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Category: Chevy
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Experience:  +15 years experience with Chevrolet. Bumper to bumper diagnosis & repair.
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I have a 1993 chevy silverado. 350 5.7 tbi.

Customer Question

I have a 1993 chevy silverado. 350 5.7 tbi. I found it in a guys yard where it had been for 2 years. I have spent a month cleaning and overhauling. Every thing is new Ac Delco. I did not change the distributor or ignition module. It starts everytime...runs like a champ at first then when driving it burps as i accelerate only at first starting off. Runs great at highway speed...then when you try to let it ideal it runs and runs..then it chokes down and dies. It will start right back up only to die at idel again....    What now?
Submitted: 5 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 5 years ago.

hi

tell me what all you have done.....does the engine light come on?if so,can you give me the codes?

thanks

Customer: replied 5 years ago.

I have new injectors...new valve cover seals...new ideal air contro..speed sensor...temp sensors..o2 sensor...wires...plugs....rotor and cap.....all codes have cleared and none show up....just wont ideal after warm up and wont take off with out burping...i have a new fuel filter and have check fuel pressure..steady at 12/13 psi

have checked timing also.( one guy here suggested distributor or addvanced timing may be sticking)..this is my first attempt to work on a car so I dont know much!!

Expert:  Tim's Auto Repair replied 5 years ago.

ok

first remove and clean the egr valve,sometimes there will be carbon under the valve ,causeing a vacuum leak from the exhaust,,,also the egr solinoid [this is what the vacuum line from the egr valve connects to]will go bad sometimes and send a vacuum to the egr valve opening it when it dont need to.....you may also want to remove the iac valve and clean it or mabe replacve it sense it has sit for a while it may be sticking

thanks

please ACCEPT my answer so I can get credit for my work.i don't receive commission unless you do ..i'm not always going to be giving you good news,so please dont let this stand in the way of you accepting my answer.it does not cost you more money.we will still be able to communicate.. Bonuses and positive feedback are appreciated!if you are not satisfied with my answer,please do not leave bad feed back,i will gladly opt out and let another expert handle the question.PLEASE ASK IF YOU NEED MORE HELP

Testing and Inspection

 

Chart C-2 - Idle Air Control (IAC) System Check
Wiring Diagram For Chart C-2 - Idle Air Control (IAC) System Check

CHART C-2 - IDLE AIR CONTROL (IAC) SYSTEM CHECK

 

Circuit Description :

The ECMcontrols idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve away from it's seat, allowing more air to pass by the throttle plate. To decrease rpm, it moves the IAC valve toward it's seat, reducing air flow by the throttle plate. A "Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air allowed (lower idle).

Test Description : Numbers below refer to circled numbers on the diagnostic chart.
  1. The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
  2. This step checks the quality of the IAC movement in step 1. Between 700 rpm and about 1500 rpm, the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 rpm), it may take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending IAC may cause engine stall. This may be normal.
  3. Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node light should flash red and green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash red and green, check the circuits for faults, beginning with poor terminal contacts.

IAC VALVE RESET PROCEDURE

  • Ignition "OFF" for 10 seconds
  • Start and run engine for 5 seconds
  • Ignition "OFF" for 10 seconds
Diagnostic Aids :

A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC "Scan" tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem.

  • Vacuum Leak (High Idle) If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 rpm, locate and correct vacuum leak including PC V system. Also check for binding of throttle blade or linkage.
  • System Too Lean (High Air/Fuel Ratio) The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. Code 44 may be set "Scan" O2voltage will be less than 300 mV (.3 volts). Check for low regulated fuel pressure water in the fuel or a restricted injector.
  • System Too Rich (Low Air/Fuel Ratio) The idle speedwill be too low. "Scan" tool IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. "Scan" tool O2 voltage will be fixed above 800 mV (.8 volts).

Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensors "Scan" voltage will be slow to respond.

  • Throttle body - Remove IAC and inspect bore for foreign material.
  • Refer to "Rough, Unstable, Incorrect Idle or Stalling". See Diagnosis By Symptoms - No Trouble Codes Stored.
  • If intermittent, poor driveability in idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.
  • A/C Compressor or Relay Failure - See A/C diagnosis if circuit is shorted to ground. If the relay is faulty, idle problem may exist.
  • Refer to "Rough, Unstable, Incorrect Idle or Stalling".
Service and Repair

 

IAC Thread Mounted 10mm Pintle

NOTES:

 

  1. The Idle Air Control (IAC) valve is an electrical component and must not be soaked in any liquid cleaner or solvent. Otherwise damage could result.
  2. The IAC valve for this application is thread-mounted and has a dual taper, 10 mm diameter, pintle. Any replacement of the IAC valve must have the correct part number, with the appropriate pintle taper and diameter for proper seating of the valve in the throttle body

REMOVAL:

  1. Disconnect electrical connector.
  2. Remove IAC valve, use a 32 mm (1 1/4") wrench.
  3. Remove IAC gasket and discard.
  4. Clean old gasket material from surface of throttle body assembly to insure seal of new gasket.

 

Idle Air Control Valve Pintle Retraction (Manually)

NOTES:

 

  1. If the IAC valve was removed during service, its operation may be tested electrically with the IAC/ISC Motor Tester (J-37027 or BT-8256K) or equivalent. Before installing a IAC valve, measure the distance between the tip of the valve pintle and the mounting surface. If the dimension is greater than 28.0 mm (1.10 inches), the valve pintle must be retracted to prevent damage to the valve. This may be done electrically using an IAC/ISC Motor Tester or manually by exerting firm pressure, as shown in the illustration, with a slight side to side movement on valve pintle to retract it.
  2. No physical adjustment of the IAC valve assembly is required after installation. The IAC valve pintle is reset by turning the ignition ON for ten seconds and then OFF. The ECM then resets the pintle to the correct position. Proper idle regulation should result.

INSTALLATION:

  1. Install the IAC valve into the throttle body with new gasket. NOTE: New IAC valves have been reset at the factory and should be installed in the throttle bodyin an "as is" condition, without any adjustment.
  2. Tighten IAC valve assembly to 18.0 Nm (13.0 lb-ft).
  3. Reconnect electrical connector to IAC valve.
  4. START engine and allow it to reach operating temperature.
Customer: replied 5 years ago.
I have a new ias and isc and a new egr already!! sorry i left that out..pretty much everything is new on top of the motor.
Expert:  Tim's Auto Repair replied 5 years ago.

ok

i will opt out and see if any of our other guys may know what is going on.....please list all you have done

thanks

Customer: replied 5 years ago.

the only thing that is not new on the truck is the distributer and the ignition moduel

 

Expert:  ase_master327 replied 5 years ago.
Hi.. Have you tried monitoring the signal getting to your computer from the TPS?
Customer: replied 5 years ago.
no. how is that done?
Expert:  ase_master327 replied 5 years ago.
Here are the voltages:
graphic
Make sure there is a smooth transition in voltages when you open the throttle SLOWLY... This test is best done with an analog voltmeter (with the needle) so you can SEE if it is operating smoothly.
... And here are some instructions on testing it:
  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the TPS or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with a high impedance voltmeter at terminals C and B.
  5. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  6. Verify that the TPS voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the TPS voltage reading.
  7. If the sensor voltage is not as specified, replace the sensor.



Click image to see an enlarged view

Fig. Fig. 2: Using a DVOM, backprobe terminals A and B of the TPS sensor to check for proper reference voltage



Click image to see an enlarged view

Fig. Fig. 3: Using the DVOM, backprobe terminals C and B of the TPS sensor, open and close the throttle and make sure the voltage changes smoothly



Click image to see an enlarged view

Fig. Fig. 4: Throttle Position Sensor (TPS) wiring diagram



Hope it helps!
Customer: replied 5 years ago.

i have changed this sensor 2 times once with a after market then went back with ac delco....is there a way to up the ideal manualy?

 

Expert:  ase_master327 replied 5 years ago.
No way to idle it up manually, as when fuel injected engines are idling, the throttle stays completely closed.

I wil have to opt out also..
We usualy test before replacing.. Helps save time, money, and frustration.

Have a nice day.
Customer: replied 5 years ago.

ok.. thanks. I dont have a ton of test equipment so I went with the put all new parts on and then see what wasnt working...now I am stuck. Cant get this one thing to work right. Stumble on takeoff and the shutting off at idel.

 

Expert:  ase_master327 replied 5 years ago.
All it takes is a voltmeter...

Anyways.. I have a few more ideas..
Is the engine light on?
or.. does it EVER come on when you first try crank the engine?
Customer: replied 5 years ago.

no ..we had a few codes early on but now with all the new parts on the codes have gone away. the check gauges light comes on when it starts to die at idel because the oil pressure gauge drops also.

 

Expert:  ase_master327 replied 5 years ago.
Some of those TPS are adjustable.. you wil want to make sure you have the proper voltage from that TPS while the engine is idling.

Other than that, there are vaccuum leaks and misfires.. Are you sure you have the firingn order right, and no cylinders are misfiring.. and that there are no vaccuum leaks?

IF so... get your voltmeter on that TPS.
Customer: replied 5 years ago.

i spent 6 hours today changing distributer and still no change....the coil is the only thing left to change. Today when I start the car it runs great..then when i drive it stumbles the goes. after it warms up it wont ideal. Seems like the idel is low. That being said now that the ne distributer is on I need to set the timming but cant. When I unplug the timming by-pass the motor shuts off. Plug it in then it runs..but will stall and shut off after a few min.

 

HELP

Expert:  ase_master327 replied 5 years ago.
I don't know.. maybe we don't understand eachother.
I am going to opt out also.

Have a nice day.

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