Jeff's DIY

Advice on doing your own diagnosis and repair


Advice from the book Can I Do It Myself?

How to Fix Poor Idle, High Idle, Stalling On Fuel Injected Gas Engines

This tip provides some generic troubleshooting and repair tips for a fuel injected gas engine that stalls or hunts at idle.

Often this condition does not result in the computer's detection of a malfunction. However, if your engine light is on, I always recommend getting a free scan at your local parts chain to help diagnose the problem.

A low idle or stalling engine can be caused by a number of things. First of all, make sure your vehicle has good spark plugs, ignition wires, and distributor/rotor (if equipped). Other possible causes include the following:
1. Vacuum leaks either in vacuum hoses or intake manifold gaskets.
2. Rich mixture caused by torn fuel pressure regulator diaphragm or failing MAP sensor (skip the check if your mixture is not rich)
3. Insufficient idle air flow due to clogged passages or failing idle air control valve/motor
4. Excess EGR flow, caused by a stuck open EGR valve
5. Excess EVAP system flow due to a stuck purge valve

A hunting idle is usually caused by a blocked PCV valve.
A high idle is generally due to a failed idle air control valve. If this is your symptom, first check to make sure the throttle cable has a little slack, is correctly attached to the throttle, and the throttle is able to close fully. See pictures below:

typical throttle cable

 

throttle body assembly

 

If the throttle is closing well but the idle is high, skip ahead to the section on IAC.

For a low idle, the above list of 5 checks are in order of difficulty and likelihood to be the cause, so this is a good order in which to perform checks, unless you have additional knowledge that makes something lower on the list more likely to be the cause. I will go thru each item for a generic engine in the order 1-5.

1. Vacuum leaks cause the engine to run lean and draw too much air for an idle setting of the throttle. Vacuum leaks can also cause an automatic transmission to shift late and can sometimes cause a vacuum powered heater mode control switch to fail, leaving your ducts in a default flow mode. An engine that will not idle due to a vacuum leak generally has a pretty good leak that you can often hear by walking around the engine. Check the big hoses first, like the one to the brake booster. Then check every other hose connected to the engine intake manifold. Check both ends of every hose for a poor connection and repair or replace any leaking hoses. Although rare, a leaky intake manifold gasket can also cause an engine to stall, but usually a leaky gasket will set a trouble code P0171 and/or P0174. To check for leaky gaskets, start the car and spray carb cleaner at the intake manifold gaskets. If the engine revs, you have a leak. If you have an upper as well as a lower intake manifold, check both sets of gaskets.

2. Torn fuel pressure regulator diaphragm: If you know your car is not running rich or you do not have a vacuum powered regulator, you can skip this step.

Many fuel injected engines use a vacuum actuated fuel pressure regulator on or near the injection rail or inside the throttle body (in the case of throttle body injected engines). The vacuum reduces fuel pressure when the engine is not in high fuel demand. However, since the intake manifold is the source of vacuum for these type regulators, when the diaphragm tears, it allows fuel to be sucked through the regulator directly into the intake manifold, thus causing a rich fuel/air ratio. The picture below shows a regulator spitting gas from the vacuum port due to a torn diaphragm.

Faulty fuel pressure regulator

 

To check a regulator on the rail, pull the vacuum hose off the regulator and plug the hose with a golf tee. Start the engine and watch for fuel to come out of the regulator vacuum port. If fuel comes out, replace the regulator.

For throttle body injected (TBI) engines, the regulator is usually integrated into the throttle body. The regulator may be on the side or actually under the flat filter interface, and the cover may be domed or flat and may or may not have an external vacuum port. Typical TBI fuel pressure regulators are shown below along with a diaphragm:

TBI pressure regulator

TBI pressure regulator

 

Diaphragm:

Regulator diaphragm

 

Remove the 4 screws and the diaphragm will be exposed. Note the orientation/appearance of the diaphragm so that you get it back in right after inspecting or replacing it. Inspect for tears and replace any suspect diaphragm.

3. Idle Air Control (IAC) Valve: Just as controlling idle speed is difficult with carbureted engines, it is also difficult with fuel injected engines. Because the engine is rotating slowly, the fuel/air ratio and flow must be carefully controlled to keep the engine running even as there are changes in temperature and load (such as the air conditioning compressor turning on and off). Fuel injected engines use a number of sensors as input and the engine computer adjusts both injector pulse and idle air flow. The flow at idle is usually controlled by the idle air control valve, which is a cone-shaped end on a stepper motor-driven screw. Much like a common garden spigot, the cone blocks or allows flow according to the automated screw adjustment. The flow paths and valve tip can become dirty over time and limit the system's ability to control the airflow as precisely as necessary. The motor can also blow a winding or wear out such that it does not step as commanded by the computer. A quick test to verify the operation of the valve is to start the engine, observe the idle speed, and pull off a vacuum hose (like the one leading to the brake booster). The idle speed should rise, and then fall as the IAC motor tries to compensate for the vacuum leak. Next, reattach the vacuum hose. The idle should drop, and then stabilize. If the engine reacted as indicated, the circuit is probably OK.

Remove the valve and clean the passages and valve with carb cleaner or a similar solvent. Do not submerge the motor in solvent. The valve may be sealed by a gasket or o-ring or both. Be sure it seals properly when reinstalled. If your valve did not react properly during the crude test, check the continuity across the stator and armature windings of the motor. If either winding shows an open or high resistance, replace the valve. Also, if the valve is loose in the motor, the bearings are probably worn out. Remember, this is a precise valve and must hold close tolerances. Some typical installations are shown below along with an IAC valve.

Idle air control valve

 

EGR valve

New idle air control valve

 

4. EGR Valve: The exhaust gas recirculation system is required by many engines to meet national or California emission standards. When the computer deems the conditions appropriate, it opens the EGR valve and controls the flow of exhaust gas back into the intake manifold to provide inert mass that absorbs the heat of combustion, thereby reducing NOx products. There are many variations of EGR systems, and the valves may be controlled by vacuum, electric motor, or electric solenoid. Many systems include a sensor to feedback either flow or valve position to the computer. Most computers do not open or extremely limit the EGR valve opening during idle because of the difficulty in controlling idle mixture. The default position of the valve is closed. However, if the valve has not closed properly from off-idle operation, the EGR flow can cause a rough or poor idle. If necessary, use a manual for your vehicle to obtain a procedure for removing, cleaning and testing your EGR valve. At a minimum, make sure the valve is able to close with no control forces applied. Some typical EGR valves are shown below. The valve is usually mounted in the intake manifold and exhaust gasses are either routed through the heads to the intake manifold exhaust passage or they are piped to the valve from the exhaust manifold.

 

EGR valve attachment

 

EGR valve attachment

 

5. EVAP Purge Valve: The evaporative emission system is designed to limit the release of fuel vapors from the fuel tank. Up to a certain design pressure, vapors are stored in a charcoal canister and are later drawn into the engine to be burned. The computer determines when conditions are appropriate for vapors to be allowed into the engine and opens the purge valve with precise solenoid pulses. As with the EGR valve, the computer stops or severely limits the flow during idle. However, the valve may become stuck, inoperative, or clogged with charcoal bits (if the canister is leaking its charcoal). This valve may be difficult to find, though it is usually mounted on the intake manifold, throttle body, along the air inlet hose or on the charcoal canister itself. If you aren't sure you have found it, trace the hose back to find the canister. Remove the valve, clean it with carb cleaner, and check that it closes on its own. If you have a manual that provides tests for the valve, run the tests to verify proper operation. Some typical installations are shown below.

Typical evap purge valve

evap purge valve mounted to canister

 

If you have performed all of these checks and steps and your car still idles poorly, consider reviewing my tips on misfires.



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