2002 Blazer 4.3l Random Misfire

[pitcrewllc]

Vehicle Info:
2002 Chevy Blazer 4.3L 4WD
146k Miles

Syptom:
Engine Idles fine, as soon as you open the throttle it begins to missfire and you cant get the rpm's over 2k

DTC's:
P0300 - Random Multiple Cylinder Misfire

Tests Done:
Fuel Pressure 56-63 PSI
Fuel Injector Balance Test - OK
MAP Sensor - OK
MAF Sensor - OK
IAT- OK
Ignition System - Has Spark at all Cylinders

Parts Changed:
Spark Plugs - Made no difference
-----------------------------------------------------------
The only history I have on this vehicle is that it had sat for months and then was driven about 500 Miles from MAS to PA in which it began to act up in the last 20 Miles.

After reading a few things on the net I pulled the distributor to check it's drive gear and found no problems there either.

Any suggestions would be much appreciated.

Thanks

[ricmorin]

My first thought would be fuel quality and possibly fuel volume. Then I'd look at exhaust restriction.

[brianp87]

fuel trims

[ricmorin]

fuel trims

Yes, yes. Fuel trims speak volumes.

[pitcrewllc]

Fuel Trim data at idle is

LT Bank 1 -2.5
LT Bank 2 -2.5
ST Bank 1 0 - (-1.5)
ST Bank 2 0 - (-1.6)

[ricmorin]

Fuel Trim data at idle is

LT Bank 1 -2.5
LT Bank 2 -2.5
ST Bank 1 0 - (-1.5)
ST Bank 2 0 - (-1.6)

Right. That looks good. What are the trims when the vehicle is acting up?

[pitcrewllc]

OK, open the throttle to about 20-30% in park engine bogs down and data is:

LT B1 -0.5%
LT B2 -0.2%
ST B1 -25.6%
ST B2 -23.8%

[liljoe]

Have you tried introducing extra fuel (carb spray) and see if it picks up? This will narow it down between fuel and ignition. Looking at the fuel trims, it looks like the pcm is comanding it too lean, what are the O2 readings when it acts up?

[pitcrewllc]

Giving it a shot of ether or carb sparay just makes it stall.

O2 Reading are around 800-900mv when its acting up.

Cant get this motor over 1800RPM

[pitcrewllc]

[ricmorin]

Put a vacuum gauge on it and see what the vac reading is at throttle. If it's real low you may have a restricted exhaust.

[pitcrewllc]

I greatly appreciate all the help guys.

I did check vacuum at idle and it was about 15in, but it jump to 0-2 in very quickly as i cracked the throttle open.

Went under the vehicle and loosened the downpipes and the engine runs great now, I'm assuming the cats are plugged.

RESTRICTED EXHAUST was the culprit.

Will comfirm fix when job completed, thanks again.

[brianp87]

Nice

[steven kiser]

sorry for jumping in late but the cats in the v6's are grenades, replace oem and they want the old one back as a core (bull) replace 02's and as it was driven as you said and started lugging at the end i would replace plugs. use only delco oem plugs. about 25 bucks a piece. i've found that anything else will haunt you. i think there was an up date to broaden the acceptable nox and 02's may have to be updated anyway.

just ine more thing, any truck or suv that comes in with a driveability issue we will pull 02's instead of dropping exhaust. avoids broken studs and hot molten slag dropping insde your sleeve.

[brianp87]

If the o2's arent easy to get at ill puch a smaller hole to check the back pressure. Usually if im doing this I am pretty sure its exhaust already. This is the most common thing that I get in that has been everywere else.

[steven kiser]

*i'll do that as well. i'm just real particular where i drill my 1/8" hole. i want to be sure i can close it with my mig. i've seen some pretty wild stuff in pipes. i had a customer complaining of a whistling noise at high speeds. it took me a while but it was from where a shop drilled a hole in the front pipe to check pressure and for some reason plugged it by drilling all the way through and running a 5/16 bolt through it, using washers on both sides, loading it with dum dum and tightening it so much it crushed the pipe. the high volume of exhaust traveling by the bolt was causing the whistle. even being in this business as long as i have (started working on cars in the late 60's) i'm still amazed by some of the things i see.

[brianp87]

Thats crazy. Thats what gives us a bad name.....

[ricmorin]

If I remember correctly, there was a kit/tool for checking backpressure that included a pointy air chisle to pop a hole in the exhaust and some stainless self-tapping screws to plug the offending hole. Never used it because I had the O2 adapter/tester and I've always thought that test was a bit invasive.

[brianp87]

I punch a hole like that but weld it up