Replaced the main catalytic converter. I didn't realize the upstream bubble is a "Pre-Cat". (Trying to avoid the major job of replacing the whole Y pipe assembly.) Seemed to make a lot of progress. Now, I still get the P0300 at start up, but after the engine gets up to temperature, all codes disappear. Any suggestions??

 

2002 Blazer LS, 4WD, 4.3L Cold start produces a rough idle, but if I increase it to 1500rpm, it smooths out, but the P0300 shows up. After warm up, and after cancelling the error code, it idles smooth and will not produce the P0300 error. Does this help or further complicate??

 

When the O2 sensors are cold the truck runs in "open loop" meaning the pcm/ecm is not yet using the voltage measurements from the O2 sensors to produce optimum air:fuel ratio (14.7:1). The pcm is kind of "assuming" and probably running a little rich (like in the old days when you would close the choke to start your car). Once the O2 sensors come to operating temp, they start sending the signals the pcm needs to get that right air/fuel mixture (closed loop). There are heated O2 sensors and unheated O2 sensors. My guess is your's are heated. P0300 combined with your symptons probably means the heater element in one of your O2 sensors is bad. You can check this with a multi-meter by measuring the resistance across the two black wires on your O2 sensor.

EDIT: The point of the heated O2 sensor is to get it hotter quicker so it can send the signals sooner then if it was only being heated by the radiant heat of the exhaust system itself.

 

Thanks ROCKP2. I already checked all three O2 sensors. One (upstream right bank) had an open heater circuit. I replaced it. Perhaps I need to check them again. I thought it strange that, when cold, it has a rough idle, but when I increase rpm's to 1500 and above, it smooths out. I will recheck them.

 

ROCKP2: I rechecked the three O2 Sensors. All three heating circuits tested OK. But, when I put my AutoTap Laptop tester while the engine was running, I found what may be the problem in the signals.


Bank 1 Sensor 1 varies between 85-855 mV (left bank)
Bank 1 Sensor 2 varies between 55-80 mV (right bank)
Bank 2 Sensor 1 varies between 75-870 mV (down stream)


The right bank sensor is way below the other two.


I had replaced the right bank sensor earlier because the heat circuit was open. The replacement is a Denso 234-4650 from Advance Auto and was a direct replacement "according to their book"


One other observation: When cold starting, something rattles. The exhaust connections are all tight. The rattling sounds like it might be coming from the "bubble" (pre-catalytic??) ahead of the regular larger catalytic converter that I replaced. The rattling disappears at 1500 rpm when cold and is non existent at any speed when warm. Spooks??

 

I think you have sensor locations out of order. I believe they are :

Bank 1 sensor 1 is left pre cat (left bank)
Bank 1 sensor 2 is post cat (downstream)
Bank 2 sensor 1 is right pre cat (right bank)

I have no experience with Denso or Bosch O2 sensors, but have read the AC Delcos are more reliable

Hope that helps

 

From my understanding, Bank 1 Sensor 2 (downstream) should hover around .5mV and should fluctuate much slower than the upstream sensors. I believe .45mV indicates a perfect mixture coming out of the cat ("not too lean, not too rich...just right" -Goldilocks). The upstream sensors will rapidly fluctuate between .1mV and .9mV But since the trucks smooths out when warm, it would make me think that the sensors are fine.

Since P0300 is the random misfire code, normally I would recommend what I learned from this forum and start with a fuel leakdown test, but I'm not sure if that is where to start since it seems to be only when it's cold. Maybe Captain Hook or one of the experts could shed light on that.

But I would take a look at a couple other things. If the EGR valve is stuck open, that can cause a rough idle. The other thing I would consider is my secondary ignition items. Are they in good shape? Is there any condensation under the distributor cap before the truck is started cold. Are the plug wires good (and are they AC Delco)? Methinks a tiny inner break in a wire could close as the wires heat up?

That's where I would start looking...just before I started pulling my hair out

EDIT: Also check for vacuum leak.

 

Any luck? I'm curious to know how you make out.

 

2002 Blazer LS, 4WD, 4.3L


First of all, let me explain. In a former life, I was a tool maker, so "I think" I can fix anything. But, when I put on my "mechanic's hat", I know otherwise. Second: All of my dumb questions "are intended to be "informational" and not "argumental" (no such word).


I can not find an EGR valve. One google answer said there was not one on the 4.3.


What is inside the "bubble" between the Y pipe and the catalytic converter? Another google search says it is a "Pre Cat". There is still a rattle in the bubble at a cold start. It rattles for about one minute and then quiets down. At 1200rpm it quiets down and disappears at warm up. The rattle comes back a little at deceleration from 1200 to idle speed @ 720.


One other observation from my AutoTap analyzer:
Short term fuel trim bank 1 = 0-2.3%
Short term fuel trim bank 2 = 0-3.9%


Short term fuel trim from O2 bank 1 sensor 1 = 0-2.3%
Short term fuel trim from O2 bank 1 sensor 2 = 99.2% (doesn't move)
Short term fuel trim from O2 bank 2 sensor 1 = 0-2.3%


I don't really want to do it, but my inclination is to replace the whole Y-pipe exhaust system from the exhaust manifolds to the muffler flange with the one piece version. This would replace the suspect "bubble". Presently, I just replaced the downstream catalytic converter.


I certainly appreciate everyone's patience and help. Joe

 

Should have added: Still getting Service Engine Light, and P0300 Random Misfire on Actron CP9580A. But, no P0300 error on my AutoTap analyzer???