Question about 1989 Chevrolet Corvette
Slimsoul, a bad fuel pressure regulator would cause a problem similar to this. You may already know this but a fuel pressure regulator kinda works backwards from the way you would think. When your car is at idle it produces the most amount of vacuum which power things like brakes. As your RPM's increase your vacuum drops. When a fuel pressure regulator recieves 0 vacuum it fully opens and allows the most amount fuel to flow through. So, you mash on the gas from idle and the car stumbles from lack of fuel. A common cause of this is because the regulator diaphram has a hole in it, which causes it to open slowly. There is a very simple test for this that, yes, even you can try at home. I'm assuming your corvette is a C4 (84-96). Open the hood, stand on the passengers side, and stare towards the back of the plenum just in front of the distributor. Now look underneath the upper plenum, and attatched to the lower manifold at an approximate 45 deg angle is a round piece that kinda resembles a top hat. This is your regulator. There should be a hard black plastic vacuum line attatched to the regulator. Now for the tricky part, unplug it and crank the car. If the regulator is in fact the problem, this should at least improve if not eliminate your problem. If it does not, my next step would be to check your fuel pressure with a fuel pressure gauge available at Autozone for around $40. A TPI motor needs at least 40 psi to operate properly. Back to the regulator, if it is in fact the problem your going to have to replace the diaphram in the regulator which is somewhat of a pain. You have to pull off the throttle body and upper plenum to get to it. Additionally the FPR is secured with special security torx bits, I believe its a T5. The plenum runner gaskets and throttle body gaskets will need to be replaced if they're original. If you're not somewhat handy I would suggest taking it to a shop to do the job.
Posted on Oct 27, 2008
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The Idle Air Control (IAC) is a bi-directional motor driven by two coils. The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in the engine load. The IAC valve, mounted in the throttle body, controls bypass air around the throttle plate. The PCM controls engine idle speed by adjusting the position of the IAC motor pintle. The PCM pulses current to the IAC coils in small steps (counts) to extend the pintle into a passage in the throttle body to decrease air flow. The PCM reverses the current pulses to retract the pintle, increasing air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load. If RPM is too low, the PCM will retract the IAC pintle, resulting in more air being bypassed around the throttle plate to increase RPM. If the RPM is too high, the PCM will extend the IAC pintle, allowing less air to be bypassed around the throttle plate, decreasing RPM.
Fig. Typical Idle Air Control (IAC) valve components
During idle, the proper position of the of the IAC pintle is calculated by the PCM based on the battery voltage, coolant temperature, engine load and engine RPM. If the RPM drops below a specified value, and the throttle plate is closed (TP sensor voltage is between 0.20-0.74), the PCM senses a near stall condition. The PCM will then calculate a new IAC pintle position to prevent stalls.
If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset. The IAC resets when the ignition switch is cycled ON and then OFF . When servicing the IAC, it should be disconnected or connected with the ignition switch OFF in order to keep from having to reset the IAC valve.
The position of the IAC pintle affects engine start up and the idle characteristics of the vehicle. If the IAC pintle is fully open, too much air will be allowed into the manifold. This results in high idle speed, along with possible hard starting and a lean air/fuel ratio. A Diagnostic Trouble Code (DTC) may set. If the IAC pintle is stuck closed, too little air will be allowed in the manifold. This results in a low idle speed, along with hard starting and a rich air/fuel ratio. Again, a Diagnostic Trouble Code (DTC) may set. If the IAC pintle is stuck part way open, the idle may be high or low and will not respond to changes in engine load.
As with most all engine control sensors used in your vehicle, a thorough and proper test can only be performed by a qualified technician using a scan tool to read the data stream from the PCM. There are a few items a non-professional should check before taking the vehicle to a qualified technician for diagnosis and repair.
Check for a poor connection at the PCM or IAC motor. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connection. Inspect for a damaged wiring harness, especially if heavy work has recently been performed (intake manifold removal, engine removal, etc.) where the throttle body has been disturbed. Inspect for a restricted air intake system. Check for a possible collapsed air intake duct, restricted air filter element or foreign objects blocking the air intake system. Inspect the throttle body. Check for objects blocking the IAC passage or throttle bore. Excessive deposits, especially on high-mileage engines, can build up in the IAC passage and on the IAC pintle. Excessive deposits can also build up on the throttle plate, so check for a sticking throttle plate. Check for vacuum leaks such as disconnected hoses, leaks at the EGR valve and EGR pipe to the intake manifold, leaks at the throttle body, faulty or incorrectly installed PCV valve, leaks at the intake manifold brake booster hose connection, etc.
REMOVAL & INSTALLATION
Fig. The Idle Air Control (IAC) valve is mounted on the throttle body-3.8L (VIN K) engine shown, others similar
NOTE Before installing a new IAC valve, measure the distance that the valve is extended. The measurement should be made from the motor housing to the end of the valve cone (pintle). The distance should be no greater than 1 1 / 8 inch (28mm). If the pintle is extended too far, adjustment is required, otherwise damage may occur when the IAC valve is installed. Adjust the IAC valve by compressing the pintle to achieve the correct length.
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