Thanks for all the tech help in the past. I have read the forum and diagnostic checklists for this problem. I will list out where I am thus far.

2000 Blazer with 4.3

Got a check engine light code P0301 was runing a little rough hadn't had a tune in about 80K miles so I thought I'd throw some cash at things that should be done anyway.

New Plugs, Wires, Cap and Rotor, Air Filter, Fuel Filter and PCV valve. Cleaned the throttle body. Ran great for 20 miles then way worse. Runs fine if I accelerate hard but feels like its starving if under load (up hill) with throttle 0-60 percent depressed.

Cleaned the MAF sensor. No change
Unplugged the MAF sensor. Runs perfectly albeit a little harder on fuel consumption
Replaced MAF sensor. (and plugged it in) Back to the same starvation and idles really rough.

Fuel pressure is at 60 with 0 bleed down at 5 minutes.
Truck now smells bad after a short trip. (doesn't with the MAF un plugged)
Revs fine and smooth in all throttle postions when in park (not under load)
Now kicking out P0300 and P0171 on my el cheapo code reader

So there's the clues as I know them. Any advice as to where to look next would be really appreciated.

Thanks for taking the time to read this.

 

You should test the egr and tps next... that's free. lol I had the same problem and it ended up being a bad coil pack.

How to test the TPS

I believe you can test the egr by just unplugging it... hell just get rid of the thing! lol

 

Cleaned and checked Egr, checked tps and still nothing. Then re-read a few postings on similar topics. The thing that jumped out was a post that mentioned the engine running better in warm up mode which is exactly what mine was doing. Replaced both upstream o2 sensors today and it is 100% improved.

A couple of curiosities: It never kicked out an O2 code. After I was done installing the sensors it still ran like crap until I put about 10 miles on it. I suppose it takes the computer a little bit to discover the change.

Thanks to everyone here. I use this resource a lot. Try to post a note when you solve a problem. A lot of threads just stop without the person reporting what the solution was.

 

So it ran well for a few weeks with the new o2 sensors and then back to the same as previously described. Throwing misfire codes etc. I took it to a shop I trust and they replaced the spider with the upgraded unit. The old one looked brutal and was bypassing. It ran well until it warmed up and then same crap. I pulled the o2 sensors (upstream) pass side still looked new, drivers side so black with crud the vents were blocked. Replaced it thinking this was secondary to the injectors. Truck ran great for 2 weeks and now the symptoms are returning. Hasn't thrown codes yet but it will any day. I fully expect it has cooked another o2 sensor.

So... something is causing the computer to believe a lean condition exists and it is dumping fuel into the left side causing it to run left side rich. I don't think it is a pre O2 sensor exhaust leak as I can't hear the whistle when you occlude the tail pipe. As it is one side only, I am doubtful it is fuel pump, temp sensor, Egr, or TPS.

My next best guesses are: one or more plugs got fouled with the previous issue and is causing a false lean. Or, I have a leak in the lower intake manifold gasket.

Any guidance would be appreciated. This is driving me nuts.

I will get this eventually just by elimination. Thanks.

 

get a new gas cap

 

You mention the truck smells bad when hot and that it would run well until it warms up. Those two items make me think of a clogged Catalytic converter right away especially if it has not has a tune up in 80K

If you cat is clogged that would cause the truck to be very under powered under any load.

That's what I think and I agree with you that this site is a great resource and that it only gets better when people post their solutions!

 

Back to the basics....

P0300 is almost always caused by a fault in the secondary ignition system: spark plugs, plug wires, distributor cap & rotor & ignition coil. Low fuel pressure & worn distributor bushings and or drive gear can also cause it.

P0171 is a lean condition on the left bank. Several things can set that DTC: a lazy oxygen sensor and low fuel pressure are the most common causes.

Did you use AC Delco spark plugs, distributor cap & rotor? Yes, it DOES make a difference.
Did you use AC Delco oxygen sensors? Again, it makes a difference.
Have you checked camshaft retard? If so what is the reading?
Have you done anything with the timing cover or crankshaft position sensor?
Have you checked fuel pressure and leakdown since last June? If so, what were the readings? 60psi is at the bottom of the allowable range and could have deteriorated since then. Also, zero leakdown is highly unusual. Fuel pressure specs are 60psi to 66psi while the pump is running, (engine off). Pressure must remain above 55psi for at least 10 minutes after the pump shuts off.

Post your results.

 

I'm having the same exact issue as you and would greatly appreciate it if you post the solution when you find it. I'll do the same if I find it first.

 

I have the same problem with p0300 can't get it to go away trying everything.. next for me is TPS

 

TPS will not fix P0300, save your time and money. See post #7, those are the likely causes.