Truck stumples at idle
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Truck stumples at idle
Hey everyone. So today I was driving my truck on the freeway for about an hour. When I got off the exit and waited for the light to change, my truck started to shudder just a tad and it felt like it wanted to turn off. Normally I would just let it do whatever it is that it's gonna do so I can see whats gonna happen, but I had some where to be ASAP so I stepped on the brake and gas so it wouldn't die on me. It only happened a couple times then it was fine. I cleaned the throttle body and the IAC carefully and put everything back together about one month ago. Aside from a bad IAC, what would cause that symptom? And how would I go about testing my IAC sensor? I'll be away from my house for a couple days. Otherwise I'd have checked my hayes manuel. Any input is appreciated! Thanks!
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From: Sioux Falls, SD
+1, I'd check the TPS.
From the Haynes manual for IAC:
Idle Air Control (lAC) valve
General description
89 The idle speed is controlled by the lAC valve (see illustrations 2.15a and 2.15b). This valve changes the amount of air that will bypass into the intake manifold. The lAC valve is controlled by the PCM and is opened and closed depending upon the running con*ditions of the engine (air conditioning system, power steering, cold and warm running etc.).
Check
90 It is recommended that a special lAC "exerciser" tool be installed in series between the lAC valve and the harness electrical connector for testing purposes. If not available, there are several tests the home mechanic can perform on the lAC system to verify operation but they are limited and are useful only in the case of definite problems rather than intermittent failure.
91 Disconnect the electrical connector from the lAC valve and listen carefully for a change in the idle. Connect the lAC valve electrical connector and turn the air conditioning on and listen for a change in idle rpm. When the engine is cold, the lAC valve should vary the idle as the engine begins to warm-up and also when the air conditioning compressor is turned ON. If there are no obvious signs that the lAC valve is working, continue testing.
92 Use a voltmeter and test for voltage to the lAC valve with the ignition key ON (engine not running). Backprobe the lAC valve electrical connector and check for battery voltage on the purple/black wire (+) and the brown/white wire (-) (see illustration). Note: Voltage should also be present when the engine is running but monitoring the voltage changes as the engine rpm fluctuates is difficult without a factory designed scan tool.
93 If there is no voltage present, have the electrical circuit for the lAC valve diagnosed by a dealer service department or other qualified repair shop.
94 If the computer is delivering voltage to the lAC valve, check that the lAC valve is not frozen or defective. Remove the lAC valve and leaving the lAC valve connected, turn the ignition key ON (engine not running). The pintie should retract (pull in) (see illustration).
From the Haynes manual for IAC:
Idle Air Control (lAC) valve
General description
89 The idle speed is controlled by the lAC valve (see illustrations 2.15a and 2.15b). This valve changes the amount of air that will bypass into the intake manifold. The lAC valve is controlled by the PCM and is opened and closed depending upon the running con*ditions of the engine (air conditioning system, power steering, cold and warm running etc.).
Check
90 It is recommended that a special lAC "exerciser" tool be installed in series between the lAC valve and the harness electrical connector for testing purposes. If not available, there are several tests the home mechanic can perform on the lAC system to verify operation but they are limited and are useful only in the case of definite problems rather than intermittent failure.
91 Disconnect the electrical connector from the lAC valve and listen carefully for a change in the idle. Connect the lAC valve electrical connector and turn the air conditioning on and listen for a change in idle rpm. When the engine is cold, the lAC valve should vary the idle as the engine begins to warm-up and also when the air conditioning compressor is turned ON. If there are no obvious signs that the lAC valve is working, continue testing.
92 Use a voltmeter and test for voltage to the lAC valve with the ignition key ON (engine not running). Backprobe the lAC valve electrical connector and check for battery voltage on the purple/black wire (+) and the brown/white wire (-) (see illustration). Note: Voltage should also be present when the engine is running but monitoring the voltage changes as the engine rpm fluctuates is difficult without a factory designed scan tool.
93 If there is no voltage present, have the electrical circuit for the lAC valve diagnosed by a dealer service department or other qualified repair shop.
94 If the computer is delivering voltage to the lAC valve, check that the lAC valve is not frozen or defective. Remove the lAC valve and leaving the lAC valve connected, turn the ignition key ON (engine not running). The pintie should retract (pull in) (see illustration).
Last edited by jasonw; May 22, 2009 at 03:23 AM.
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From: Sioux Falls, SD
NOTE: On part 94 there, I found that to be incorrect for at least 2001 model years. I have two perfectly good IACs, one brand new and the other 68k miles, and neither moved with key turned to on position. Both work perfectly when engine is running.
Here is the last part from the Haynes manual (wouldn't fit into above post):
95 There is an alternate method for testing the lAC valve. A scan tool is available from some automotive parts stores and specialty tool companies that can be plugged into the ALDL for the purpose of monitoring the computer and the sensors. Install the scan tool and switch to the lAC position mode and monitor the steps (valve winding position) (see illustration). The scan tool should indicate between 10 to 200 steps depending upon the rpm range. Allow the engine to idle for several minutes and while observing the count reading, snap the throttle to achieve high rpm (under 3,500). Repeat the procedure several times and observe the scan tool steps when the engine returns to idle. The readings should be within 5 to 10 steps each time. If the readings fluctuate greatly, replace the lAC valve. Note: When the lAC valve electrical connector is disconnected for testing, the PCM will have to "relearn" its idle mode. In other words, it will take a certain amount of time before the idle valve resets for the correct idle speed. Make sure the idle is smooth before plugging in the scan tool.
Here is the last part from the Haynes manual (wouldn't fit into above post):
95 There is an alternate method for testing the lAC valve. A scan tool is available from some automotive parts stores and specialty tool companies that can be plugged into the ALDL for the purpose of monitoring the computer and the sensors. Install the scan tool and switch to the lAC position mode and monitor the steps (valve winding position) (see illustration). The scan tool should indicate between 10 to 200 steps depending upon the rpm range. Allow the engine to idle for several minutes and while observing the count reading, snap the throttle to achieve high rpm (under 3,500). Repeat the procedure several times and observe the scan tool steps when the engine returns to idle. The readings should be within 5 to 10 steps each time. If the readings fluctuate greatly, replace the lAC valve. Note: When the lAC valve electrical connector is disconnected for testing, the PCM will have to "relearn" its idle mode. In other words, it will take a certain amount of time before the idle valve resets for the correct idle speed. Make sure the idle is smooth before plugging in the scan tool.
Last edited by jasonw; May 22, 2009 at 03:24 AM.
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Champion
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From: Bay Area, CA
Answering questions this early in the morning?! You guys are all star pros!!! thanks fellas! I got a couple of graduations to go to this weekend but I'll be sure to check that out. I was just checking out the DIY section. Does the TPS fix work? You know, cleaning it?
Last edited by J415; May 22, 2009 at 03:34 AM.
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From: Sioux Falls, SD
As far as the cleaning fixing it, most people that I've seen do that it either worked temporarily for a few weeks or months and then completely quit on them, or it didn't work at all.
From Haynes manual for TPS:
Check
48 Locate the Throttle Position Sensor (TPS) on the throttle body. Disconnect the TPS electrical connector and using a voltmeter, check the reference voltage from the PCM. Install the positive (+) probe onto the purple/white wire terminal and the negative (-) probe onto the black/blue wire terminal. The voltage should read about 5.0 volts.
49 Next, install the electrical connector onto the TPS and check the TPS signal voltage by backprobing the electrical connector with straight pins. With the throttle fully closed, connect the positive probe of the voltmeter onto the orange/dark blue wire terminal and the negative probe onto the black/light blue terminal. Gradually open the throttle valve and observe the TPS sensor voltage. With throttle closed, it should read about 0.5 to 1.5 volts. As you slowly open the throttle valve, you should observe a change in voltage as the sensor travels from "idle" to "full throttle". The maximum the voltage should reach at fully open is 3.5 to 4.5 volts. If readings are incorrect, replace the TPS.
From Haynes manual for TPS:
Check
48 Locate the Throttle Position Sensor (TPS) on the throttle body. Disconnect the TPS electrical connector and using a voltmeter, check the reference voltage from the PCM. Install the positive (+) probe onto the purple/white wire terminal and the negative (-) probe onto the black/blue wire terminal. The voltage should read about 5.0 volts.
49 Next, install the electrical connector onto the TPS and check the TPS signal voltage by backprobing the electrical connector with straight pins. With the throttle fully closed, connect the positive probe of the voltmeter onto the orange/dark blue wire terminal and the negative probe onto the black/light blue terminal. Gradually open the throttle valve and observe the TPS sensor voltage. With throttle closed, it should read about 0.5 to 1.5 volts. As you slowly open the throttle valve, you should observe a change in voltage as the sensor travels from "idle" to "full throttle". The maximum the voltage should reach at fully open is 3.5 to 4.5 volts. If readings are incorrect, replace the TPS.
Site Moderator
Joined: Jan 2008
Posts: 8,374
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From: Sioux Falls, SD
I know when I disconnected my TPS out of curiosity the other day, it caused major hesitation initially and responded sluggishly to throttle. I'd clean the connectors first though and reseat them as well (unplug, clean, replug). If its intermittent, it might just be a loose electrical connection.
