Ive got a 95 s10 blazer 4wd 4.3 vortec and have replaced cap, rotor, wires, plugs, tps, lower intake gasket, cpi spider injectors, plenum gasket, fuel filter, pump and fuel tank. Still after all that i have a smooth idle and revs pretty good. When I go to take off it wants to hesitate, backfire and pop until I mash it to the floor then it comes alive like a monster. Any suggestions on what else it could be. Im at a lost at this point..

 

Your best bet is to eliminate and isolate the problem because it can be many things. There is more than one order but first you need to figure out if it’s fuel delivery, ignition, air or engine mechanical and then get to the cause. It’s not air if it can run strong at full tilt. It’s a transition problem under maximum load. It idles ok and it runs strong at full output but it doesn’t handle the transition so the most likely problems are the sensors or the systems ability to react to changing sensor values. Examples of sensors not reporting correctly are the TPS, O2 or MAF. Problems not reacting correctly are ECM, fuel pressure, injectors, strong enough spark, etc.

One example of why you stumble under acceleration is that the MAF sensor under reports or the O2 sensors are sluggish so when you present a load the feedback loop is slow and it takes a while for fuel delivery to stabilize. A simple sensor example is that your TPS sensor does not produce smooth correct output so the initial fuel pulse signal is wrong but the O2 sensors eventually get the fuel delivery corrected. A few things to start with to see if you have any clues:

Did this problem start after one of those repairs or is this the problem your trying to solve from the beginning?

Have you recently done a compression test to rule out a weak cylinder or two?

Did you verify proper fuel pressure and leak down after the fuel system repair?

Have you checked vacuum?

Have you checked the strength of the spark at a plug end?

Do do you have any codes?

Have you looked at fuel trims?



Depending on any starting clues from the above and your test equipment, things to consider testing:

Check fuel pressure and leak down at key on, idle and under acceleration

Check the strength of spark


Do you have a scanner that can look at live data and graph sensors?

look at sensor values for TPS, MAF, IAT, MAP, ECT and O2. Example, is the TPS output nice a smooth from idle to full pedal with Key on engine off. Do MAP, ECT and IAT give ambient readings?

look at fuel trims at idle and then watch what happens when you accelerate and stumble. Do you go lean or rich? What do the O2 sensors outputs look like?

Look at the MAF values under different conditions.


Thats a lot but it gives you an idea of how to test, eliminate, identify and repair your problem. You don’t have to do it all at once but you need a plan. The alternative is parts swapping which is only cost effective if you get lucky and guess right early on.

If I can help you to get started let me know and we can break this down into something manageable for you.


George

 

It started with misfire and I changed the cap, rotor, plugs and wires. Then found out it was spider injectors #3 and #5. It didn't change anything so I started swapping out the other stuff. I don't have a MAF sensor. Also I'm trying to find a good picture of vacuum diagram for my 95 4.3 vortec

 

I forgot you said ‘95. Some more of what I said may not be possible with that transition year for ODB. Let me know if I can help any further.


George

 

You just substitute a speed density calculation for the MAF btw. The ECM still has to take a best guess at the proper initial fuel delivery injector pulses based on the IAT and MAP values. Then like all systems, the O2 sensors continue to correct to maintain optimum emissions. That’s why fuel trims are valuable in your case. Does the truck start lean or rich and does it go lean or rich under acceleration and that helps point to the problem.


George

 

It seems rich both ways

 

What are your fuel trim numbers? If not measured, why do you think your running rich?


George

 

58 with key on drops to 30 when off idles at 55 goes to 62 when I rev it up

 

Your leakdown should hold at 50 psi after 10 min and you are at 30 so there is a leak somewhere in the system. This may or may not be causing your stumble when accelerating. To rule out fuel pressure as the cause of this issue you can monitor pressure while accelerating under load with the gauge taped to the windshield or using a remote sensor. To isolate the leak you remeasure at the fuel filter outlet.

George

 

Ok thanks for the info 👍 I'll see what happens tomorrow