Misfire at idle

[packardman]

Hey everyone,

I have a '99 Infinity Q45 with 98,000 miles in prestine condition. The car ran flawlessly until about 80,000 miles when I decided to change the spark plugs. Twenty years ago I was an ASE certified line mechanic in a large dealership for 10 years. So, changing plugs should have been within my ability. It was. However, I did not use the original brand of plugs, but a cheaper plug. Well, the car would barely run when driving it. I realized immediately my mistake and that I was going to have to buy the $9.95/ea. plugs. I did and replaced them immediately.

Since then it has a minor, intermintent miss at idle only. Sometimes I can go days, even weeks without it. Then it returns. It does occassionally pop up a code -- O2 sensor, Bank 1, Sensor 2 (sometime bank 1 and bank 2, sensor 2). I know that the O2 sensor is not going to cause this kind of miss.

The strange thing is that the car runs flawlessly at every other speed and load. Normally, a miss like this is caused by a secondary ignition problem (coil, dist. cap, wires, etc.) and under a load, not at idle.

I'm confident it is a secondary miss, but don't know how to test the coils, which is all there is to the secondary ignition on this car. So, if you know how to test the coils so I can find the bad one, I would appreciate it. I have tried using an ohm meter across the terminals, but I get readings I don't understand.

Any help will be appreciated!

ThanksPackardman

[Q45tech]

The ecu uses the O2 sensors to guess at a misfire [rich when it should be lean] in the 20 degrees after that exhaust valve opens.

An idle misfire may be a partial burn so things get confusing.

The coils [actually the jumper from coil to plug] are not as robust as the 90-95 units and seem to fail when the plugs are changed............if you are not delicate in your removal.

Measuring the resistance thru the jumper and the insulation resistance looking for a punch thru in dielectric might help.

Just using the wrong plug might damage the insulation........theory?

Years ago we would connect a Sun Machine to the plug and examine the secondary waveform.............almost impossible to day..........you might build a high voltage extension and cabling to bring the coil out of the well.

We usually solve with injector cleaning [BG44k] and finding the weak coil by looking at the individual O2 waveforms synced to timing.

I spent 8 hours once deciding on which coil was worse than the others.

Remember how very very lean warm idle really is so things must be kept perfect!!!!!!!!!

Most dealer techs just change all 8 coils as few customers will authorize 8 hours of diagnosis. Especially since all the diagnosis may weaken others and sequential failures forever.

Buy one coil and swap then around take good notes!

[Q45tech]

A power balance test runs at 1300-1500 rpms with locked IAC and advance so it doesn't really show true idle..................I have done manual tests loaded................AC on and in gear [brakes on] to try a lower leaner rpm...............some success?

[Q451990]

The coils [actually the jumper from coil to plug] are not as robust as the 90-95 units and seem to fail when the plugs are changed............if you are not delicate in your removal.

Are the coil boots available separately for the Y33, like on the G50? That might be an inexpensive place to start.

Heath

[maxnix]

A misfire will throw an OBD II code. Switch around coil packs one at a time once you have identified the offending cylinder to determine if it is the coilpack or something else. Good thing you returned to the PFRG plugs.