1991 chevy Silverado 5.0 305 engine,has a rough idle,wants to stall at stop signs and bogs down when accelerating.I've replaced the following-egr valve,temp sensor,map sensor,vacuum solenoid,fuel filter,no difference.Then I replaced the throttle sensor and the idle air control valve and now it won't idle at all......

SYMPTOM 1: ROUGH IDLE IN THE COLD

The engine will not idle smoothly, or it stalls during idle when the engine is cold. When the engine is cold and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine, or at least it runs more smoothly.
Possible causes:

1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
   The Fix: Replace accelerator pump or replace the carburetor.
2. There may be a vacuum leak.
   The Fix: Check and replace the vacuum lines as required.
3. There may be some type of ignition problem.
   The Fix: Check and replace the distributor cap, rotor, ignition wires, and spark plugs.
4. The ignition timing may be set wrong.
   The Fix: Adjust the ignition timing.
5. There may be a fault in the computerized engine control system.
   The Fix: Check engine control systems with a scan tool. Test the circuits and repair or replace components as required.

1. The EGR valve may be bad.
   The Fix: Replace the EGR valve.
2. The engine may have mechanical problems.
   The Fix: Check the compression to determine the engine's condition.
3. Idle speed is set incorrectly.
   The Fix: Set idle speed to the car's original settings.
4. The fuel injectors may be dirty.
   The Fix: Clean or replace the fuel injectors.

SYMPTOM 2: ROUGH IDLE WITH WARM ENGINE

The engine will not idle smoothly, or it stalls during idle when the engine is warm. When the engine is warm or hot and you take your foot off the gas pedal, the engine runs very rough and may even stall. When you run the engine at higher speeds, it seems to run fine.
Possible causes:

1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
   The Fix: Replace accelerator pump or replace the carburetor.
2. There may be a vacuum leak.
   The Fix: Check and replace vacuum lines as required.
3. The fuel pressure regulator may be operating at too low a pressure.
   The Fix: Check the fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (This is not really a do-it-yourself kind of job.)
4. Idle speed set incorrectly.
   The Fix: Set idle speed to specs.
5. There may be some type of ignition problem.
   The Fix: Check and replace distributor cap, rotor, ignition wires and spark plugs.
6. There may be a fault in the computerized engine control system.
   The Fix: Check engine control systems with a scan tool. Test circuits and repair or replace components as required. (This is not really a do-it-yourself kind of job.)
7. The EGR valve may be bad.
   The Fix: Replace EGR valve.

1. The engine may have mechanical problems.
   The Fix: Check compression to determine engine condition.
2. The fuel injectors may be dirty.
   The Fix: Clean or replace fuel injectors.

SYMPTOM 3: FAST IDLING

The engine idles too fast. After the engine has run long enough to become warm, the idle speed does not come down to normal. You really notice it when you come to a stop and must have to push ******* the brake pedal to keep the car from moving.
Possible causes:

1. If you have a carburetor, you may have a bad accelerator pump or power circuit.
   The Fix: Replace the accelerator pump or replace the carburetor.
2. The engine may be overheating.
   The Fix: Check and repair the cooling system.
3. The fuel pressure regulator may be operating at too low a pressure.
   The Fix: Check fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (This is not really a do-it-yourself kind of job.)

1. The ignition timing may be set wrong.
   The Fix: Adjust the ignition timing.
2. There may be some type of ignition problem.
   The Fix: Check and replace the distributor cap, rotor, ignition wires and spark plugs.
3. There may be a fault in the computerized engine control system.
   The Fix: Check the engine control systems with a scan tool. Test the circuits and repair or replace the components as required.
4. There may be a vacuum leak.
   The Fix: Check and replace vacuum lines as required.
5. You have a bad idle speed control unit.
   The Fix: Replace the idle speed control unit.
6. The alternator may not be working properly.
   The Fix: Replace the alternator.

SYMPTOM 4: STALLING UPON STOPPING

Car stalls when stopped quickly. You are driving along and everything is just fine ... until you let off the gas pedal and apply the brakes. The engine starts shaking and may even stall. Not a good thing to happen because you lose power steering when the engine dies and could risk an accident.
Possible causes:

1. There may be a serious vacuum leak.
   The Fix: Check and replace the vacuum lines as required.
2. There may be a fault in the computerized engine control system.
   The Fix: Check the engine control systems with a scan tool. Test the circuits and repair or replace the components as required. (This is not really a do-it-yourself kind of job.)
3. Broken linkage.
   The Fix: Repair or replace the linkage as required.

Idling issues can be very frustrating, but with some patient troubleshooting, you'll have a real chance at figuring it out. Remember to always check your engine idle with the air conditioning and defroster turned off, as both of these systems are designed to change the idle when they are on due to the air conditioning system's demands on the engine.

If you have 68 lbs of fuel pressure you need to rebuild your TBI. Should never have 68lbs on TBI system

I'll assume you can feel the engine misfire by the engine idling rough or "looping". I've had a lot of experience with chevy's "phantom" problems with engine idle,stalling and error codes. This would be the easiest approach to start with. Disconnect the battery for at least 30 minutes to an hour. Reconnect the battery and start the engine. It should idle rough, disconnecting the battery will literally erase the learned idle settings. It will take between 15-20 minutes running in "park" to relearn idle settings, should default to factory settings.You'll need a scanner to erase all the stored error codes to see if the problem reccurs. Another approach would be to check the altenator output and battery voltage, the computer needs a minimum amount of dc voltage to operate correctly. I've been through the filter changes, injector power flushing, fuel pumps, injector rail pressure testing, coil packs, MAF sensor cleaning etc. Its been my experience with 98' and newer chevy trucks, its usually something simple. The ABS light is the one that throws me off, that tells me its electrical. I personnaly would start at the battery-altenator verify voltage. Check PCV valve these were a problem chevy had a technical service bulletin on these. An old school trick, start the engine and let it idle, take a can of carb cleaner and spray around the intake manifold gaskets. if theres a vacum leak the engine will gulp the carb cleaner and stutter briefly. I've had the misfire code on my 2001 4.8L ext. cab in the past. It was always electrical, throttle body, EGR valve related. some cases its the engine crank angle sensor. Consider checking the fuel pressure regulator, on the V-8s its mounted on the fuel rail.

I got rid of my 01' Silverado and traded it in on a 07' 4.8L ext. cab, before i did that I looked under the hood. They've eliminated a lot of the fuel and emission control devices that had all of us scratching our heads and paying through the nose to diagnose and correct. I hope this helps provide some direction, we've all shared your pain at some time.

yea id definately try cleaning ur throttle body 1st. My 03 1500hd had the exact same bad idle after my battery went bad it would die at lights unless i kept foot on gas i cleaned throttle body runs perfect now took 30min cost 8$ for cleaner at autozone and theres how to videos on you tube

Sounds like a ruptured diaphram in the fuel pressure regulator. Fuel is being sucked into the motor through the vacuum line. remove vacuum line off regulator and see if there is fuel, or odor of fuel in the line , if so , the regulator is bad. Good Luck

First check for any cracked pipes leading into the intake manifold If all OK (with no codes showing it probably is OK) try checking the fuel pressure regulator. If you smell fuel from the pressure regulator vacuum side arm (when the vacuum line has been disconnected) it needs to be replaced.

You may have a clogged up egr valve. When the egr valve gets clogged up with carbon, the truck will run rough, surge stall, and be very anemic on power. Is the service engine soon(check engine) light on? I've dealt with this issue before, and those 4.3 v6's tend to get egr issues. Try this: when it bogs and runs rough, stop truck, open hood, and tap on egr valve base( where it bolts to engine) if rough running clears up, there's your problem. Egr valve is located on front of engine just to the rt of alternator bracket.

The CAMshaft position sensor is faulty.

Here is the most common cause of surges and stalls at stops and low idle speed and rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

if you have a vacuum gauge you can check egr to see if it holds vacuum if it don't replace