Hey guys,

My car vibrates at idle, especially when the AC is running. The vibrations go away when I press the gas, but when I am sitting a light I get this vibration that is not terribly rough but is rapid. Please let me know if you guys have any ideas what it could be. I took it to a shop and they could not find them problem, they said that the sparkplugs are fine, but the Infinity dealership told me the sparkplug coils are bad ( a couple of them), could that cause it? Please let me know.

Eugene

 

No, that would set off a check engine light. But get your fuel injectors checked.

 

Faulty ignition coils will certainly cause a rough idle, but they will usually turn on the Malfunction Indicator Lamp and set a Diagnostic Trouble Code in the range 0603-0608. You do not report the MIL.

4Gen engines are equipped with two fast idle control solenoids which maintain a constant idle speed despite fluctuating engine loads. One solenoid responds to power steering loads and the other to air conditioning loads. When either solenoid is energized additional air enters the intake manifold to raise the idle speed. A fault in this system will not be detected by the OBD system.

These solenoids are mounted on the rear face of the upper intake manifold, flanking the Idle Air Control Valve, just downstream of the Throttle Body. Please see the Haynes repair manual (page 6-18) for Check and Replacement instructions.

 

Your rough idle could be caused by a sticking fuel injector. There are two tests the home mechanic can make, the sound test and the resistance test.

Sound test...
This test is performed with the engine idling. Use a mechanic's
stethoscope or a length of rubber vacuum hose, and listen to each injector.
All injectors should sound alike. If you find one which makes a different
sound (or no sound at all) you have found a problem.

Resistance test...
This test is performed with the engine off. Use an ohmmeter to measure the
resistance of each injector. This does not require removing the injectors.
I don't know the correct resistance value for the injectors on your model,
but they are typically a low number such as 16 ohms. The important thing
is they should all be equal. If you find one injector with substantially
higher resistance than the others, it is bad. If you find one with zero
ohms (short circuit) that is truly unfortunate, because the injector is bad
and it may also have damaged the Engine Control Module (the computer)
because of excessive current drain.

Measuring the resistance of the front bank of cylinders is easy because the
injectors are in plain view. Disconnect the injector electrical connector
for cylinder #2, measure the resistance, reconnect the connector. Repeat
for the cylinders #4 and #6.

Measuring the resistance of the rear bank of cylinders is almost as easy,
but it is difficult to reach the injectors. The resistance may be measured
at a conveniently located electrical connector. Notice the largest,
thickest electrical harness at the top of the engine. This is the Engine
Control Harness and it is shaped like a U, with the open top of the U at the
driver's side of the car. The U has two corners. Look at the corner
nearest the passenger seat. Just inside that corner you will find an 8-pin
electrical connector. This is connector F131. Disconnect this
connector. Now look at the male part, the connector half with the pins
exposed. They are arranged in two rows of four pins.
The pins are numbered 1 - 4 (top row) and 5 - 8 (bottom row).
Measure the resistance of:
- injector #1 between pins 1 and 2.
- injector #3 between pins 1 and 6.
- injector #5 between pins 1 and 5.
Be sure to measure the PINS, not the female receptacles.

The dealer's shop is equipped with high-tech diagnostic instruments. These are wonderful devices but they are expensive and the dealer has to recover his cost by charging you for diagnostic time. Sometimes the home mechanic can do legitimate diagnostic work with nothing more than a ohmmeter and a rubber tube.

 

What year is your I30? I have an I30 and have a recurring rough idle. The rough idle in my car usually smooths out with the ac on, so its most likely not the same situation as I am in. Dealer's first response was to put the base idle up to 700rpms, and then later, they adjusted the target idle on the ecu. when i had my car in for the 30K service (including injector clean), the roughness went away and is usually kept away when the weather is nice and warm (once its cold, car runs like crap).