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jon_050, Auto Mechanic
Category: Car
Satisfied Customers: 67
Experience:  Domestic and Imports
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86 Suburban: Need test procedure to confirm functionality of A.I.

Customer Question

86 Suburban C20 3/4 Ton 454 Federal/HD emissions, 2WD AT. Need test procedure to confirm functionality of A.I.R. management (diverter) valves and decelleration valve. Symptoms: traced manifold vacuum loss to diverter valves, valves fail to hold vacuum or redirect air flow to air cleaner when vacuum applied by hand pump. Seems suspect that both valves exhibit same behavior, suspect there may be some deliberate vacuum bleed designed into the valve, since the diverters are connected directly to manifold and carb manifold vacuum sources and would be subjected to high vacuum for prolonged periods (especially at idle)Replacement valves costly and require special order, so I'd like to to confirm component failure and explore repair options prior to replacement. Verified continuity across solenoid, but have not verified voltage delivered to valve.

Specific questions:

- Expect diverters to hold vacuum- valid?
- How to validate operation of solenoids?
- Relationship of vacuum signal and solenoid regarding valve operation?
- If valves determined to be defective, repair/rebuild options?

Any other info on diverter valves from experience welcome and appreciated.

Submitted: 11 years ago.
Category: Car
Expert:  Marc replied 11 years ago.

 Can't find any info on holding vacuum but they should redirect air when vacuum is applied.

Check the out put of the pump those pumps would fail if they sat 2 weeks in the rain without running.  

  1. Pull off the vacuum line to the top of the valve with the engine running. There should be vacuum in the line. Install the vacuum line and inspect. No air should be escaping with the engine running at a steady idle.

  2. Open and quickly close the throttle. A blast of air should come out of the valve muffler or air cleaner for at least one second.

  3. If the truck is equipped with a electric diverter valve go to the next Step. If not equipped with the electric type go to Step 6.

  4. Disconnect the harness connector from the solenoid and then turn the ignition on. Connect a voltmeter or test light between terminal A on the harness connector and ground.

    1. If 10 volts or greater is measured or the test light lights, the solenoid is getting power.

    2. If less than 10 volts is measured or the test light does not light, the solenoid is not getting power due to an open or short to ground in the wiring.

  5. Measure the resistance across the solenoid by connecting an ohmmeter between the two terminals on the solenoid. Make sure that the solenoid is disconnected or a false reading may occur. The resistance of the solenoid should equal about 20 ohms.

  1. If the resistance is less than 20 ohms, the solenoid is shorted. Replace the solenoid and the Engine Control Module (ECM), as required.

  2. If the resistance is more than 20 ohms, the solenoid is open. Replace the solenoid.

  1. If the valve must be replaced, use a new gasket at the valve mounting on the pump and torque the bolts to 85 inch lbs. (9.5 Nm).

  2.   I'll post back a link to check any other questions you might have


  4.      Regards Enginboy   


Customer: replied 11 years ago.
Reply to enginboy's Post: Thanks for the reply. This info basically matches the info I have from AllData, but it's the subtle details that I need to complete the diagnosis. The info you provided (and the data obtained from AllData confirms) indicate that the engine should be equipped with either electric solenoid actuated diverter valves, or vacuum operated valves. The diverter valves I'm testing appear to be a hybrid - both electrical and vacuum connections. I'm assuming that the electrical connections are for an actuation solenoid, but it's possible they are actually a sensor - though I haven't found any reference to sensors or position indicators associated with the diverters. Since the electric/vacuum combination isn't referenced in either of our docs, I think it may be a less common valve configuration. AllData, Haynes, and Chilton all have significant gaps/errors regarding the emission control system of this particular vehicle, including references to components such as O2 sensor, mixture solenoid, ALDL connector, and catalytic converter which are clearly not included in this particular design. I think the Heavy Duty/Federal Emissions designation represents a slight design variation from the standard 86 C20 spec, and must be small enough top be overlooked by the three reference sources (4 including yours) that we checked. I'm stumped on this one, but do appreciate your response.

Expert:  Marc replied 11 years ago.

I'll bet you don't see alot of those in Calf. Your right it's the weight that makes them an exception Try going back a few years late seventies maybe you'll find what you need.
Customer: replied 11 years ago.
Reply to enginboy's Post: This is the first I've worked on personally, and although the theory hasn't changed, the mostly mechanical vs electronic engine management and feedback systems are a little diffent to troubleshoot. Thanks for the assist, perhaps a veteran Chevy Tech can fill in final details.

Expert:  jon_050 replied 11 years ago.
 What are you doing with this truck anyway, trying to pass tailpipe emissions?
Customer: replied 11 years ago.
Reply to jon_050's Post: The original issues reported were rough idle, hesitation on accelleration, backfiring on decelleration at highway speeds, and dieseling. Engine was in good mechanical condition at 130K mi, and plus, wires, cap, and rotor had been recently replaced providing little improvement in performance. Vacuum leaks leading to improper function of distributor advance, EGR, and EFE were resolved by replacing leaking vac advance actuator and replacing several vacuum lines. Engine timing (initially 12 BTC) was adjusted to 4 BTC specification (VECI). Mechanical accellerator pump replaced, and EGR valve cleaned to remove accumulated carbon. Rough idle corrected, dieseling eliminated, and hesitation on acceleration and backfiring on decelleration improved, but not eliminated. There is 20" of vacuum at the diverters at idle, but vacuum test shows they don't hold vacuum, and do not appear to actuate on decelleration. This seems to be a potential cause of both the remaining hesitation and backfiring issues. I'm not sure of the failure to hold vacuum test of the diverter valves is conclusive, and given their cost and limited local availability, I'd like to get a 2nd opinion before replacing.

Customer: replied 11 years ago.
Problem resolved. Diverters are in fact hybrid vacuum/electric, and one had a bad solenoid. Finally got access to original GM shop guide along with wiring diagram, and easily traced the source. Thanks Enginboy for taking the time to suggest the initial diagnostics.


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