5.7 wants to stall when stoped when HVAC is on

[SuperTramp]

I could use the experience of the members on a problem with my 1998 5.7; 160k miles; complete tune up 10,000 ago. It wants to stall when stopped (in Drive -foot on peddle) ONLY when HVAC is on and ONLY when the engine has warmed up. The RPM drops to 500 and oil pressure drops. RPM increases back to 750 when I put in PARK. Any thoughts? Thanks in advance.

 

[SUBURBAN5]

When you say stall does the engine hit 500rpm and shake or hesitate?

 

[SuperTramp]

I put it in PARK before it stalls. Engine shakes/shudders when it dips to 500 RPM

 

[SUBURBAN5]

Have you checked fuel pressure?

 

[96-2D-Hoe]

If it's only when the AC is on it's possible you have problems with your compressor. Otherwise it's probably a weak idle for some reason.

Could check for vacuum leaks. SUV's, not pickups afaik, have a vacuum controlled heater valve that is active when the AC is on. The hose to that on mine was perished.

And your brake booster could also be failing and losing vacuum as you say it's when you have your foot on the pedal.

 

[Supercharged111]

The PCM compensates for the load of the compressor. It's possible the compressor is a bit tight as previously mentioned and that this correction factor is no longer sufficient. It's also possible the IAC is being a POS. Also possible weak idle as mentioned before. How does the compressor feel if you turn it by hand.

 

[stutaeng]

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.

 

[HotWheelsBurban]

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.

 

[98 Nitro]

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

 

[Erik the Awful]

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.