i have a 1998 s10 blazer 4.3 liter and it's kicking a po300 & p1404a code and runs ruff at idle, smokes, loads up and long term fuel trim on bank one at idle is +25% with bank 2 at -0.8%. i replaced the intake plenum upper and lower gaskets because cylinder 1&3 were showing miss fires on scan tool. below is a list of items replaced within the last year.
Fuel pump
Spider injector system
Distributor
Cap & Rotor
Plugs & wires
Intake gaskets twice now.
EGR valve twice and installed a screen gasket under egr valve the last time.
the 1404 code has been coming on since doing the injectors and intake gaskets about a year ago.
I'm seeking some ideas of a direction to go to maybe diagnose this vehicle my self. I'm a 30 year veteran mechanic but lack in the drivability field. the truck is sold but really need to get it running right first.
i'll answer all the questions i can to help inform anyone of the running problem.

 

Did the problems start after the work or has it been getting worse over time?

 

did u use ac delco parts when replacing? these truck are very pickey

 

it was running fine for the longest time then it started running ruff at idle in gear. scanned it and it showed cylinder 1&3 misfiring. replaced the lower plenum gasket yesterday and runs the same plus now cylinder 5 is now showing misfires.

 

no i did not, trying to keep the out of pocket price down so i can get it fixed and sell it. did use the better of the 2 available Borg Warner cap and rotor on it. also used Fel pro gasket kit on intake.

 

when i get to work tomorrow I'm going to start pinching off vacuum hoses and see if there's any difference. then if no change I'm going to order a used spider injector assembly from my local salvage yard and try that Monday. $75.00 apposed to $300.00. plus i just replaced that thing about 15 months ago with an Auto Zone $300.00 unit thinking it might give me an extra mpg or so.

 

really need to stick with ac delco. ur wasting ur money

 

yea that's what I've been hearing a lot lately. i've put so much money in this truck getting it safe to drive and running correctly over the last 2 years its crazy how many issues this truck really had and the car dealer said it was a SAFE GOOD running truck. NOT!!!!! but that's a whole other story.

 

Additional info, installed oem gm cap no change. It runs smooth the first 3-5 minutes of driving then it starts the idling ruff problem. Pinched all vacuum hoses off today but no change there either.

 

If it runs smooth for the first 3-5 minutes, it's probably not the injectors causing the problem. When the engine is first started, it operates in open loop: The PCM ignores the oxygen sensors and uses data from the MAP, MAF, IAT, ECT and TPS sensors to determine what fuel mixture and ignition advance to apply. When the PCM switches to closed loop, it uses the oxygen sensors to monitor the fuel mixture. If the oxygen sensors reaction time is slow, or inacurate, it can cause what you're describing.

Does the Service Engine Soon light come on with the bulb check, and remain off while driving?

If the SES light remains on while driving, or flashes, post the DTC's in the order they are retrieved.

Do you have access to a scan tool that can monitor streaming oxygen sensor data?