Question about 1999 Ford F150 Regular Cab

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Intermittant Idle problem

I have seen similar problems posted here, and mine may be the same (the IAC), but has additional symptoms not listed, so here it is...

Sometimes, usually when outside temperature is cold, it idles very low. So low that it will not stay running. It starts with no problem, but then drops to a very low idle. If the idle is still high enough to keep running, it will start pulsing, with the RPMs climbing as if I tapped the gas pedal, then dropping lower than it was before, then (if it didn't die) climbing slowly back to where it was just to do it again.

If left like this for a while, it may suddenly rev up to around 3000 RPMs for a second or two, and then act completely normal. It may fix itself like this, or it may not, which requires me to drive it with one foot on the brake and one on the gas so that I can keep the idle high enough at stop lights.

Also, if I put fuel injector cleaner in the tank, it will correct the problem until that tank runs out and I refuel. Then the problem comes back. Two different mechanics cleaned the fuel injectors to fix the problem, however it came back within a week or two.

Could it be completely gunked up spark plugs, where the cleaner is knocking just enough gunk off to let it operate normally? And when it fixes itself, it is just knocking build-up off a fouled spark plug? It doesn't seem to be missing, just idling too low, however.

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Try this:
Here is the most common cause of surges and stalls and low idle, it is the idle speed control air-bypass valve and throttle valve, they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Posted on Jan 13, 2009


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Definition 'IAC'?

Idle Air Control usually as in IACS S is for solenoid. don

Jan 04, 2017 | Cars & Trucks

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Inermittently when coming to a stop the car accelerated like wide open throttle with foot on the brake.

Is the check engine light on , or any other lights ?

Chart 5

  • Unique Idle Concerns:
    • Fast Idle
  • Additional Drivability Concerns:
    • Diesels/Runs On
Note: If vehicle runs normally after the ignition key is turned OFF, check for damaged ignition switch, IGN RUN circuit short to power, VPWR circuit short to power, etc. Refer to applicable Wiring Diagram and/or Workshop Manual.
SYSTEM/COMPONENT REFERENCE (Section 5 Pinpoint Test unless noted)
  • Base engine air leaks, including proper sealing of intake manifold and components/vacuum lines attached to intake air (such as the PCV, EGR or IAC valve/vacuum lines).
  • Visual, Engine System - General Information Section 303 of the Workshop Manual.
  • Verify engine operates at normal temperature.
  • Visual (refer to Symptom Index, or Engine Cooling Section 303 of Workshop Manual, to diagnose any cooling system concerns that are present).
  • Fast idle concerns:
  • Key on, engine off, monitor TP MODE PID while wiggling TP sensor circuits. TP MODE PID can also be monitored during vehicle drive. With throttle closed, TP MODE PID must be C/T (closed throttle).
  • TP Mode PID is not C/T with throttle closed: NOTE: At vehicle start, the TPREL will begin at about 1.25 volts, and count down to the lowest TP V value seen since engine start. If the TP V value goes below the "normal" range, then increases again, TPREL will set to the lower voltage. If TP V is about 0.04 volts greater than the TPREL value at closed throttle, the PCM will go into part throttle mode.
  • Monitor TP V and TPREL PIDs for sudden changes while checking for intermittent TP circuit/connector concerns. Also check for loose/worn throttle plates. If no concern is found, GO to Z1 in Section 5.
  • Intake Air System (air leaks)
  • GO to HU1
  • Idle Air Control (IAC) System
  • GO to KE1
  • Additional Testing
  • GO to Z1
Do you know what these thing's are that I have listed above ? Mostly the TP - throttle position sensor . These are known to cause this type of problem . Has it ever been changed before ?

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1 Answer

2005 Dakota will not stay running

Here are some tests you can try. The list is generic and you may have tried some of these already. It sounds like a vacuum line issue to me. For other info try this website:

Repair Guides Symptom Diagnosis no Codes What To Do When There Are No...

Preliminary Checks
Prior to starting this symptom test routine, inspect these underhood items:
  1. All related vacuum lines for proper routing and integrity.
  2. All related electrical connectors and wiring harnesses for faults (Wiggle Test).
  3. Check the throttle linkage for a sticking or binding condition.
  4. Air Intake system for restrictions (air inlet tubes, dirty air filter, etc.).
  5. Search for any technical service bulletins related to this symptom.
  6. Turn the key to off. Unplug the MAP sensor connection and restart the engine to recheck for the idle concern. If the condition is gone, replace the MAP sensor.

Test 2: Rough, Low or High Idle Speed Condition (Continued)
Test 2 Chart
Step Action Yes No 1 Step Description: Verify the rough idle or stall Does the engine have a warm engine rough idle, low idle or high idle condition in P or N- Go to Step 2. Fault is intermittent. Return to the Symptom List and select another fault. 2 Step Description: Verify idle speed & timing Verify the base timing is within specifications Verify that the base idle speed is set properly Are the timing and idle speed set properly- Go to Step 3. Set the base idle speed and timing to the specifications and then retest for the symptom. 3 Step Description: Check AIS / IAC Operation Check the AIS or IAC motor operation Inspect the AIS/IAC housing in throttle body for restricted passages. Clean as needed. Set the parking brake, block the drive wheels and turn the A/C off. Install the Scan Tool. IAC Motor Tester - Turn the key off and then connect the IAC tester to the IAC valve. Start the engine and use the IAC tester to extend and retract the IAC valve. ATM Test - Start the engine. Use the tool to change the speed from min-idle to 1500 rpm. Did the idle speed change as commanded- Install an Aftermarket Noid light and check the operation of the PCM and AIS or IAC motor circuits. Check the motor for signs of open or shorted circuits. Replace the IAC motor or PCM as needed or make repairs to the IAC motor wiring. If all are okay, go to Step 4. If the AIS/IAC motor passages are clean and engine speed did not change as described when the AIS/IAC motor was extended and retracted, replace the AIS/IAC motor. Then retest for the condition. 4 Step Description: Check/compare PID values Connect Scan Tool & turn off all accessories. Start the engine and allow it to fully warmup. Monitor all related PIDs on the Scan Tool. Verify the P/N switch input in gear and Park. Check the O2S operation with a Lab Scope. Are all PIDs within normal range- Go to Step 5. Note: An IAC motor count of over 80 indicates the pintle is extended and an IAC count of (0) indicates the pintle is retracted. One or more of the PIDs are out of range when compared to "known good" values. Make repairs to the system that is out of range, then retest for the symptom. 5 Step Description: Check the Ignition System Inspect the coils for signs of spark leakage at coil towers or primary connections. Check the spark output with a spark tester. Test Ignition system with an engine analyzer. Were any faults found in the Ignition system- Make repairs as needed Go to Step 6. 6 Step Description: Check the Fuel System Inspect the Fuel delivery system for leaks. Test the fuel pressure, quality and volume. Test the operation of the pressure regulator. Were any faults found in the Fuel system- Make repairs as needed Go to Step 7. 7 Step Description: Check the Exhaust System Check Exhaust system for leaks or damage. Check the Exhaust system for a restriction using the Vacuum or Pressure Gauge Test (e.g., exhaust backpressure reading should not exceed 1.5 psi at cruise speeds). Were any faults found in Exhaust System- Make repairs to the Exhaust system. Then retest the symptom. Go to Step 8. Repair Guides Symptom Diagnosis no Codes What To Do When There Are No...

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Troubleshooting p0506

p0506 is idle control system low-- probable causes. = If there are no other codes, inspect for vacuum leaks, restrictions, and damage. If there are no symptoms other than the DTC itself, just clear the code and see whether it returns.
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