Diagnostic Dilemmas: Snow-Plowing Chevrolet
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Diagnostic Dilemmas: From Here To There In A Snow-Plowing Chevrolet

In this installment of Diagnostic Dilemmas, we’re diagnosing idle air control problems by pursuing a no-code, cold-idle speed control condition on a 2002 Chevrolet 2500HD four-wheel drive pickup. Owned by a small local construction company, this U-code, 6.0-liter Chevrolet hauler performs admirably during the summer towing trailers and hauling heavy construction tools in its roomy, all-steel utility bed. During the winter, it plows snow in local commercial parking lots and on private mountain roads. Due to their simplicity and reliability, we still see thousands of these formidable workhorses performing their duties across our great country.

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Photo 1: This 2002 Chevy 2500HD earns its living by plowing snow in the winter and hauling heavy loads in the summer.

This current installment of Diagnostic Dilemmas might sound suspiciously like my February installment in which I pursued a no-code, hot idle speed control condition on a 2003 Ford Explorer. This month, we’re doing a different take on diagnosing idle air control problems by pursuing a no-code, cold-idle speed control condition on a 2002 Chevrolet 2500HD four-wheel drive pickup.

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Owned by a small local construction company, this U-code, 6.0-liter Chevrolet hauler performs admirably during the summer towing trailers and hauling heavy construction tools in its roomy, all-steel utility bed. During the winter, it plows snow in local commercial parking lots and on private mountain roads. Due to their simplicity and reliability, we still see thousands of these formidable workhorses performing their duties across our great country (see photo 1).

And therein lies the tale of a 2500HD commercial-specification Chevy truck with wind-up windows, manual 5-speed transmission, and manual two-speed transfer case with a cold-weather, cranking/no-start and engine stalling problem. Not something found on a typical dealership lot, this heavy-duty Chevy is missing a number of on-board modules normally found on consumer-grade trucks. That said, diagnostics on a commercial-spec vehicle aren’t always a case of just following the numbers.

NO-CODE COMMON SENSE
I’ve had worn-out GM fuel pumps not activate during cold weather due to low battery voltage. A shot of starting fluid usually spins the engine and alternator fast enough to activate the worn-out fuel pump. Once the engine is warmed up, the fuel pump will run the rest of the day. This is a rare condition, but it prompts me to always test key-on fuel pressure before cranking the engine on a cold winter morning.

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Although this 2500HD had a recent battery replacement, I needed to eliminate the possibility of a low-voltage cranking problem. To do this, I correlate the voltmeter reading on the instrument cluster with the key-on, engine off (KOEO) voltage I see on my scan tool data stream. Next, I connect a voltmeter to the battery terminals. KOEO, the difference between readings from the digital meter, the cluster meter, and the data stream should not exceed .5 volts. If the data stream reading doesn’t coincide with the other readings, the PCM could have a data processing problem which requires further testing and possible replacement. During cranking, the battery voltage remained above 10.0 volts, which brings me to my next no-code diagnostic step.

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NO-CODE CRITERIA: Looking at enable criteria, the basic IAC codes for this application are P0506 (idle speed too low) and P0507 (idle speed too high). Both require two consecutive ignition cycles to produce two identical failures. Intake air temperatures must be above 14º F and coolant temperatures must fall between 140º to 241º F before either code will set. Barometric pressure must be at least 65 kpa (19”Hg). The vehicle speed sensor (VSS) must also indicate less than one mph before a code will set. Given that the truck probably wasn’t meeting these enabling criteria during start-up on cold December mornings, I could see why the PCM wasn’t setting P0506 DTCs.

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If you’ve followed Diagnostic Dilemmas over the years, you also know that the first step in any modern diagnosis is to poll all modules for communications issues and to record all codes. In this case, I retrieved three U-codes from the body control module (BCM): U1255 (class 2 fault), U1016 (lost class 2 communications with PCM), and U1064 (loss of class 2 with BCM/DIM/SDM). While the U-codes aren’t relevant to the idle speed control problem, they need to be noted on the repair order because they can be indicative of a worn ignition switch or bad battery. The U1XXX codes did not reset after clearing.

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NO-START, NO IDLE
A local shop found no powertrain DTCs stored in the 2500HD’s code histories and was therefore reluctant to pursue this rare driveability complaint. The shop also found no leaks when it performed a smoke test on the intake manifold, gaskets and mass air flow (MAF) sensor ducting. The driver became so frustrated with the engine stalling each time his right foot transitioned from throttle to brake to reverse gears, he parked the truck until it was repaired. Unfortunately, the stalling complaint persisted after a new idle air control (IAC) was installed and the throttle body was cleaned, which is why my phone rang the next morning.

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OPERATING STRATEGIES
Most cable-type General Motors throttle systems control idle-speed functions with an IAC valve operated by a two-phase, bi-polar stepper motor. The windings in the motor are pulse-activated, meaning that electrical power is delivered in pulses rather than in a steady state. The motor is geared to the pintle valve, which allows the motor to incrementally move the pintle to or from a seat machined into the throttle body. The stepper motor is highly accurate and can be rotated by the PCM in steps or “counts.”

GM’s method of measuring pintle position is different than other brands. The IAC pintle’s fully bottomed position equals zero counts. With a clean throttle body and no other mechanical problems, the idle air at 625 no-load RPM should be somewhere between 25 and 35 counts at operating temperature. Lower counts usually indicate a vacuum leak, while higher counts usually indicate a reduced base air flow caused by carbon accumulation on the throttle plate and bore.

Photo 2: Idle speed is 417 rpm while commanded idle speed is 350 rpm with the IAC commanded into a closed position. This indicates the IAC pintle valve is completely closed.

When the ignition is turned on, the PCM immediately seats the pintle to a closed position inside the throttle body, which is called minimum authority (see photo 2). This provides a reference point for the PCM to begin counting back to a predetermined open position. At key-on during a cold start, the PCM includes engine coolant and barometric pressure parameters to calculate the initial start-up count. With our 6.0-liter engine, the IAC begins at a fast-idle position of 310 counts at around 50º F. and 22.6” Hg. The IAC quickly reduces the fast-idle speed of about 1,100 rpm to 625 rpm at operating temperature with no accessory load. At part-throttle, the IAC is in the open position. As the vehicle slows to a stop, the IAC gradually closes to prevent engine stalling.

NO INTAKE AIR
let’s go back to the cold cranking, no-start complaint. The company technician said that he could start the engine only by “feathering” the throttle. This description of the problem indicated to me that the IAC pintle valve is stuck closed, allowing only base air to enter the intake manifold. Feathering the throttle to get the exact amount of idle air for starting is pretty “iffy” thing to do because the initial starting mixture will usually be too rich or too lean to initiate combustion. In any case, the engine did feel alternately rich or lean while I was “feathering” the throttle.

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GETTING FROM HERE TO THERE
Since I was the third technician to tackle the cranking, no-start and engine stalling complaint, it’s hard to say how this problem began. I would assume the cranking, no-start problem gradually materialized due to the throttle plate and IAC passages becoming clogged with blow-by residue flowing from the adjacent PCV air inlet. I’m also assuming that the first new IAC, which was installed last winter, didn’t remedy the complaint because it was mechanically defective.

The basic IAC diagnostic procedure consists of entering “functional tests” on the scan tool menu to perform a bi-directional control test on the IAC. As I suspected, the engine speed didn’t respond to the commands from my scan tool to increase or decrease idle speed. Removing the four-wire IAC connector, I measured 10.9 volts on each power wire and .4 bias volts on each ground wire. With the power and ground circuits verified, I knew that the IAC motor was sticking in position due to heavy sludge contamination from the positive crankcase ventilation (PCV) tube exiting inside the throttle body.

Photo 3: This voltage graph indicates that rpm, TPS, MAF, and IAC are responding smoothly and concurrently as the throttle opens.

BASE AIR FLOW
During normal operation, the idle-speed counts will vary due to minor variations in what I call “base air flow,” which is pre-set during the manufacture of the throttle body. Base air flow occurs when the IAC passage is fully closed and idle speed is well below hot idle specification. The throttle plate opening is adjusted with a stop screw located next to the throttle shaft (see photo 3). Since this adjustment had been tampered with, I had to reset base air flow as best I could by first cleaning the throttle plate, throttle bore and IAC by-pass air passages. Next, I adjusted the throttle stop until the throttle position sensor voltage indicated .59 volts. The .59 volts remained constant after snapping the throttle closed several times, so the adjustment was permanent.

Photo 4: The throttle stop screw had been tampered with, so I adjusted it as closely as possible using throttle sensor voltage and IAC counts.

IDLE SPEED COUNTS
Adjusting the base air flow around the throttle plate and replacing the badly fouled IAC resulted in a fast-idle speed of about 1,200 rpm at 32º coolant temperature. After a thorough warm-up, the idle speed reduced to 625 rpm and a hot-idle count of 25 with no accessory load. Following the IAC bidirectional test on my scan tool, I measured 410 rpm with the IAC count at zero and 1,500 rpm as the IAC count increased (see photo 4). A snap-throttle test indicates that the IAC is stepping down incrementally as it should.

The basic principles of idle air control are nearly the same for all engines and they’re not complicated. But to diagnose any no-code driveability condition, you have to understand how the system operates before you can find the failed part.

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