i have a 99 with a 4.3, 4wd, automatic. prior to developing the p0300 code it would ocassionally run rough, ie hesitation between at shift points and "missing" at highway speeds but no code. when p0300 first occured, replaced plugs, distributer cap and rotor. misfire continued. replaced fuel filter and vacuum lines and misfire continued but wouldnt always throw the code. eventually took it into a neighborhood shop, mechanic diagnosed the tps and bad muffler since the cat appears to have been recently replaced. replaced tps and muffler. code went away but still had the misfire. now have developed the p4020 code. any suggestions would be greatly appreciated, i dont have much extra time to work on it. thanks for your time everybody
ps, the warmer/dryer the weather the better it runs. on cool damp mornings the hesitation is greatly pronounced. and for some reason, if i'm not getting an active p0300 code it runs worse than when the code is active. ie no code runs worse than when the code is active. hope that makes sense

 

I just wrote up a long response and lost it because the site is acting hinky..
Read this whole thread and the links in it about those codes.

https://blazerforum.com/forum/2nd-ge...de-help-29437/

Note the OP found disconnecting MAF and MAP connectors EACH resulted in better driveability.. Thats because the stored values are set to run a little richer.
If I were betting with odds..I'd say it's your distributor is worn.. OR your coil bad.

 

i just got done reading that one. seems that there are alot of different theories around but i'll go with the distributor when i get back home. thanks

 

Okay, it's not just theories.. there ARE several different things that can cause it.. you just have to find YOURS.

 

yeah i didn't say that correctly

 

What brand cap & rotor did you install? Did you put silicone dielectric grease on each terminal, inside and out? What brand spark plugs are in it?

The 2 most common causes for P0420 are an exhaust leak and a dying or dead catalyst.

As pettyfog mentioned, the distributor or the coil, I agree. Worn distributor bushings or gear will cause a misfire. Usually on the same cylinders due to the way that the bushing and gear wears. Keep in mind that P0300 is almost always caused by a fault in secondary ignition: distributor cap, rotor, spark plugs, spark plug wires and ignition coil. Low fuel pressure will set P0300 also.

 

Capn Hook beat me to the one puzzler...
A new cap rotor would fix the misfire for a short time until the cap got contaminated again and I would think there would be a specific cyl shown at least for short time.
But with it having started immed after new cap.. could well be coil AND wobbly shaft/ worn gear.

When you check the dist look for the vent holes in the base and make sure they are clear.
{cool damp morning is clue. That may point to contamination inside cap, also to coil}

Actually if we were in a classroom {Since this IS sort of a classroom to me.} I'd say this is classic example of either two related faults or one starting from same source.
Without KNOWING the source code/algorithm for misfire detection you could assume that the Cam position sensor signal would have to be somewhat regular for it to go into the detect loop.
The same wobbly distributor shaft could create misfire by eating at the rotor cap interface and creating the misfires in the first place.

OTOH, I could be fulluvit.
ADDED: One think we havent addressed here is the EGR valve being dirty and stuck open a little. Very common and causes similar driveability problems but usually described as hesitation/sputtering.
This will result in a leaner than normal mixture with all sensors operable because there's less O2 in recirculated exhaust it present a false 'rich' to the O2 sensors and alters fuel fuel trim. And again engine would run better if either MAF or MAP disconnected.

Aslo note THIS:
This isnt the first time this has shown up on here and points out that any new part CAN fail. I would bet if I had that rotor I'd find a cold mold flow problem. That is it was near first of injection mold run. Close look, there will be a faint swirl pattern near the punch through point. And the pinhole{s} were where flows met.

 

Pettyfog, you're absolutely right! If the bushings are worn, it will affect the camshaft position sensor accuracy as well. The cam & crank sensor data are both used for misfire detection and cylinder identification. I've seen camshaft position sensors that have been beat up due to distributor shaft wobble. Replacing the distributor, (comes with the cam sensor) fixes the root cause of both problems. I often wonder, when I see someone on the forum saying that they had to replace the cam sensor... is the root cause worn distributor bushings?

 

Well an update. Installed new distributor this morning, old one the gear was badly worn. Drivability is greatly improved. Both codes are completely gone. However, a new code, p1345 (crankshaft to crankshaft position coorelation fault). Any ideas????

 

P1345 means the distributor gear teeth are not meshed correctly with the camshaft teeth. You're off by one or more teeth, one direction or the other.