I was thinking that would be a good idea as well, I'll get to work on a summary,and............Thanks to everyone! You all, oops (y'all) are great!

(I LOVE THE BLAZER FORUM!!!!)

Gator

 

we as a forum will give you all the ammo you need to help get this taken care of don't worry the more we narrow it down without it costing you alot to no money the better sorry its taking so long to help but just to let you know were here for you

 

so the missfire only happens under light trottle at 55-65 mph? Does it happen in any other gear at the same rpms?

Is this a 4x4, if so, drop it into 4lo and see if you can get the missfire to happen at the same rpms?

It could the the coil. Have you tested it, and did you replace it, if so with what Brand?

I doubt its your timing, could be a streched chain, but not likely. Typically with timing, it works or it don't, its not really an intermittent problem.

Did you replace teh cap, rotor, and plug wires?

 

The following is a summary of components tested and or replaced while searching for the cause of a DTC. Code PO300 (random multiple cylinder missfire)

The subject vehicle is a 2001 Chevrolet s-10 blazer 2wd with 103000 miles
H.O.4.3L v-6 (L35)
NV3500 5sp (M-50)
8.5 rear axle with 3.42 (GU6) and positraction (G80)

The following has occured within the past 6-9 months (approx)

Ignition module replaced (defective)
Ignition coil replaced ( weak)
Spark plug wires replaced
Spark plugs replaced (x2 no change)
Distributor cap replaced
rotor replaced
Inlet air temp sensor replaced (tested marginal)
TPS replaced (flat spot)
MAP sensor replaced (tested marginal)
Fuel filter replaced (x3 nochange)
Fuel system cleaner added (x3 nochange)
SCFI (spider assy) MFI upgrade
Distributor shaft end play corrected (shim kit)
Pick-up (distributor) replaced (no effect)
EGR valve/ supply tube cleaned (no effect)
Vacuum lines (hoses) replaced ( dry rot)
O2 sensor(s) disconnected (no change)
' ' reconnected&cleared codes
Ground wires inspected/cleaned
Positive cable inspected/cleaned
Electricle leads/connect inspected/cleaned

The above is the short list (ya right!) of parts inspected,tested,or replaced while searching for the cause of the missfire. Bear in mind this missfire occures during light/ part throttle (cruise) and doesn't occur at idle or while accellerating. At times it is barely noticeable, other times it can be quite severe (can't narrow down specific temp/wheather conditions that if at all apply) Dry verses wet, hot verses cold, etc. Fuel consumption is not to bad, approx 20mpg (but I used to get on average approx 25mpg, believe it or not) when I can FINALLY solve this missfire issue I know the mileage will return. The plugs appear fine no fouling, good color (for unleaded fuel anyway) no excessive deposits, no signs of detonation, etc. As for fuel I have tried different brands, (although I haven't tried a higher grade) and to this point have noticed no improvement. I could be mistaken but I don't believe I have a fuel delivery problem (pressure or volume) as it starts, idles, and accellerates fine. I suppose I should test for pressure if for nothing else to rule it out. Still haven't found my gauge ( I guess the best way to find it is go out and buy a new one LOL)
I have worn out my best fine tooth comb looking for the cause of this missfire. The only code that has ever come up is PO300. Maybe I should just bite the bullit and go buy a really good scan tool like what kyle linked to me, but I really hate to because this is the only vehicle I own that even has a computer. But what is the alturnative, pay someone else every time (I really hope this is the last) I have a problem, or keep throwing time, money, and parts at it. Would I really be time and or money ahead if I had just taken it to a dealer in the first place? Maybe yes,.......maybe no,..... that remains to be seen. As I have said before everything that has been done up to this point has helped, except for the missfire. To be honest I think it runs better now than it ever has, and it was purchased new from a dealer with 3 miles on it.

I think I've covered everything up until now, so I guess we'll see what happens! Sorry again for the longgggggg post.

Thank you all for your continuing help and support.

Gator

 

The missfire seems dependant on rpm not mph, same rpm spread in all gears (1-5) I agree with you about the timing issue, and I don't think it's a worn Timing chain. Short of pulling the cover an checking, or hooking up a timing light and watching for a traveling timing mark (at the harmonic balancer) I could not say with 100% certanty(sp). It's something to think about but,........not very likely. I'm willing to entertain any ideas at this point, and will not refuse any.

Thanks
Gator

 

Have you attempted an upper intake tract cleaning? A bottle of seafoam (or GM top engine clean which is more expensive) will only cost you a few dollars and shouldn't hurt anything that you have already done. I posted the procedure in another thread. You really should change the oil after words. It may help.

 

When I did the MFI conversion I cleaned the living stuff out ofthe intakeall the way down into the heads with carb cleaner and engine cleaning brushes. It wasactually pretty cleani.e. no carbon etc. just the typical synthetic oil discoloration.Removed the excess rags, shop vac,( waited until everything evaporated first--didn't want to blow myself up)and finally compressed air, looked into each one with a light and a mirror. When I got done it looked brand new like the day it was cast. Then I pulled all the spark plugs and spun the engine over to make sure I got anything that might have gotten past a valve pushed out. Re installed the plugs and changed the oil and filter. So do you think I should dump some Sea Foam , or G.M. top engine cleaner down it any way? If you think it would help I'll do it.

Thanks

Gator

 

The top engine cleaner (Seafoam or GM stuff) will clean the valve stems in the intake & exhaust tracts as well as clean up the combustion chambers. The carbon buildup in the combustion chamber is if interest here. If there is carbon buildup, it could cause preignition which the computer could misinterpret as a misfire. But this should be more promenant the harder you push the motor... I'm just running out of ideas here.

 

i agree with swartlkk its worth a shot right? i don't know what else to advise about and swartlkk even said it himself runnin outta ideas but hey were the blazerforum we can do it just have to dig deep into everything that could be linked to a misfire. try the seafoam like swartlkk said and see if it changes anything if it doesn't work let us know we will try something else hopefully by then we think of something else

 

just some other feedback.
http://www.obd-codes.com/trouble_cod...e-detected.php
http://www.aa1car.com/random-misfire/

Since your sure of the other stuff.
Check that cat, vacumm and do a compression test.