2003 S10 4.3 runs rough past 1/2 throttle

One of our Lemons cars is an 03 S10 2WD, it has the 4.3 V6 and 5 speed, and it is the first year for the MPFI fuel injection instead of the old Vortec Spider.  

We have been chasing what was an intermittent problem that we are now nailing down.  The truck runs rough when more than about 1/2 throttle, it stumbles and bucks and just won't go.  At lighter throttle you can get it to run right up to redline without a problem.  The gear doesn't matter.  The driveline is mostly stock, just with a gutted cat and free flow exhaust, cold air intake, ECU reflashed to remove the security system and speed limiter and adjust speed sensor. 

Here is what we have replaced, but certainly any of these could still be bad, even though we tried to use quality parts:

-Fuel pump with OEM Delphi unit

-MAF sensor

-Throttle position sensor

-Crank position sensor

-Intake air temp sensor in the intake tube

-Distributor

-Plugs/wires/cap/rotor

-Ignition module thing

Any ideas on what to check next?  We have a race this weekend and need to get this fixed and are out of ideas!

I'm guessing you've already done this if you've done the fuel pump, but fuel filter? Or in-tank pickup pre-filter sock if present?

If it'll rev, but it doesn't like load, that sounds to me like it stands a good chance of being an inability to provide enough fuel for the present air. High RPM at half throttle is still half-throttle.

I think we did the fuel filter, but I’ll double check as that is cheap and easy.  Sock on the pump was clean, and it is a plastic tank so no rusting. 

TPS is what comes to mind.  They are cheap enough to swap out and see if that’s it.

Yup, we had the same idea, so that was the last thing we changed.  No difference. 

It doesn’t look like we ever changed the MAP sensor, so maybe we will try that, a Delphi replacement is less than $50. 

Have you checked timing?

Not since we installed the distributor, can’t hurt to check again and it is easy, will do. 

With the list of parts you changed, did you replace all of those chasing this problem, or did you change them earlier and they are just newish parts?

Mostly chasing this problem.  The distributor was worn and being inconsistent so that was something else, and the plugs/wires were just good measure when we were doing other things.  Most of it has been chasing this issue, which used to be more intermittent, at times it ran great!

Where's the pump relative to the pickup on this thing? Is it in-tank, or outside? I'm wondering whether you've got a leaky junction that's sucking some air along with the fuel... Though that sounds less like something that would be sporadic.

Think think think... Definitely load related, not RPM related.

Plug type and/or plug gap?

Pump is in tank.  The replacement was an all in one Delphi.   

Plugs are NGK standard v-power. 

Thanks for for trying to help think it through!

My brother had an s-10, with the 4.3. Bought it and drove it home. Fell on its face acceleratin and wouldn't go above 35. He thought he bought a turd. Turns out the fuel filter was the 13 year old original. New filter and it ran stuipd hard.

Sounds like a fuel supply issue,  so check fuel pressure under WOT and higher rpm.

Is it over half throttle at any RPM, or just higher RPM?

You said the problem used to be intermittent... So it used to run fine under load sometimes and sometimes not? But now it always falters under load?

I'm still curious about plug gap. I'm also curious about your coil. Of course the only dying-but-not-totally-dead coil I've ever had mostly affected idle. But it's about the only piece not swapped out in your list...

I like Vigo's test.

MAP changed... Not better

Fuel filter changed... Not better. 

The fuel flow seemed un restricted it either direction. 

Bluetooth odb module shows action on all sensors and no faults except o2 warming p0155 (not a code just a pending) 

The break up is very consistent between 3200 and 3500 rpm if the throttle is pressed more then the halfway point... At neutral or driving (my neighbors love me)... It also seems to clear at higher revs... Like you can get through it if you back off or just hold the throttle down long enough 

Frustrated and cover with gas I gave up yesterday and started packing for the race! 

Knock sensor?

I had a problem I chased around in my ‘96 1500 350 for a few weeks.  I replaced the Vortec spider, but re-used the old distributor.  Pulling it out and putting it back in was the last straw for the shaft bearing, and it had a slight stumble from about 3k rpm on.  Replaced with an aftermarket distributor, which was good for a few days, until it rained.  Then pretty much the same problem as before.  The aftermarket caps are horrible about corrosion inside the cap, and in a humid state like Florida, that’s no good.  Back to a factory cap, and all was good again.

The factory cap screens can get plugged up too, and not allow the condensation to escape as well.

Fuel volume along with pressure.   eye sight checking of volume not valid.   Measure it.    You can have pressure but not enough volume.

Sensors related to fuel,  exhaust and temperature.

Is the exhaust clear ?

I'm not the mechanic guy, but just wanted to say thanks for jumping in on this.  I'm one of the drivers of this fine LeMons machine and will be behind the wheel on Sat/Sun.  I drove it last time we raced it, which was late last summer.  I wish my memory was better, but I recall we were having what we felt may have been heat soak issues.  I realize this current issue isn't heat soak, but just giving more background on the truck, in case that helps at all.  I got in after it had been on track for a few hours (I think I was the second person to drive it).  When I first got in, it simply wouldn't accelerate.  I was a rolling chicane.  For the first lap or two, it was so bad I nearly brought it in because it just simply refused to go past about 45-50mph.  But it slowly started to clear up and after about 5 laps, I was up to full speed.  It ran like a raped ape for the rest of my shift.  I don't recall specific engine temps, but I know it wasn't "overheating" when I climbed in, just hotter than it is when running on track and getting air.  It did the same thing for the next driver.  From there, my memory is fuzzy and I don't remember when it developed this consistent issue. 

In reply to Klayfish :

That sounds like you could have a coolant temp sensor giving bad readings to the ECM.  Looking at the list of parts replaced that seems to be the only one that hasn't.  They are cheap, get an AC Delco unit and make sure all the air is out of the cooling system after it's replaced.  Worth a shot..

I've seen GM vehicles be very temperamental to plugs.  I'd put the proper 41-993 (I think) plugs in it first.  The V power plugs are good, and they have their place, but I think you need the OE plugs in it.  They're probably made by NGK anyway.  

We have three not-totally-inconsistent but not-totally-aligned descriptions of the behavior:

1. Over half throttle, mostly RPM-independent
2. "The break up is very consistent between 3200 and 3500 rpm if the throttle is pressed more then the halfway point ... It also seems to clear at higher revs... Like you can get through it if you back off or just hold the throttle down long enough"
3. Wouldn't run well after sitting hot, cleared up with... airflow? (this one may be historical and not totally applicable to the issue as it exists now)

Having a look at fuel pressure when it's happening seems like a really good way to rule fuel in or out.

Am I crazy to be the only person pondering the coil given the list of what's been changed, the time involved in chasing this, and the modest cost of swapping it out?

In reply to Kramer :

Oh good, I feel less crazy to be pondering the spark being put out under load with a second voice!

Ransom said:

We have three not-totally-inconsistent but not-totally-aligned descriptions of the behavior:

1. Over half throttle, mostly RPM-independent
2. "The break up is very consistent between 3200 and 3500 rpm if the throttle is pressed more then the halfway point ... It also seems to clear at higher revs... Like you can get through it if you back off or just hold the throttle down long enough"
3. Wouldn't run well after sitting hot, cleared up with... airflow? (this one may be historical and not totally applicable to the issue as it exists now)

Having a look at fuel pressure when it's happening seems like a really good way to rule fuel in or out.

Am I crazy to be the only person pondering the coil given the list of what's been changed, the time involved in chasing this, and the modest cost of swapping it out?

Yeah, the issue we had with heat may be 100% unrelated, so I don't mean to muddy the waters.  But that's my memory of it from last time we raced it, which I think was September 2017 (maybe August).  Just hopeful we can identify it before Saturday morning.