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Miss fire #3 cyld. compression 125psi idle speed 2500 rpm

New plugs wires ,believe faulty injector. idle very high,code po507, maybe bad iac motor.can you help?

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  • Anonymous Mar 25, 2014

    engine miss fireing, only 4200 rpm, dtc p2, injector replace, fuel pump replace.ecm replace all wire ok but same ..........



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A faulty IAC would cause a random misfire rather than a misfire in a specific cylinder, as would an intake air leak, both of which would cause a high idle amongst other things.

Run the engine until its good and hot, disconnect the injector harness plug so the injectors are disabled, remove all the spark plugs, disable the ignition by unplugging the wiring connector(s) from the coil pack(s), and then carry out another compression test on al the cylinders.

If the compressions are now all relatively the same then you very highly likely have a faulty injector in cylinder 3.

Very best regards


Posted on Jun 28, 2008

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Were the plugs you took out all the same? if not I think you're right.
If the idle is smooth (ish) fuel is getting burnt. But with a faulty injector - too much in that cylinder. You may be able to pull it and test it by cranking over without power to the plugs, but this will just show it working. It must work between limits to be effective - which can only be done on a testing bench.

You could try refitting a 2nd hand one - or a set to see any difference, or get them tested and reconditioned to be sure.

Posted on Jun 28, 2008


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Extracted from

Code P0507

Possible causes

- Dirty throttle body

- Electric throttle control actuator misadjusted or damaged

- Faulty Electric throttle control actuator

- Intake air leak

- Poor electrical connection to the Intake Air Control (IAC) valve

help.jpg What does this mean?

Tech notes

If there are no intake leaks, clean throttle body, adjust engine idle and erase engine code. If the code comes back you may have to replaced the Electric Throttle Control Actuator (Motor) to repair the problem.

There is a factory service bulletin for the following Chevrolet models:

2006-2008 Chevrolet Trail Blazer

2007-2008 Chevrolet Avalanche

2007-2008 Chevrolet Express

2007-2008 Chevrolet Silverado

2007-2008 Chevrolet Suburban

2007-2008 Chevrolet Tahoe

Chevrolet Factory Service Bulletin OBDII Code P0507

What does this mean?

When is the code detected?

Thi P0507 code sets when the Powertrain Control Module (PCM) detects an engine speed outside of the Idle Air Control (IAC) valve range of control.

Possible symptoms

- Engine Light ON (or Service Engine Soon Warning Light)

- Engine hard to start

- Rough or erratic idle speed

P0507 Chevrolet Description

The Idle Air Control (IAC) valve is a Powertrain Control Module (PCM) controlled stepper motor located on the throttle body. The stepper motor drives a valve pintle which protrudes into a passage that bypasses the throttle plates. The PCM commands the IAC valve pintle to extend to decrease the idle speed. The bypass air flow is reduced and the idle speed decreases as the pintle approaches its seat. The PCM retracts the IAC valve pintle away from its seat to increase the idle speed. The retracting of the IAC valve pintle allows more air to bypass the throttle plates. One of the PCMs uses for the IAC system is to maintain a desired idle speed. This DTC sets when the PCM detects an engine speed outside of the IAC\'s range of control.

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