Low/Rough Idle Driving Us Crazy!!

[mmm98]

I have a 1994 Chevy C1500, 5.7L engine.

My truck idled high and I finally got around to replacing the idle air control valve. This didn't fix the problem and it continued to idle high. I determined the issue to be a vacuum leak, so I replaced my vacuum hoses and checked connections. I tested by spraying carb cleaner and the engine still bogged down at this so I cleaned the throttle body and made a new gasket for the throttle body because that one was falling apart. After doing this, the truck began idling extremely low. It threw the EGR code, so I have since replaced the EGR valve and EGR solenoid. Neither of these fixed the problem. Next on the list is to replace the throttle body gasket along with the distributor cap and rotor. Additionally, my truck will also struggle to accelerate. 

The confusing part is this- there seems to be no rhyme or reason as to the low idle. Sometimes, the truck idles in park at 800-900 RPM and is able to adjust the idle to the load of the AC being turned on. Other times, it idles in park at what is most likely 300-400 RPM because it doesn't even register on the gauge. It does not seem to have any consistency as to if the engine is hot or cold- it just seems to be whenever it feels like it. This evening, while working on it, it started straight up once and idled great at 800 or so RPM and handled AC and headlight load. While the AC was running, we unplugged the wire that makes the AC compressor run- this dropped the RPM down to around 700 and it never recovered from that. The truck was turned off and started again. This time it idled around 700. Truck was turned off and started again- this time it idled around 500-600. The truck was turned off and now it had to be given gas to start and idled low, around 400-500. 

Any ideas to explain what is causing this? We are all going mad trying to troubleshoot this and figure it out. We've checked so many things and nothing seems to be causing the problem. Any help would be greatly appreciated.

[richard wysong]

could be a bad ecm. what does the scanner say for desired idle, IAC counts etc? maybe just a defective IAC? Possible fuel pressure regulator gone bad early Vortecs had problems with them

[mmm98]

5 hours ago, richard wysong said:

could be a bad ecm. what does the scanner say for desired idle, IAC counts etc? maybe just a defective IAC? Possible fuel pressure regulator gone bad early Vortecs had problems with them

I checked the IAC again this morning and I had good resistance on it- are there any other checks I should to do make sure it's functioning properly?

 

How about checking the fuel pressure regulator? I checked my Haynes Repair Manual and they had no suggestions for testing it, only replacing it, so any advice would be welcomed.

[richard wysong]

could be a bad ecm. what does the scanner say for desired idle, IAC counts etc? maybe just a defective IAC? Possible fuel pressure regulator gone bad early Vortecs had problems with them

[richard wysong]

a scan tool that does live data will show IAC counts that change depending on desired idle speed and throttle position. they do get stuck occasionaly. to check the fp reg put a fuel pressure guage on it, run it up to operating temp, shut it off and it should hold pressure for at least 5 minutes. have you changed the fuel filter?, when you do dump it out and see if theres water in it

[richard wysong]

fuel pressure should be 50lbs or better