So a bit of history first, I had misfires earlier this year with multiple codes. I replaced the cats (one was clogged) all the oxygen sensors as well as the EGR valve. Car has been running smoothly up until recently where at about 1/2 - 3/4 throttle it is misfiring. No codes at all although it's been happening for about 2 weeks now. No other symptoms I can tell.
The only thing I can really think of is coils? Or maybe the spark plug seals? I've read in other posts that after a while they can crack and oil can leak onto the spark plug.

Car is a 2005 STR

 

Short answer is yes, but I haven't heard that they are a known weak part. My wife's Jetta needs them changed every couple of years. I would try to track down the misfire to a specific cylinder or it will get expensive guessing. You are also correct that oil can get in the spark plugs, I had that issue which I fixed when I replaced the valve cover gaskets. One test you could do is get 1 new coil and rotate it to each cylinder, but this will only work if only one is bad.

 

Coils - main suspect BUT first check the battery is top notch, not old and getting low. They start the car but can cause coils to misfire.

When checking coils peer in the plug wells and make sure no oil/water.

You may have pending codes to hint at which coil(s).

Other thing to do is check LTFTs. Hot engine, park, idle and rev to about 2500. Trims that are out might explain the previous trouble.

To some extent coils are a maintenance item as they get baked and suffer high voltages. At 10+ yrs not all may be good any more.

 

If you buy coils, stay away from Lucas brand ones. I've had bad luck with them, just my experience.

 

I don't think they make them for the STR (or do they?).

 

Thanks guys! Battery is fairly new (less than a year old). I need to check all the plugs for oil/water, but I may do the coils while I'm at it. Any suggestions from the rockauto (looking at the standard motor products brand). For reference, car has about 160k km's on it or 100k miles.

 

Ah... you're due new plugs at 100K miles i.e. now.

NGK IFR5N10

 

+1 on replacing the spark plugs at that number of kilometres.

Given the age of the vehicle, it may be best to replace the coil units as a set since misfires can lead to catalyst damage without setting a DTC or turning on the MIL.

 

On my '02 V6, I've got three Beck/Arnley and three Motorcraft coils. No problems with either brand.

 

This'll jinx 'em - I'm on all 8 OE coils at 128K miles and nearly 13 yr old car.

 

Thanks for all the responses. So spark plugs I had replaced with the NGK Iridium plugs at like 60k miles. Hopefully those are still ok? Or should I replace again along with the coils?

 

The plugs I posted are good for 100K so leave 'em in (on the basis you did use them and did not damage them - the centre electrodes are very delicate - and they come pre-gapped).

 

My 2005 STR still has all 8 factory coils at 126K miles. Now my 2003 Lincoln LS went thru a number of coils and had a special factory 10 year extended warranty on them.

If you are getting misfires that should give you some error codes indicating which cylinders are causing it.

I would also suggest a plug change first.
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