Idle drops in relation to the canister purge valve solenoid

  • 4.7L
  • V8
  • 4WD
  • 160,000 MILES
My vehicle has a idle drop that can almost cause the vehicle to stall in relation to activity with the purge valve. Let me explain.

I recently changed timing chains and valve cover gaskets so I disconnected vacuum lines going to the intake by the throttle body and the line going to the charcoal canister.

The car starts and runs fine. While parked I start the car and let it run. After some time I begin to hear the canister purge solenoid beginning to tick or pulse. While this is going on the car still runs fine. However, at a point the canister purge valve solenoid will stop ticking. At this point, the idol of my engine drops out, almost to a point of stalling. Then after about two seconds the idle comes back up to normal and the canister purge solenoid begins pulsing again. I have not tested anything with the EVAP system. The car did this before I worked on the timing chain. I suppose it is possible that I could have put the vacuum lines on backwards from the intake manifold and the charcoal canister, but I did not think that would even matter since I thought the canister purge solenoid simply opened and did not draw a vacuum, but just allowed the intake manifold to pull fuel fumes into the manifold. There is a laundry list of repairs I have done to this vehicle since I purchased it, including the idle air control valve.
Any ideas on what could be causing my problem?
Do you
have the same problem?
Monday, July 10th, 2017 AT 5:51 PM

1 Reply

I suspect when the pulsing of the purge valve stops, that is simply another clue related to what is causing the drop in idle speed. Most commonly that is when the AC compressor cycles on. The first thing to consider is if the battery was recently disconnected or run dead, the engine computer will have lost its memory. Fuel trim data and other information will start to be rebuilt right away, but it needs to relearn "minimum throttle" before it will know when to be in control of idle speed. Until then, the engine may not start unless you hold the accelerator pedal down 1/4", you will not get the nice idle flare-up to 1500 rpm at start-up, and it will tend to stall at stop signs. To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.

If there is still a problem related to idle speed, you will need a scanner to view live data and see which "step" the computer has placed the idle speed motor at. Out of 256 steps, step 32 is typical for a properly-running engine. You will see it at around step 50 when one cylinder out of eight is misfiring. If you find it at step 0, minimum throttle hasn't been relearned.

The value of observing the idle speed steps is to see if that corresponds to what the engine is doing. If you see it at step 15, for example, when idle speed is too high, the computer is trying to bring it down, but without success. If you see the step number go real low when the idle speed also goes low, the engine is responding as it was commanded to do, and you have to look at the numerous inputs to figure out why the computer wants that lower idle speed.

You also might try pinching off the purge hose with a hose pinch-off pliers. The canister purge system is a controlled vacuum leak, but it is mainly fuel vapors that are going through it. The engine computer will open that valve at times to see how the oxygen sensors respond, and it will close it at times to verify it has control over it. Either condition, lack of fuel vapors, or a prolonged vacuum leak, will affect idle speed.
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Monday, July 10th, 2017 AT 7:55 PM

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