ive been on this site for the last 3 weeks on my phone and havnt found the awnser to my problem. plus my phone wouldnt let me sign up so i had to come to the library to put the random ? in (LOL). i have a 1999 chevy blazer 4.3 4-door 2-wheel drive. its having trouble at idle and has the god awful p0300 random misfire code. runs great as soon as you give it gas and has power going down the road just has trouble at idle. good news is i have a 1997 gmc jimmy 4x4 that runs great(but looks horrible, its my mud toy ) that ive been switching parts with. everything ive takin off the jimmy and put on the blazer has made no differnce and when i put the blazers parts on the jimmy it runs just as good as ever. ive replaced all the sensors i can find put new plugs wires cap and rotor on. swapped out the throttle body, distributer, and spider injection system. i dont have the tool to check the fuel pressure and no money to rent/buy one. ive checked every vacuum line i can find and found 2 cracked, fixed them and no change. i hate to start a new thread but all the ones ive been reading since this started has not led me to the problem. please help.

 

I had the same code a long time ago and gave up looking for an answer. Sometimes ran good, others bad. Then all bad. The problem turned out to be a bad plug wire which had partially come off the plug, middle, passenger side. After a time, it corroded and forced me to go through the whole thing again. It is easy to have wires not seated properly in the distributor cap as well. Inspect each wire carefully for corrosion and be sure each is seated properly.

 

Poor idle quality can be a sign of sticky poppet nozzles. You could try to run some Chevron Techron Fuel System Treatment Plus through it. A double dose of the 20gal treatment might just get them cleaned up. If that doesn't work and a full tune up consisting of AC Delco plugs, cap, rotor, and a good set of wires has been done, then you may want to consider changing out the fuel injection spider with the new MFI spider.

 

Everything I put on it came off my jimmy and then I put what came off the blazer back on the jimmy. Blazer still runs rough and jimmy still runs great. Would there be that much difference going from the 97'rear to the 99'? But why would the jimmy run just as good with the blazers parts on it?

 

Sorry, I had trouble reading your post due to the run on nature of it with no real paragraph structure. Missed the point that you had swapped the spider.

Have you tested the fuel pressure?

 

I might try unplugging the MAF and see if that helps your idle. and if that doest help save up to rent the tester.

 

Sorry, they have timers on the computers in the library. I only had 5 minutes to type up the first post. Which one is the maf?

 

the MAF sensor is the sensor in the middle of your intake, the big round medal one well medal and plastic (not the sensor that is plugged into the intake hose further up. the MAF is actually part of the hose). unplug the connector on the top of it then drive around. it might be hard to start it, but if it runs better with it unplugged then you can either clean it (theres a thread on here that explains how to do that) or you can try swapping the one from your jimmy into the blazer.

 

If you want to get to the bottom of this, you need to do the diagnostics that are requested. Not doing so could result in more lost time/money which, according to your post, you don't have much of either. Do you have money to throw at parts without definitively proving they are at fault for your condition? If you want our help, you have to help us by doing what we ask. Please don't waste our time and we'll be sure to not waste yours.

In lieu of a fuel pressure test... How do the plugs look?

Have you checked spark? If so, how does it look?

Unplugging the MAF sensor may or may not result in meaningful results (results that conclusions can be drawn from). It will set the PCM to open loop status which can mask other problems. If it runs better, it doesn't necessarily mean that the MAF is bad.

 

Plugs have a little browning but other than that look like when I put them in.
Spark is the same on all of them.
I tried unplugging the MAF and and it made no difference.

Should I be able to hold in the fuel with my finger during priming and while running? It wasn't hard. I thought it was supposed to be somewhere between 50-60 psi?