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Q: heater problems on 1996 Dodge Ram Van 1500

the heater only blows hot air on acceleration,already changed the heater control valve,it is full of coolant,and can't find any vacuum leaks anywhere around the engine, could it be the vacuum canister that is causing the problem ?
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I think there's a vacuum check valve in the dash somewhere that fails (passes air both ways) and produces the symptom you describe. This is a typical Chrysler complaint so I think that's what it is.
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You can purchase a hand held vacuum pump at your local auto parts store to check individual acutators and hoses for leaks. Test at about 10" Vacuum. The push button mode control switch operation test will determine if the vacuum circuits, electrical circuits and controls are functioning correctly. It is possible that a vacuum control system that operates perfectly at engine idle may not function properly at high engine speeds. Before starting this test, stop the engine and make certain that the vacuum source hose at the engine intake manifold and vacuum reservoir are connected. Disconnect the vacuum hose at the engine intake manifold. Insert the vacuum tester hose prod into the source hose leading to the push button mode control switch. Place the vacuum gauge on the cowl so it can be observed from the driver's position as the push buttons are operated. Start the test by pushing the Heat button. The vacuum gauge needle will drop until the actuator has operated. Continue to push buttons: Off, Max A/C, A/C Norm, A/C Bi-Level, Defrost and Heat allowing time for the actuators to operate after each button is pushed, and for the vacuum to stabilize.Inspect the hose connections to the actuator involved. Then test whether the actuator or hose is at fault. Use the test hose on the actuator involved Vacuum Actuator Test

A leak in a hose may be detected with the vacuum test set. Run your fingers along the hose and watch the vacuum gauge reading. The vacuum reading will fluctuate when your fingers contact the source of the leak. The tube can be repaired by cutting out the leak and inserting the tube ends into a 3 mm (1/8-inch) inside diameter rubber hose. Wet the tubes with water to aid assembly.

A vacuum drop in excess of 3 kPa (3/4-in. Hg.) below the 27 kPa (8 in. Hg.) needed in this test will not interfere with engine operation, although there may be a rough engine idle. However, excessive vacuum drop may interfere with the proper operation of the controls under heavy engine load conditions.
Also check the vacuum reservoir and vacuum check valve for leaks.

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