You should do the GM OBDII Drive Cycle test. That way every circumstance is met for any of the sensors to throw a code, if the are going to that is. Here's a link (do the GM one at the bottom): OBD II Drive Cycle (reset car diagnostic monitors )

 

Ok so i have replaced the O2 sensor after the cat.

I replaced the fuel filer, which looked as if it was the original filter from 2000, I also replaced the vapor canister vent selonoid that sits above the gas tank and checked the hose, which was in good shape, still plyable and no cracks that I could see.

Although, the fuse on the secondary intake system blew again, so I am thinking that the motor is probably shot also. although, that shouldnt create a misfire situation. from my understanding, the secondary air intake system only comes on when the engine is cold at stat up.


I cleared the codes and ran it for a couple days and these are the codes I have got now and its still misfiring, although its getting better...

Code P0300
absltt tps 22.3
speed 2367 rpm
calcload 23.1
maf 40.05
map 63
coolant 194
st ftrm1 -.8 ( thats negative.8)
lt ftrm1 17.1
st ftrm2 -.8 (negative .8)
lt ftrm2 18.7
veh spped 73 mph
fuel systm 1 & 2 closed

I/m Monitors , all applicable are ready except the evap and secondary air sys are not ready.

I am thinking of removing the new cat, putting in a test straight pipe and see if the codes change. as, with a new cat and new o2 sensor, i should not be getting a code for the cat being below efficiency

at this point, i think I am going to replace the MAF sensor, as after spraying it, the misfiring did get better... any ideas?

 

The best I can suggest is to go this thread and go thru the diagnostic steps best you can: https://blazerforum.com/forum/diagno...4-5-6-a-35153/

 

ok, so this is what I have down so far:
I replaced the Cat. as the honey combs had come lose and where clogging the exchaust. I replaced the gas filter, all plugs, wires, distributor capt and rotor, I replaced the O2 sensor after the Cat, MAF sensor, I replaced the vapor canister vent selonoid that sits above the gas tank and reset the coeds.
after about a week of driving, the misfiring is still happening, although not as bad. it seems to be more of a fluttering after travelling at 50+ for 5-10 mins or more and its random..


My engine light popped back on and the code again was P0300.
the freeze frame data showed the following.


Code P0300
ABSLT TPS 45.4
Eng Spd 2997
Calc Load 59.6
MAF 99.19
MAP 81
Coolant 194
ST FTRM1 -7.1
LT FTRM1 -4
ST FTRM2 10.1
LT FTRM2 -3.2
Speed 54
Fuel sys 1&2 Clsd


I did however, noticed that the one elbow for the PCV valve was spongy and sucked it with a small crack in it. So, I just replaced that along with the PCV valve and reset the codes.


Anyone have any suggestion on the current data?

 

well after replacing the secondary air injection pump, the misfiring stopped for a couple months. until recently, i noticed when the gas tank got down to baout 1/4 tank the engine light started flashing and it would start misfiring. it would be random and only started after hitting 65-70 mph

after filling the tank it stopped for about two weeks, now its doing it again,
I get teh same P0300 code
the freeze frame showed the flollowing data:

abset tps 52.1
2602 rpm
calc load 51.7
maf 90.63
map 95
temp 199 deg
st ftrm1 -1.6
lt ftrm1 -6.3
st ftrm2 3.9
lt ftrm2 1.5
spped 79 mph
fuel sys 1/2 clsd

Any ideas?
I've replaced just about everything i could think of at this point.

 

Fuel trims can be tricky to understand but they are a big help, really sucks that your code reader cant do live data. Heres some things to look at #1. Maf, remove it and take the 3 pieces apart so that you can actually see the element and clean it with the maf cleaner and an old toothbrush VERY GENTLY!, allow to dry and reinstall. This is the first component that will cause goofy fuel trims. If the maf has gunk built up on it it will tell the computer that there isnt a lot of airflow (when there actually is) so it wont deliver much fuel, this gives lean o2 readings and the the computer commands the fuel trims high positive numbers but still wont run well because it has mixed signals on what kind of load condition is actually present, thats why i had you disconnect maf, when you do that the computer uses throttle position, engine rpm, and manifold abolute pressure (vacuum) readings and calculates how much fuel and timing to give it based on predetermined values, it totally ignores the maf and O2 readings, its a good quick test to see if you have a maf or O2 sensor problem. However, i think your first problem may have been your air pump allowing unwanted air into the exhaust stream somehow which caused it to think it was lean and made it add lots of fuel which may be what hurt your original converter, it may have overheated or become partially clogged then. But now what you have may be an ignition system problem, your negative fuel trim number on bank 1 could be caused by a cylinder on that bank not firing and sending raw fuel to the o2 sensor which tells the computer its rich and commands the fuel trim negative to pull out the excess fuel, even though the other cylinders on that bank were running ok they were also commanded less fuel and now will make less power because they will be lean and they will also misfire, yes their lean condition would combat some of the non-firing cylinders rich signal but remember a lean misfire still means that some fuel wasnt burned during the mis and will also contribute to the rich o2 signal, the "false" rich o2 signal will command the fuel trim negative for that bank which can cause the P0300 misfire, in some cases on single bank engines the computer can actually lean it out on a false rich condition to the point that all cylinders are starved enough to cause it to stall. Another scenario is that you have an injector sticking open and thats causing the false rich condition.
Hopefully I didnt mess that all up and its understandable.
I say dissasemble maf and make sure its clean first, then if you havent done anything with the ignition system for a while i would look into that, plugs, wires, cap and rotor of good quality.

This picture is the maf from my blazer, it ran like crap before i cleaned it and was actually throwing O2 Sensor "LOW signal voltage" codes for both banks. Once cleaned the thing instantly ran like a champ.

 

Ok,
when this first started happening. I had received codes for the cat and evap system. I replaced a evap valve the mounts to the gas tank and that seem to correct the evap code. The cat has since been replaced, along with the O2 sensor after the cat. I also installed new plugs, wires, rotor and dist cap. The maf is also new it was one of the last things i replaced and none of that corrected the problem. until the secondary injection pump was replaced. So, i do not think its the maf, unless it got that gunk up after a few weeks of running before i replaced the injection pump. So that leaves the O2 sensor on the drivers side, which had been replaced months before this started to happen, as we had got a code for that sensor being faulty. so that leaves the O2 sensor prior to the cat.

But, i will try your maf idea anyway, i could nt hurt. Other then that.. im at a lost... i can bring it to a shop, they want $175 to hook it up to the diagnostics machine, but i pay that even if they dont find the problem.. So, i rather try everything i can do first.

 

Fuel trims are not consistent, which means there are a couple of things going on.
What brand and part number are the spark plugs?
What brand is the distributor cap & rotor?
Has the distributor ever been removed, (intake gaskets replaced etc)?
If so, was camshaft retard checked and adjusted afterward?

 

distributor cap and rotor are AC Delco
as far as I know, the distributor has never been removed
the plugs are Bosch Iridium plugs plug number 9601
we have owned this car for three years and it only started to happen within the last few months. first this I did was replace plugs, wires, cap and rotor which didnt fix the problem at all. cleaning the MAF seemd to help a little but not completely, so, the MAF was replaced, but that didnt fix it. the only thing that seemed to work temporarily was replacing the secondary air injection pump. that last about 2-3 weeks, then the misfiring came back

 

Time to start over I think....in all of the parts that youve put on it, there could be one thats bad and youre assuming that its good, happens all the time, i swear that junk parts are engineered into the aftermarket world just so that you go back next week to buy everything else because one of the first parts you replaced failed and we just assume that its good because its new. 10k miles down the road and all of these others will be failing too bacause they arent made with the same quality standards that originals were.
I dont know how many tools you have but i think its time to go back to basics...
FUEL (Misfire under heavy throttle, looks like your mis is on bank 1, the negative fuel trims means the computer is trying pull out fuel to compensate for a rich O2 Sensor reading, rich o2 is either (the plug not firing), (the injector stuck open and spraying too much fuel) or the (cylinder is not sealing well enough to compress the mixture) need snap throttle fuel pressure readings, also run some cleaners through it B-12 Chemtool is good, seafoam is just hype IMHO, the injector poppets could be sticking intermittantly. What kind of fuel do you run? Ethanol blend? Get an ohm meter and check resistance of fuel injectors through big plug on top of the intake.
COMPRESSION (need to be sure we dont have a low cylinder or cylinders)
SPARK (Coil is still original correct?)
CYLINDER SEALING (Valves-Leakdown test) Plugged exhaust could have burnt some valves.
Also check wiring to crank sensor, could have been damaged during initial "Event" when this started.

Chances are that whats wrong is so stupid that you will want to break things and pull your hair out when you find it....this is why im losing mine...

One other thing, make sure that you have no leaks behind the mass airflow sensor, make sure intake tube is tight and has no holes in it, seal to the throttle body is intact, make sure crankcase fresh air tube is plugged into the intake tube/hat above the throttle body and tightly into the passenger side valve cover, make sure pcv valve is in good condition and hoses connecting it to the intake plenum are good, also make sure oil fill cap fits snug, any of these components will cause a "False air" reading to the mass airflow because it has not been accounted for and will in turn throw off fuel trim numbers.