Have you considered that you may have a cam lobe or collapsed lifter issue? I realize that a collapsed lifter typically causes a tapping at the rocker arm, but a wiped lobe may not. If the exhaust valve is not opening fully, it can cause an overly rich condition as well as a solid miss fire.

I would pop the driver side valve cover off and put a dial indicator on the valve then turn the motor over by hand. Compare intake and exhaust valve opening distances between all of the bank 1 cylinders.

 

So that others may learn from our misdeeds, I wanted to post a conclusion to our search for the cause of our P0305 code. We did remove the valve cover in an attempt to check lifter range, but we lacked the dial gauge to accurately measure any differences in lifter movement. We did check the lifter rods to make sure they were not bent/warped, and were able to eliminate them as our cause. After reassembly, we somehow got back to checking compression. As you may recall, our cranking compression seemed good on all cylinders. This time, we decided to look at running compression. The results were quite revealing. At idle, we were lucky to see 30 psi on cylinder #5, while performing a similar test on cylinder #1 gave us about 70 psi. Repeating the test gave us really inconsistent results - one reading was zero for cylinder #5, another got as high as 95. Cylinder #1 was consistent at 90. Our conclusion is that we have a mechanical problem with #5... probably a cracked ring. We suspect that at cranking speeds, there is enough oil infusion to seal the rings and give us decent compression readings. At idle, there is enough movement that the seal is not maintained. With the low pressure, the fuel mixture does not burn well, and results in a quickly fouled plug. And while it did not look like oil fouling the plug, that may be a contributing factor as well. We just never saw any evidence of oil in the exhaust.
We do not know how we are going to proceed at this point, but we are convinced that it would be a major effort to repair, and significant $$$. Thanks to all who offered advise. It has been a real learning experience.

snakebit

 

I am glad you have found the problem although I am not glad for your situation.

A leak down test also can show this. I like to perform a leak down test at various piston positions as I have had some engines have odd wear in the bore (sloppy at the bottom).

 

i to have had that dreded code and mine turned out to be a bad seal between the transmissin and the transfer case in turn over filling the t case and blowing transmission fluid out the over flow on to the top of the O2 sensor and incase any of you dont know transmission fluid reaks havoc on a sensor i replaced the same sensor 4 times and three different shops before finilly figureing this out. good luck

 

Need some help identifying why I am getting a number four misfire. The truck also sputters when it starts to rain or after a night of high humidity.

Items I have already replaced

MAF senor
ERG Value
Spark plugs
Spark plugs wires
EGR value
Distributor Cap and Rotor

After reading a few forum posts, I know there is a possibility, it could be a bad wire set. Plan to put on a new set of wires this weekend. Just looking for anything else that could be causing the misfire.

Thanks in advance. Everyone on this forum makes it a lot easier to be a do-it-yourselfer.

 

Read through this thread from pg 1 to the end. Pretty much all of the causes are exposed here.

 

Like many of you my 2000 Blazer has been giving me the P300. Misses and runs like it's at half power at will. I notice it more at cold start-ups. I've been at loss with this for I've switched out about everything at least twice. Bought a new fuel pump, spider unit, plugs, wires, cap, rotor,, all AC-Delco or GM parts, replaced the intake gaskets with the better Felpro aluminum ones, checked every plug and used dielectric greese. Time and again same thing runs good for a limited time and back into the P300 I go. I did the compression test and was about to have a motor rebuilt thinking that I had a valve problem. The rebuild was going to cost me $1,300. I only gave $900 for the Blazer, but got a lot more in it by now.
By chance I stopped over a buddies (grease monkey like most of us) and he told me that a nieghbor of his had stopped in with a question.
Why is it that my Blazer misses like crazy, but if I unplug the MAP sensor it runs perfect? Buddy told him to buy a new sensor. He said it was the third new on in his truck. Buddy told him to leave it unplugged. He said that would be fine but gas mileage sucked. Anyhow I go home to my Blazer (been parked for a week due to issues). Start it up and sure enough idles rough and misses if I try to rev it. I repeate this multiple times to confirm the miss. Without turning the motor off, I reach up and unplug the MAP sensor. Idle clears up and upon reving it, it acted more like a full race motor. Snappped up to above 5,000rpms and in an instant idled back to 550-600rpm????? Plugged it back in and started running like crap again. Switched it out twice and finally found one that ran good. ????? Drove the Blazer all night and ran flewless. Got in it the next day (cold) and there it was again, unplugged it and cleared up, once warmed up it ran good with it plugged in????
Today I took the time to double check the wires on the MAP sensor plug, I pealed back the factory tape and found nothing that looked bad. Did however take notice that the MAP wire was lying close if not against the coil terminal/wire that leading to the cap. I relocated the MAP wire so not to come anywhere near the coil and "DAMB IT RUNS REAL GOOD". took it for a drive while out, it started raining and still ran good. Now just hope it runs this good in the morning when I start it for work. I'm not saying I fixed mine but lets cross our fingers. If you've got the P300 give it try.