There are a few possibilities for this;
but first have the trouble codes pulled before moving forward with the diagnostics below as it will give you the best direction becasue it will use the On Board Diagnostic or OBD system to tell you what problems there are with the truck. Any Advance Auto or Auto Zone will do this for free. The Malfunction Indicator Lights do not need to come on to have a trouble code come up in a scan, so still have it done first.
There could be a vacuum or exhaust leak, some cruise controls use engine vacuum to modulate the throttle and a vacuum leak would also explain a problem with stalling. So look for a vacuum leak by listening for a sucking air or hissing sound under the hood and try to locate any possible vacuum leaks. If there is an exhaust leak near an O2 sensor especially. The O2 sensor will the the computer that the air/fuel ratio is lean and to richen it by adding gas. A failed O2 sensor or an exhaust leak that allws air to get near an O2 ensor will cause gas milage, surgin and sometimes stalling problems. You might also check the intake and exhaust tracts for blockages.
Another reason the idle may surge and you have poor fuel economy and stalling is a failed or failing Idle Air Control Valve, IACV, which is located near the throttle body on the intake manifold. It will have coolant lines running through it in many cases and either an electrical connector or vacuum lines. The electrical connector or vacuum move a pluger or solenoid that changes air ports in the intake manifold to allow air around the throttle body and change the air/fuel ratio to keep the idle steady. They often become diry with carbon depostis with the mileage you have and either the ports get clogged or the plunger has problems moving. You can check it by removing it from intake, some do not have coolant lines so it is easier if it does you can leave the coolant lines intact for testing, leave the vacuum or electrical connection hooked up. Start the eninge and see if the plunger moves at all and inspect it and the ports it covers for carbon deposits. If it does not move, and the engine may want to stall so it should react to this, putting your fingers over the ports may keep it from stalling or revving the engine, then the IACV may be bad. Re-install it and start the engine again, this time reomve the electrical connector or vacuum, you will have to cover vacuum lines but it will react immediatly, the engine will change in RPMs in reaction to the IACV mot working at all. This test works better for electrically controlled IACVs than vcacuum as the vacuum being taken off makes it hard to judge the reaction the engine has to the reomval of the IACV control. You have an electrically controlled IACV I beleive. This test will tell you if the IACV still has some functionallity as the engine idle changing when the elctrical connector is removed stops the IACV from working and the engine changin in idle means that the IACV is still working at least to some extent. If it does react to this test and the plunger moves from the other test, the IACV can be placed lower on the probable cause list as a candidate for the cause of the problem.
Lastly, you may have to check the fuel pressure. Auto Zone or Advance Auto will have test gauge kits on a loan basis. The test port for fuel pressure is on the fuel rail and the kit will have an adapter to test it or you can have it check for a small fee at a repair shop. Remeber to allow time for fuel pressure to subside by letting it sit without running for 2 or more hours and then when you begin to remove the test port plug, be wary and do it slowly as there may still be a great deal of pressure behind it as fuel injection systems use high pressure fuel systems. You can also bleed off pressure if there is a fuel fitting or filter you can get to that is easier to loosen slowly and put a rag around to let off pressure. Usually letting it sit without turning the key to the ON position as this primes the system or not starting it is enough to allow the fuel pressure to drop off to eventually nothing.
When you are finished with this test check the fuel pressure regulator near the return fuel line on the fuel rail. Remove the vacuum hose, the fuel pressure and Idle should go up. This tests the regulator to make sure it works as no change indicates it may have failed.
Let me know how things turn out.
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Thursday, December 15th, 2011 AT 5:09 PM