2002 4.3 V6 Auto- Multiple Misfires

[shawnmd]

I have a 2002 GMC 1500 with the 4.3 L V6 auto. I have a weird issue. At idle (800 rpm), it runs smooth. Rev it up to 1300 or above and it starts missing. Drive the truck it feels fine, until you let off on throttle and it starts to "buck" or miss. Its especially bad if the truck is in cruise and it throttle's down (going downhill). I have replaced the plugs (AC Delco platinum), cap and rotor, wires, fuel filter and fuel pressure regulator. I have checked the fuel pressure and it was between 50-60 if I remember correctly. It has thrown check engine codes; Multiple Misfires. What should I check next?

 

I should also mention that I'm getting about 16 MPG on highway, which I think is bad.

[Jsdirt]

I had the same issue with an '00 Jimmy. I got so frustrated after 2 years of getting nickeled & dimed to death, that I parted it out - although I just installed the engine in a '89 S10 Blazer, sans all electronics. Runs great now! Nice & simple now - no more failure prone OBD-II.

 

As to your problem - with mine having the same issues, it was narrowed down to either the crank position sensor, or the spider injector assembly under the plastic upper intake.

[shawnmd]

Is there a way to test the existing spier injector assembly (bench test)?

[Jsdirt]

Check this out: http://troubleshootmyvehicle.com/gm_fuel_i..._injector_1.php

[shawnmd]

Check this out: http://troubleshootmyvehicle.com/gm_fuel_i..._injector_1.php

 

 

Thanks for the great article. I did the leak down test and everything checked out. I did not loose any pressure after shutting off the truck, so the spider injection system isn't. What else can I try?

[Jsdirt]

If the nylon lines aren't leaking at all, & your certain all the spark plug wires are on there good, the next thing I'd try is checking for vacuum leaks - check all hoses (if 4x4, don't forget the ones that run to the transfer case), then between the lower intake manifold & cylinder head. Easiest way to check intake gaskets is with a scan tool & same can of cleaner - spray between manifold & head, & watch o2 sensor activity - if voltage suddenly goes up with each application, you've found the vacuum leak.

 

Next, try cleaning the MAF sensor - takes 5 minutes to remove it & hose it down with brake cleaner, or similar solvent.

 

If you still have a problem, before doing anything else, clear the code & see if it comes back. If it's still stumbling at this point you can disregard that & proceed to a compression test.

 

Good luck & keep us posted.