5.7 wants to stall when stoped when HVAC is on

[SuperTramp]

2x on the scanner. I would also look at the throttle position sensor and the idle air control valve. Check out all your vacuum lines too. It's probably something simple, it's just going to take patience to sort it out. Good luck and post updates.

I've replaced the IAC. I'll look into the TPS and vacuum leaks, which I'm not looking forward to. I will employ a scanner.

 

[SuperTramp]

Sitting in your driveway with the engine warmed up, put a pair of vise grips on the big vacuum hose to the brake booster. See if your RPM dips when you press the brakes. Take the vise grips off and see if your RPM dips when you press the brakes.

If the diaphragm in your brake booster is torn, the vise grips will seal off the leak, and your idle won't dip with your foot on the brake.

Thanks! I'd of never figured that one out.

 

[SuperTramp]

You can check for vacuum with an inexpensive guage. They are around $25.

Otherwise you need a scanner that reads live data to monitor what sensors are reading/reporting. You can check just about all sensors on these engines with a DVOM, except MAF sensor (required DVOM with frequency) and O2 sensors. But scanner is way faster and shows a lot of graphs that can use to your advantage.

Thanks!

 

[SuperTramp]

I was thinking mine did the same thing when my alternator was going out.

Test with a voltage meter?

 

[stutaeng]

We should have asked if you had any codes first?!

If it is a vacuum leak, your should have something like a P0171/P0174 codes, depending on which bank its from. They are lean codes. But it could also a single injector giving you a lean code, .i.e., injector stuck closed.

And look at fuel trims. A fuel delivery problem would show up as lean on BOTH banks.

Just all sensors on the Vortec can be verified with live data stream on a handheld scanner.

I helped my nephew once with his 4.3 Vortec S10. The truck felt like it had no power. He was going to go down the rabbit hole of changing parts. "...oh, I read on an online forum someone had the same problem and it was the TPS..."

Anyways, I said let me take a look at it, just cause. He had a shift solenoid CEL and half of his sensors were not reading anything. After an engine shutdown everything was ok, but we went for a drive and quickly went back to original sensors shutting down.

We quickly concluded ECM was going into "limp mode." I think ECM goes in speed density mode and MAF sensor shuts down, along with about half of the other sensors.

Long story short: you need an advanced diagnostic scanner. Don't go down the senseless route of changing parts without concluding they are bad. It will save you a lot of frustration!

 

[SuperTramp]

UPDATE: (1998 k2500 5.7) The only thing I've had time to do to fix this problem is to add Seafoam to the gas tank. Within 10 miles, the SERVICE ENGINE light came on. Four days and 100 miles later, I checked the codes which were; "PO174 Bank 2 System too lean" and "PO154 Oxygen Circuit No Activity Detected (Bank 2 Sensor 1)". The system was reset and it hasn't thrown the codes again. The engine idle is smoother but still has room for improvement.

Does anyone have any interpretations of the codes and suggestions on what needs to be inspected and possible work that may need to be done? Has anyone ever experienced inaccurate codes sent by the computer?

Thank you in advance for your help.

 

[96-2D-Hoe]

Surprised nobody replied to this so I'll revive it.

If seafoam in the gas tank helped your issue and you had lean code you probably had low fuel pressure / worn fuel pump / clogged fuel filter. Change the fuel filter if it's not been changed any time recently. Also grounding can affect the fuel pump performance and just about everything else on these trucks. Might check voltage to ground at the pump. I literally check my battery cables every time the hood is open as they seem to loosen off over time.

The codes are set when there is an issue. If the issue is resolved they won't come back after a reset, but will stay in the system until they are reset.

I have no idea of the reason for the O2 sensor code but maybe someone else will. This is from a Google search for "chevy truck PO154"

Possible causes of this error code include: 1. Damaged or faulty Bank 2 Sensor 1 Oxygen Sensor 2. Ground or short to voltage on signal circuit 3. Holes near the exhaust of the O2 sensor 4. Open or high resistance in signal circuit 5. Broken, burnt or frayed wiring due to excessive rubbing on exhaust components 6. Intrusion of water or oil at the Oxygen sensor connector 7. Loose terminals or broken lock on O2 sensor connector 8. Fouled oil or coolant O2 sensor

If you have an intermittent problem it could be a wiring issue/short

 

[stutaeng]

UPDATE: (1998 k2500 5.7) The only thing I've had time to do to fix this problem is to add Seafoam to the gas tank. Within 10 miles, the SERVICE ENGINE light came on. Four days and 100 miles later, I checked the codes which were; "PO174 Bank 2 System too lean" and "PO154 Oxygen Circuit No Activity Detected (Bank 2 Sensor 1)". The system was reset and it hasn't thrown the codes again. The engine idle is smoother but still has room for improvement.

Does anyone have any interpretations of the codes and suggestions on what needs to be inspected and possible work that may need to be done? Has anyone ever experienced inaccurate codes sent by the computer?

Thank you in advance for your help.

I had those same codes on my K3500 with 5.7. I foolishly changed the O2 sensors, all 4 of them! That wasn't it. After weeks and weeks checking sensors with voltmeters and checking for vacuum leaks (on and off, since it's not a daily driver,) I bit the bullet and bought a scan tool with live data.

Looking at fuel trims on bank 2 (passenger side) and seeing a lean condition being the same at idle as revving up the engine I was 99% at that point it was injector problem, NOT the infamous intake manifold gasket leak, which is what I kept thinking it was. Next I did an injector balance test and #6 was stuck closed! Just to be certain I checked for injector pulse at said injector and it did have pulse.

Problem: Spider Injectors!

New MPFI assembly solved the problem. Good luck.

BTW, my SES light came and went on the dash, but the code was always stored when I had scanned it for codes before. So its more likely these were the issues, but maybe the ECU had not detected the lean code and that's why now the SES light is on. I doubt Seafoam did any good (or bad.)

 

[Curt]

+1 for replacing the spider injectors. I had an identical issue to yours (minus the CELs) and my truck would tend to intermittently die at red lights when the A/C was on. My little trick was to put the truck in neutral once the revs dipped instead of putting the truck in park. I replaced the CFI injectors to the upgraded MPFI injectors and the problem has not come back since.

 

[SuperTramp]

Surprised nobody replied to this so I'll revive it.

If seafoam in the gas tank helped your issue and you had lean code you probably had low fuel pressure / worn fuel pump / clogged fuel filter. Change the fuel filter if it's not been changed any time recently. Also grounding can affect the fuel pump performance and just about everything else on these trucks. Might check voltage to ground at the pump. I literally check my battery cables every time the hood is open as they seem to loosen off over time.

The codes are set when there is an issue. If the issue is resolved they won't come back after a reset, but will stay in the system until they are reset.

I have no idea of the reason for the O2 sensor code but maybe someone else will. This is from a Google search for "chevy truck PO154"

Possible causes of this error code include: 1. Damaged or faulty Bank 2 Sensor 1 Oxygen Sensor 2. Ground or short to voltage on signal circuit 3. Holes near the exhaust of the O2 sensor 4. Open or high resistance in signal circuit 5. Broken, burnt or frayed wiring due to excessive rubbing on exhaust components 6. Intrusion of water or oil at the Oxygen sensor connector 7. Loose terminals or broken lock on O2 sensor connector 8. Fouled oil or coolant O2 sensor

If you have an intermittent problem it could be a wiring issue/short

Thanks for your response and suggestions.

1) I changed fuel filter about 6k miles ago and the fuel pump about 3k miles ago. Both were AC Delco.

2) Am I correct in thinking that in order to check the ground at the pump, you either need a head of a small child or loosen the tank straps and lower the tank? When my pump went out, I have 30 gallons of gas in the tank. I ended up having to unbolt and slide the bed back. It was x#n?1!

3) I'll check the O2 sensors; wiring and the presence of liquid.