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Discussion Starter · #1 ·
I think I have a bad coil pack because at idle the engine sometimes vibrates and hesitates. Occasionally when stopped, I hear and feel a thunk like another car ran into me at low speed.

I just got a new coil from FCP but I'm trying to figure out the best way to test. I see a lot of recommendations to unplug each coil and see how the engine sound changes. But pumping a bunch of unburned fuel onto the catalytic convertors seems like a bad idea. I've also seen one post recommending unplugging the injectors one by one. Does that work just as well?

BTW I have new OEM plugs already
 

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What model/year? Mileage?

Most cars, if you have a bad coilpack, you will either get a CEL or at least a stored code. Use a code reader and it will tell you exactly which cylinder is having the problem. Clear the codes, then move the coil pack to a different cylinder and see if the problem moves with it. If it does, you know that coil pack is bad, if it doesn't, you have another problem.

Did you check the gap when you installed those plugs? OEM plugs should be gapped correctly, but I've heard stories where they were not.
Hesitation sounds like a bad fuel pressure sensor or a dirty throttle body, or both. Could even be a dirty MAF.
Vibration is odd... That sounds more like a worn engine mount, which is often only noticeable at idle and not while the engine is under load and cruising. Does the vibration lessen if you put the car in neutral?

A thunk that feels like a rear ending, that is really concerning if that's coming from the engine! Does it happen as you stop, or while you're stopped? That could also be an engine mount or a multitude of other suspension components on the go.
 

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Discussion Starter · #3 ·
It's a 2005 V70 2.5T with 75k miles. No CEL and no codes show up when I press the button to view codes on the screen

I did check the gap on the plugs

Thunk is only when stopped. It's not very loud. As I understand that's what happens when you have a misfire

I'll check if there's any difference in neutral
 

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You'll need a real OBDII code reader to see if there are any pending codes being stored, what the car can read does not suffice.
Any autopart store can check for codes for free.
 

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It has pending codes?? I have a Scanguage but I can't find the damn thing
Yeah, it's kind of strange that it can have some codes will trigger a CEL and get stored, while other codes just stay as pending...

Though honestly, if there was a misfire problem with a coil pack, that should have trigger a CEL to let you know which one.
 

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All good ideas from above.Never pull a wire to check.It is dangerous #1 and it will cause an open in the circuit that will fry the pack you pulled the wire from.The current at the pack generates very high voltage and it must go to ground to save the pack.That is why a very old worn out spark plug can damage a coil pack.
 

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Discussion Starter · #8 ·
All good ideas from above.Never pull a wire to check.It is dangerous #1 and it will cause an open in the circuit that will fry the pack you pulled the wire from.The current at the pack generates very high voltage and it must go to ground to save the pack.That is why a very old worn out spark plug can damage a coil pack.
Pulling plugs from packs is exactly what I was trying to avoid, but I want to know if pulling the plugs from the injectors is OK
 

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Pulling plugs from packs is exactly what I was trying to avoid, but I want to know if pulling the plugs from the injectors is OK
Nope.Really,whats that going to tell ya?It might isolate which cylinder is miss-firing but you still don't know why.At 75,000,it is doughtful that you have a bad pack unless you have been running with a bad plug for a while.If it was miss-firing for a while,it would set off the Cel and set a hard code that any obd2reader can detect.
 

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The "thunk" when stopped is what gets me. Can't see that being related to ignition personally. So maybe you have a bad motor mount too. About dumping fuel into the cat, consider that that is how any mechanic is going to test. Its not like you should drive with the coil unplugged to test or anything. You just get them all unbolted and pull them out 1 at a time until you find the one that DOESN'T make it worse. Then you turn off the engine and SWITCH that coil with a different cylinder and do it again. The "no-change" condition should follow the coil. You have idled for maybe 2 minutes total. No worries. Your cat should be able to withstand much more unburnt fuel than that.
 

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Discussion Starter · #11 ·
The "thunk" when stopped is what gets me. Can't see that being related to ignition personally. So maybe you have a bad motor mount too. About dumping fuel into the cat, consider that that is how any mechanic is going to test. Its not like you should drive with the coil unplugged to test or anything. You just get them all unbolted and pull them out 1 at a time until you find the one that DOESN'T make it worse. Then you turn off the engine and SWITCH that coil with a different cylinder and do it again. The "no-change" condition should follow the coil. You have idled for maybe 2 minutes total. No worries. Your cat should be able to withstand much more unburnt fuel than that.
Why would a motor mount thunk just once?

People refer to what the engine does with a misfire as "stumbling". So what is stumbling and what does it sound and feel like?
 

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Discussion Starter · #12 ·
I noticed something different last night-- Idling when stopped the car sways slightly, like there is some harmonic. But it doesn't sway in neutral. Does that indicate engine mount needs replacing?
 

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Any kind of body movement that happens when the engine is running and there are no external factors can be an indicator of bad motor mounts. The most common failures are the torque mount down btw the trans and the radiator, the upper engine mount that attaches to the black bar btw the strut towers, and then the mount by the crank pulley and the front mount on the subframe. I have replaced the first 3 and the left side mount was the trickiest to diagnose. It caused weird things not the usual steering wheel or door mirror vibrations or loose change clanging.
 

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Discussion Starter · #14 ·
I have that Wells Fargo warranty that I got when I bought the car, so I'm gonna have my mechanic look at the mounts and probably request to get the mount replacement covered by the warranty
 
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