If all you are looking at is the codes, then you are missing 99% of the PCMs output.

 

I agree ^^ The DTC's are a clue telling you what to look for in streaming data. DTC's alone never tell you to replace a part. Granted, a number of things can cause P0300, that's why fuel trim data is used to determine if it's a lean or a rich misfire. A rich misfire is almost always caused by a fault in secondary ignition. Most commonly caused by a lack of maintenance, or installing "economy brand" parts.

The OP's misfire is caused by a lean condition: O2 sensor DTC's and positive fuel trims confirm that. Low fuel pressure is pretty much the "smoking gun" here. Next step is see if the pump can supply proper pressure and hold it. If if fails, the pump is faulty. If it passes, the problem is in the plenum.

 

On my 99 I had a pinched wire on my spider injector under the upper intake that was causing a rich issue. It started out periodically then just a constant. Drove me crazy till I seen that wire. Just an idea maybe check your spider assembly.

 

So here's a question that I hope fits as part of this thread. Week or so ago I ran across a Technical Service Bulletin that relates to engine rattle at the RPM's listed by the OP. I was going to offer it up as possible consideration. In reading it I notice that the fix refers to installing a new "Timing Chain Tensioner". It dawned on me that I did not have a "Timing Chain Tensioner" either on the engine I pulled or the reman engine I'm installing. So the question is...is the "Timing Chain Tensioner" an add on part to make the fix, or was it something that was original with every engine from the factory? You can see the bulletin in this thread https://blazerforum.com/forum/engine...5/noise-13461/ in post #4. Mind you, the thread takes off into a completely different direction.

 

TMM217, all of the injectors are supplied battery voltage when the ignition is in the start and run positions. The PCM supplies ground to open them. Makes perfect sense you'd have a rich misfire if the injector is open It would show negative trim numbers on the bank with the shorted injector, and normal trims on the other bank. Another perfect example of using fuel trims to locate a problem.

rockp2, The addition of the timing chain tensioner is strictly to reduce/eliminate noise from the balance shaft, (due to excess slack in the timing chain) and it does not affect engine performance.

 

Yeah you're right Swartlkk ... my brain was thinking "codes" .. and not thinking "parameters for scan tools" when I made that reply to Captain. The ol' brain ain't what it used to be. Years of beeeers .....

But, I still like carbs better. If carbed, the "code" would've never happened to begin with!

 

I just spent 4 weeks with a misfire problem. I won't go through all of the symptoms and troubleshooting procedures since there could be many reasons for the misfire. I will just explains my fix. It turned out to be an electrical connector. With the engine running and holding about 1500 rpm... Wiggling the ignition module connector caused the missfire. I cut off the connector and hard wired it with spade lug connectors. No more missfire. This can be a quick check. 2000 Chevy 4.3 150k miles.
I have a 2000 Chevy Blazer with 170k miles on it.

It started misfiring recently which caused the SES light to come on. I tried nearly everything and still cannot diagnose the problem.

I get the following codes:

P0301 - most of the time

P0300 - some of the time in lieu of P0301

P0171 and P0174 - some of the time

P0302 - came on once

Obviously, the codes change without any rhyme or reason.

The car runs but shakes a lot around 1800 RPM. It seems that it gets smoother when it's over 2000 RPM.

Gas mileage is around 9MPG.

Some days it runs better than the others.

When under load (usually over 3000RPM), the SES flashes.

The fuel pressure is good, pressure remains the same after shutting the car off as supposed to.

At idle fuel trims are over the roof. STFT is frozen at 50 and LTFT is at 25.

There are no vacuum leaks.

So far I changed spark plugs, wires, distributor cap, rotor. Still the same.

What bothers me the most is the inconsistency with which it switches from P0301 (Cyl#1 misfire) to P0300 (Multiple Cyl misfire).

Anybody had the same symptoms? What could it be?[/QUOTE]