still no power? The rich condition is gone now that the miss is fixed?

The miss is gone (2 plug wires were crossed) but the rich running condition is still present. I am beginning my diagnosis all over. Starting from scratch. We got new AC Delco plugs, new cap & rotor, base timing is at zero before top dead center (which is what is called for), new fuel filter, pcv valve, AC Delco O2 sensor, cleaned all the grounds, new ECM, replaced the MAP sensor, and coolant sensor. I disconnected the exhaust system, checking for restricted exhaust - no luck there. The engine has 21 inches of vacuum when it runs good, indicating good sealing ability for the mechanical condition of the engine. Fuel pressure is at 12 psi.

The issue is that as the engine warms up, the O2 sensor reading goes to .90 - 1.11volt and as soon as the ECM goes into closed loop, the Block learn reading plummets to 90 and the engine runs rough, surging, with the IAC trying to control idle speed. Eventually if I don't catch it, the engine shuts off. If I can keep it going by pushing the accelerator pedal further down, I can increase the rpms but the block learn numbers and the o2 sensor readings remain the same. After a minute or two like that, the ECM goes back to open loop. After another minute it reenters closed loop and it may repeat the former sequence or it may run fine after that. At times a code 45 may appear. Yesterday code 33 also appeared. I can connect my sensor simulator to the o2 wire, start the engine and it runs fine as can be making it appear like a faulty o2 sensor but with 3 new ones tried there obviously is another issue. In my 19 years of auto work I have never seen something so frustrating. I suppose that one thing I have not focused on is the 2 injectors on the throttle body but I tend to think they are not at fault because with my sensor simulator connected the system is real happy and the engine runs fine.

With your sensor simulator connected, does it ever go into closed loop?

ricmorin wrote:With your sensor simulator connected, does it ever go into closed loop?

Yes, once the internal ECM timer has expired and the coolant temp is up to specs the ECM goes into closed loop with my simulator connected and operating, the engine runs perfect. Block learn numbers stay in the 122-134 range then. However, when I connect the o2 sensor, then I am back to where I started.

Is the o2 readings you posted what you are seeing at the scan tool? I would scope the sensors directly at the sensor plug and at the pcm to see if there is a difference between the two or what is displayed on the scanner. You may have already done this and I missed it.

protraxrptr17 wrote:Is the o2 readings you posted what you are seeing at the scan tool? I would scope the sensors directly at the sensor plug and at the pcm to see if there is a difference between the two or what is displayed on the scanner. You may have already done this and I missed it.

No actually I did not do that but I will when I get back to working on it again. Thanks for the suggestion. Are you thinking there may be a connection issue in the o2 sensor circuit?

Yes. I'm not familiar with how the simulator works, but it might let you vary the voltage far enough to overcome a circuit problem. Just a guess.

Ok. i am back to this thing again. I noticed that when the engine is acting up, the spark advance is pegged at 40 degrees advanced. I am wondering what controls the advance on this system. It does not have a spark control module like many have but it does have a knock sensor that is reading "no" activity on the scanner reading.

I also connected my lab scope to the O2 wiring. One lead at the sensor and the other at the PCM. Identical readings. So, we ruled that option out.

The distributor and timing are on? Id look closely at the distributor or for a ground problem

brianp87 wrote:The distributor and timing are on? Id look closely at the distributor or for a ground problem

Well, I checked base timing. And with the timing connector disconnected the timing is at zero before top dead center which is what it calls for. I admit to being somewhat suspicious of the distributor in regards to the 40 degrees advanced timing, but I am not sure what. Coil pickup? Module? Or what?

You mentioned a ground problem. What circuit did you have in mind?