JeffAhn

~the first 100degreeF day of the season (~week ago) I started getting a really bad engine vibration on throttle.

This happens only when the car is fully warmed up, and the vibration turns into violent shaking as it's driven more, like a cylinder is off timing or some sort.

This doesn't occur on idle; only when on the throttle (even the slightest bit).

Now, I'm a 1.8QTip with 54xxx miles. I changed the plugs to NGK coppers, and only fill up with 93, so that most likely rules out the plugs and bad gas.

I had to get it towed to the dealer and they changed the MAF sensor and a check valve. Went to pick the car up today and the same symptoms remain (I know, what a fantastic job huh?)

I'm focusing more on getting this issue resolved, than dealing with the dealership's work/buttload of swirlmarks on the paint upon inspection today.

So if it's not the plugs, bad gas (little possiblity), MAF sensor or a simple check valve - any ideas from you all? I am going to double check they tried coilpacks (honestly, that's what the engine feels like when on throttle) but it's fine at idle so it makes me wonder.

I'd love some input from the knowledge here. Bad coilpacks only on engine load and not at idle? Any other ideas? *scratches head*

diagnosticator1

When a coilpack fails, the coilpack cannot develop enough secondary voltage to cause a spark to jump the plug gap with higher cylinder pressures that exist with larger throttle opening angles or when boost is present, thus the misfires worsen as load increases. At idle, or very light loads, the cylinder pressure at the ignition point is comparatively low, so the voltage required for a spark to jump the plug gap, is below the voltage break down limit of the faulty coilpack. There will be a misfire DTC for the cylinder involved.

JeffAhn

but this seems to be the culprit, and I hope it's simply this. They did pull codes, which did include misfire codes.

BTW, would the vibration being more violent in APR 1+ program be consistent with a coilpack failure? Logic tells me there are going to be increased cylinder pressures on different programs, at the same throttle percentage, even out of boost...

Another interesting note is that no flashing CEL, like many have reported, just a CEL that's been on and off for a month prior to these symptoms.

diagnosticator1

And misfires are less likely to be the cause without the misfire DTC that normally accompany them, but it is possible that misfires are occuring without misfire DTCs. It's a stretch, but it's possible the misfire recognition is disabled in your ECU, thus no misfire DTCs.

character zero

yup- sounds like cp's classic symptoms before they completely fail.