Greetings all,
I will try to make a long story short, but I'm afraid that it's not going to turn out that way. But here goes anyway. First things first, I am attempting to locate the cause of a DTC #PO300 (Random/Multiple Cylinder Misfire). I know, it's the absolute worst code to diagnose but it's the hand I've been dealt.
The vehicle in this case is a 2001 s-10 Blazer 2wd with a 4.3 (W) 5 spd (NV3500) with 103000 miles. My wife bought it new just before we met so I know the complete history of this vehicle. About six months ago on the way to work it suddenly started to buck and surge and then just died. It felt like a fuel delivery issue, likethe pump quit. It would crank fine, but would sputter and die and wouldn't rev over about 5-600rpm. trailered it home and started looking for the problem. Turns out it was a bad ignition module and a weak coil. (no codes and the pump testedfine)Replaced bothalong with thespark plugs, cap and rotor (all ac delco parts) and plug wires (msd) Figured while I was at it might as well change the fuel filter.
About 4 months ago I started to notice a slight intermittent miss (no codes) from then until now it has gotten progressively worse. Now I have a serv eng soon light, and the missfire can be quite severe at times, but it seemsto happen mostly under part/light throttle conditions, and not so much if at all under acceleration, or idle. It was a little sluggish from idle to about 1000 rpm but would pick up from there all the way to red line. But at cruising speed (55-65 mph, 1900-2400 rpm) you could feela surge, almost like a cylinder dropping out then refiring. So Thinking maybe the fuel system was getting dirty I changed the filter again, ran some fuel system cleaner through it (3times with a filter change in between each) and got no improvement. Given the fact that I live on the gulf coast where humidity is a real BIG issue I also ran some HEET (water evaporater) through it as well.
That didn't cure the problem (not that I REALLY thought it would) that would be too easy.
I have checked the following as well, theTPS, MAF, and MAP sensorsI replaced the IAT sensor thinking that be cause the problem was more pronounced at a lower ambient air temp it might help,....NOPE!
This weekend I upgraded to the MFI Spider (THE BEST THING I HAVE DONE TO THIS TRUCK THUS FAR, BAR NONE!!!!!! THANKS BLAZER FORUM!!!!!!) No I haven't checked my fuel pressure yet, (can't find my guage) but with the cost to have the injectors etc. pro cleaned, plus parts I figured I'd be money ahead.( Was it necessary,......WELL, I won't argue with the end result, It runs better than it ever has!) The throttle responce(sp) is incredible,....got the old girl sideways last night, WHOO HOO! Anyway the part throttle misfire is still there. Cleaned pcv and the EGR valve and tube (not that it needed it, was really quite clean actually) No improvement, as of yet I have found no vacuum leaks (BTW don't vacuum leaks SUCK?! ;p I pulled the spark plugs and they looked ok, (minor amount of high speed glazingn ,not bad but I will replace them)
So I'm thinking I either have a unlocated vac leak or may be a bad O2 sensor or two. It seems that after theinitial warm up,....for the O2 sensors (couple of mins) the missfire starts,but as I said it's only at part throttle/ cruise rpm, not under accelleration. I'm assuming (Ya, I KNOW)that the truck is going into open loop in reasponce to bad O2 sensor(s). How often should they be replaced I mean is there a scheduled replacement interval, or is it a replace at failure deal. Speaking of which how can I test the sensors with a multimeter ( I didn't have much luck earlier, not my strong suit sorry to say) Please correct me if I'm wrong but If I were having issues with fuel pressure or volume wouldn't accelleration be affected more so than part throttle/ cruise? I guess I'm just trying to find a way out of having to buy a new guage to test my pump.
I'm sorry that this post has turned into the latest version of War and Peace but I just want to give you all the information that I can in one shot in the hopes that WE can solve this issue without having to post/ reply muliple times.

Thank you in advance for ANY ideas you my have to help me resolve this issue.

Gator

 

Looks like you've got a lot of experience trouble shooting vehicles! You've done pretty much everything right. I would have inspected the O2 sensors before replacing the spider, but with 103k miles on the clock and your justification (I did the same thing), the new spider was money well spent.

For me, it's easy to take a look at the sensors and what the vehicle is doing while the misfire occurs, but I have a scan tool capable of showing me whatever it is I want to see. For this type of problem I would look at the O2 sensor trace, cross counts, as well as long and short term fuel trims. This information would tell me if a sensor has become lazy to a point of affecting performance/efficiency or maybe point me in another direction.

Without a scan tool capable of delivering this data, your choices are really quite limited. AC Delco O2 sensors aren't exactly cheap, but in my experience with O2 sensors for GM vehicles, there isn't another that beats it. At 103k miles, the O2 sensors very well could be the culprit.

Hope this helps!!

 

Boy does this thread seem familiar. [8D]

My P0300 turned out to be cap, rotor, and plug wires. All replaced within a year of the problem. Don't rule them out. Still got the old wires, swap them and see what happens. Pull the cap off and look for corrosion. Try spraying the cap with water while the engine is running, see if she purrs differently.

I need to do some research into the MFI change thingy. Anybody got a link to a thread on it?

 

Thanks for the quick reply,

Just for giggles, if I were to disconnect the O2 sensor(s) (upstream of the cat; for diagnositic purposes only) would that tell me if I had a bad or marginal one? I.E. force the computer to stay in open loop. Short of taking it to a dealer to diagnose them and not being sure exactly how to test them with a multimeter am at a loss. I really don't want to through money at the problem until I stumble on the cure.

Just a straw to grab

Thanks again,

Gator

Thanks,

Gator

 

I had a similar problem with my blazer, P0302 code I believe, Cylinder 2 misfire, I had just changed the plugs and wires so I just ruled them out. Turns out the problem was a faulty wire or plug, I just changed both and the truck has never run better. Dont over look the simple crap, thats what I did for 2 whole months. [:@]

 

yea, you could disonnect the O2 sensor. The puter only read the sensor after the engine is up to operating temp. Once up to temp it reads the sensor and makes adjustments to the air/fuel ratio to hit that perfect ratio. If the O2 sensor is disconnected teh puter will stay in open loop, trip a code, and go into some default programming. If it runs smoother, your O2 is at fault. If nothing changes, you haven't found the problem yet.

Disclaimer: with the O2 disconnected the engine will get horrible gas mileage, and if you let it go long enough it could plug up a cat.

 

Thats what I was thinking, and yes I am aware of the adverse effects of disconnecting the sensors. I would only disconnect them to rule them out as a possible cause for the current condition. But as I'm sure you are aware O2 sensors ( AC Delco) aren't CHEAP. I'm wondering though in order to make it a valid test I have to disconnect all (3) at once correct? If it clears up how do I determine which one is the culprit,...replace two of them, buy a third and just keep changing it's position ? Or should the smart thing to do be just bite the bullit and replace them all at once. Gee that was a STUPID question !!!! (Tripping over dollars to save nickels)
Thank you so much for pointing out the detrimental effects of disconnecting the sensor(s) as I realize that others who may not know also read these post! I'm am so greatfull to everyone on this site for their help. As soon as I can figure this mess out I will most definately post the results so that others may benefit as well.
Please keep the ideas coming.

Thanks again,

Gator

 

Only the upstream (B1S1 & B2S1) sensors influence fuel delivery and these sensors are located before the catalytic converter. The post-cat or downstream sensor B1S2 is a catalyst monitoring sensor only and should only be replaced if a code is present and repeatable (ie. comes back if you clear it).

So since you are experiencing an engine drivability concern, the only sensors of interest would be the upstream sensors, B1S1 & B2S1. You can disconnect them independant of each other. Driving around for a day with an O2 sensor disconnected should not significantly impact the life or performance of the catalytic converter.

 

Thank you so much for clearing that up for me. I'll try it and see if it helps.

I did run across onther post in a different section that sounds carbon copy to my situation, that I will reinvestigate (i.e. TPS) I did check it but I might have missed a flat spot. HUMMMM come to think about it my rolloff truck (MACK) did the same thing and it was the TPS. I'll get back to you

Thanks again,

Gator

P.S. Nice reply to goat6500 : )

 

Nuts,

Disconnected all (3) O2 sensors, 1 at a time and,.........no change. Oh well, I guess it's not all bad. I haven't found the problem,............yet, but at least I don't have to spend big$$$$ on O2 sensors. The battle continues.

Gator