Hey guys. A few days ago i got the dreaded P0300 on my 97 blazer 4x4. I thought at first it was the cap and rotor cause it happened after i drove it to work in very heavy rain. The new cap and rotor made no difference. A friend of mine had me swap out the coil and ignition mod from his truck, and also no change.

After a good few hours of searching here, i decided to bring it to a shop because they would have allot more testing equipment than i do. After a few hours they called back with there diagnosis, saying i need 3 injectors. They said its missing on cyl 4 and 6 at idle, and then just 5 once you add some rpm. Also said they ohm'd the injectors to see that they are bad.

Does this make sense to you guys? I just dont want to spend the money to replace the spider if its not going to fix it.

 

I talked to the guy that is working on the truck. He said he Ohm'd the injectors, 1 2 and 3 ohm at 12.3v-12.5v, and 4 5 and 6 ohm at 13.5. So that is why hes saying they are bad.

 

Well, i went and got the truck from the shop it was at. They wanted to throw injectors in it, but i am still not convinced that is the problem. I had some time today so i ohm'd the injectors myself, they all come out at 13.5. I also had got a fuel pressure tester from my neighbor and tested that. Key on it get 55psi, and running is at 52. If i goose it, it will go to 60, if i ease on to the throttle it will go down to 50.

Does anyone have anything that may help me solve this??? Please??

 

You say you changed the cap and rotor, what about the plugs, wires, and fuel filter? As for the fuel pressure, have you worked through the steps in:
96+ Fuel System Diagnosis?

 

Yes i did change the cap and rotor, plugs and wires i did about 5k ago when i did the manifold gasket.

Heres what i find odd, i pulled the upper intake last night to get at the injectors. Mt thought was to change the injectors from side to side, and see if the misfire follows the injector swap or not. When i got the intake off, i noticed that it is full of carbon build up, but only around cyl 1, 2, and 4. Around 3, 5, and 6 is spotless. So im wondering if fuel is leaking in there or something.

I found the 96+ fuel system diag at about 3 this morning, so thats the plan for today. I am also going to do the fuel filter today, but i want to make sure there isnt a leak in the intake first.

 

Clean intake typically indicates leaking spider hoses to the injectors. ITs about a $300 part.

When I had the P0300 I farted around iwht it for days and finally took it to the dealer. My problem was the "new" plug wires and cap and Rotor were defective. These are high performance engines and need the best parts you can afford. Actually the best parts are AC Delco when it comes to the ignition system.

 

Alright folks, heres an update. I started today by checking further into the fuel system. I got my neighbors psi tester again, and found that my psi is at 60 with key on. But, it drops pretty quickly to about 30. From what i have read, and how i understand it, it shouldnt drop more than 5 psi in i think it was 2 min, and its dropping 30 in about 45 seconds.

I then replaced the pressure regulator, and the truck runs much better, but i still have a misfire. This time the code is a p0304. With the truck running i grounded out that plug wire with no rpm change. So I pulled the plug to inspect and it is jet black.

Im thinking either that plug or wire is bad, i will pick up new ones tomorrow and hopefully the truck will be back to normal.
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Yet another update. Im lost, changed the plugs and wires, still have no cyl 4. Took it for a short drive to get it to full op temp and reset the code to get the freeze frame data as i cant scan live. Im really not sure what it all means but i have seen it asked for in other misfire posts so i will go ahead and post it.

Fuel Sys 1 CL
Fuel Sys 2 CL
Calc Load(%) 7.45
ECT (*C) 89
STFT B1(%) -3.91
LTFT B1(%) -7.03
STFT B2(%) 11.72
LTFT B2(%) 14.84
map(kpa) 48.00
Eng RPM 1276
Veh Speed (km/hr) 65 <--- Not sure why its in metric??
MAF(g/s) 15.36
TPS(%) 6.27

 

If you use a spark tester, does the tester illuminate when it's on the end of the #4 plug wire?

You may want to test the compression on that cylinder as well as the rest of them for comparison.

 

Ok guys, i do have spark on cly 4. However, i checked the compression. I dont think its good.

On cyl 4, while cranking, i have 125psi. At idle, i have 30 psi. I then tested cyl 5, and got 120psi while cranking, and while barley running, i got 110psi. So i think that solves my misfire issue.

 

You tested the compression with the engine running?? You shouldn't allow it to start...

But from the sounds of your results, you may have a wiped cam lobe (or lobes) affecting #4 that is not fully opening the valve(s).