Misfire problem

Hello gurus,

I have found some good information here in the past, including some concerning misfiring problems. I figured I would post my specific problem and see what people think.

At about 166k (2/3 highway miles), I replaced the spark plugs in my 2007 Prius. I wasn't paying attention when I left the store and had accidentally bought platinum plugs. About 1 week and 500 miles later, it started misfiring on three cylinders. Error codes were the normal misfire codes, plus one for the catalytic unit. I pulled the plugs to check if they were seated properly, check for carbon build-up etc, which is when I noticed that they were platinum plugs. I went back to Advance for iridium plugs and put those in the car. Normal mpg range. Three+ weeks and 1500-2000 miles later, it has started misfiring this morning. I haven't had a chance to check the codes this time yet. Plugs were always torqued correctly.

I was careful when pulling the ignition coil/tube off the plugs and out. Still, I suppose it is possible I put too much pressure sideways, but that would mean that they are pretty fragile. Yes/no? If these need to be replaced, is there somewhere to buy them for less than +/- $100 each?

Some history on the car:

Purchased in October 2009 with 55k from Toyota dealer. Since then, regular oil changes, only using Mobil One or Castrol Titanium synthetic and either a Bosch or K&N filter. Every oil change is accompanied by a Seafoam treatment in the oil and fuel. Air filter cleaned regularly. No issues whatsoever until the misfiring problem.

Any ideas? Anything else I should try?

Thanks for any and all help,
Kris

 

Thanks for the reply.

The plugs that I took out looked to be in normal shape for having been used for a long time. Only white soot.

No corrosion that I could see. Yes, I am using dielectric grease on the connections.

 

Thanks for the reply, Roger.

Engine was not misfiring at all before changing the plugs. As mentioned in the original post, I have driven the car over 110,000 miles with zero problems. I'm fairly certain the misfiring is caused by something that happened when I changed the plugs.

Has the oil issue with K&N air filters come up before? I've never had problems in my other cars, but if it causes issues in Prii in general maybe I'll switch filters.

 

Thanks for the reply, maestro.

The first time it misfired with the platinum plugs it was cylinders 1, 2, and 3. I haven't had a chance to check the codes since it misfired this morning.

I had the same thought about swapping ignitors. Hopefully this weekend.

 

Thanks for the reply, Critic.

If I remember correctly the codes were P0301, P0302, P0303. (Misfire on cylinders 1, 2, 3) I don't have any information on fuel trims at the moment. I'm hoping to find time this weekend to do some investigating. When it misfired this morning the CEL did not come on.

 

I used Pulstar Pulse BE1I. The platinum plugs that I used the first time were the same plug, different tip. The platinum plugs caused the original misfiring and did turn the CEL on (after about 500 miles). The misfiring with the iridium plugs started after 1500+ miles (no CEL).

I was careful to pull the coils straight up and out, but I suppose it is possible that I put too much sideways pressure on them on accident.

Yes, I used a torque wrench. I checked the torque again a couple days after I put the iridium plugs in and they were still tightened correctly.

I'm not sure what to think at this point other than I must have damaged a coil(s) somehow and it takes some time for driving vibrations to separate a connection.

 

I put the NGK plugs in shortly after Critic's reply to my misfiring problem. About two months and several thousand miles later, everything is fine, not a single misfire.

Thanks to all who helped.