Another help make my 454 Vortec run thread!

[coachhick]

I have a 1997 Crew Cab K3500 454 Vortec with an NV4500 and 4.56's and 148k miles. My dream truck - go figure! I owned it for 77 days in 2020, but sold it to a buddy after multiple random issues. A month ago, he sold it back to me. He had just put $3,200 into the front end and got it back running like garbage. He replaced the mass airflow sensor then decided he was done with it. I have read and read and read over these forums the last month and your old threads have been great help SO FAR, but I'm not sure where to go next with it. Here's the skinny:

- Running lean - fuel pressure gauge showed 15 psi
- I replaced fuel pressure regulator (twice - Delphi fpr10021 the 2nd time) , fuel pump, fuel filter

Was running MUCH better, but had a high idle. Fuel trims were within range at idle and speed, but when clutching to slow she would hang around 2,500 rpm at idle until you shut her down. While looking for the problem, I found the most incredible crankcase vacuum that I'd ever seen (pulled the air filter, took off oil fill cap, took out PCV valve, took off the pass side valve cover vent, plugged the air intake..... and it was still running...)

- replaced lower intake gaskets with a MAHLE set
- replaced spark plug wires, dist cap, rotor from Summit since I was under the plenum AGAIN
- replaced thermostat, flushed, new coolant


She still has a high idle, BUT fuel trims now creep and once they reach 50% & 25%, she purrs like a kitten. You clutch from highway speeds and she idles high until the trims get back high and she settles down. (odd how I had a high idle previously but the fuel trims read normal) - - - fuel trims at highway speed are good
BUT ALSO, now I have what feels like a miss in the lower rpm range..... like the truck bucks/vibrates like I've slung a tire weight but you can tell it's short on power when it's doing it (tach dances when this is happening). You get it wound up in the rpm range and its smooth as silk.

No misfire codes, only codes are IAC higher idle than expected and another lean code

I checked fuel pressure today - 58 psi key on, 50 psi idling - 55+ when you romp on it - PPPO installed new AC Delco injectors in 2019 (less than 10k miles ago)


Where do I go? What do I look for next?

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[Keeper]

Did someone adjust up the throttle stop screw? Maybe trying to get a better idle when the fuel pump or injectors started dying? Left field guess.

 

[coachhick]

Did someone adjust the throttle stop screw? Maybe trying to get a better idle when the fuel pump started dying?

I saw the throttle stop screw on one of my multiple plenum removal adventures. Would you think that maybe its adjusted too open and the computer is having to add fuel to compensate? But that wouldn't explain it being a normal idle once the trims are maxed out would it? It's idling around 1,000 rpm currently, feels normal for a standard.

 

[Keeper]

I saw the throttle stop screw on one of my multiple plenum removal adventures. Would you think that maybe its adjusted too open and the computer is having to add fuel to compensate? But that wouldn't explain it being a normal idle once the trims are maxed out would it? It's idling around 1,000 rpm currently, feels normal for a standard.

There are much better people on here to address the computer responses and why. That idle screw is easy to adjust to check. It just sounds like a computer trying to adjust for something and making a pretzel out of itself. I know that on my vortec 454's, my idle issues have been due to fuel pump, injectors, fpr, or a screwy coolant temp sensor (the one by the thermostat). New o2 sensors also helped with idling and mileage.

 

[coachhick]

I replaced the cts by the thermostat also. No change.

The codes I have are P0171 and P0507. There is no code for a misfire

On the ride home today she felt and sounded like she was running on 3 cylinders the entire time. RPMs didn’t help. Struggled to keep highway speed. Would sometimes surge. So…. That condition is worsening it seems

 

[BeXtreme]

I replaced the cts by the thermostat also. No change.

The codes I have are P0171 and P0507. There is no code for a misfire

On the ride home today she felt and sounded like she was running on 3 cylinders the entire time. RPMs didn’t help. Struggled to keep highway speed. Would sometimes surge. So…. That condition is worsening it seems

Check for an exhaust leak before the 02 sensors. That can drive the trims way out of whack thinking that the system is super lean and dumping fuel to compensate. That can cause the P0171. You could also have an intake leak somewhere(upper or lower gaskets or injector O-rings) or a stuck open Purge or EGR valve causing it to be able to suck in air from somewhere. If its an intake leak, it could cause both codes. Exhaust leak will cause it to throw the code but be VERY rich instead.

The second code is just telling you that the IAC can't get the RPM to drop like it thinks it should. That usually means either a bad IAC valve, intake leak, or the idle set screw is turned in too much. If you have a scanner you can make sure the cam retard value is right and then adjust the idle screw until you get a small IAC count at idle(~30-100)