2005 5.4L Rough idle and chugging under load
 

I am hoping that you guys can help remove my confusion.

I have a 2005 5.4L 4x4 with 170,000 miles and am having an issue with a rough idle when engine is below operating temp. There are not codes at the moment but there have been codes regarding a misfire one of the cylinders. There is also a hesitation and lack of power until the engine heats up a bit.

It also exhibits an issue where it runs perfectly fine when at normal operating temp for the most part. The issue comes up when I an running at highway speed and go to pass someone. If the trans drops down and I keep my food in it, it will start to lose power and eventually start bucking a little. The CEL starts blinking and there is a loss of power for a short time. After 30 seconds or so, it is back to running normally unless I put it under that load again. This is when it generates the misfire code.

I have done a lot of searching and reading but am a little confused as to where to start. It acts like the cats are not working right (the rear O2 sensors voltage jumps all over the place) but could also be injector, coil packs, or plugs. I do not want to start throwing things at it randomly so any suggestions on where to start? The plugs were all changed out about 12,000 miles ago.

Change the boots and springs on the coils.

Thank you. I will go through and verify all of them. I did a little more looking and the rear O2 sensor on Bank 2 is the one that has the voltage jumping around. I was also able to get the CEL to come back on and the code is P0306. I am assuming am issue with the coil pack could cause issues like I am describing and potentially found a catalytic converter? I am planning on changing the muffler (do not like the flowmaster that is on it) and thought I would do the cats while I am at it. Is there more of a definitive way to check the cats other than the O2 sensors? Or should they be alright to leave if it is not throwing a code?

Sorry about the formatting in the previous post. I tried to break it into 3 paragraphs but it keeps removing the breaks.

A blinking CEL is telling you , you have a mis-fire condition.. what brand plugs were installed and like Stephen mentioned ..did you change boots and springs ? With a 170K miles on the motor it's probably time for COP's.
If this mis-fire has been going on for some time you may have burnt up a cat from the raw fuel being dumped into it.

P0306 indicates a misfire on cylinder 6. The only way Im aware of to check the cat is to see if it is getting excessively hot, which would indicate it is plugged. I would start with just changing the boots and springs on the coils ($50ish). Very rarely does a coil go bad. You can check them with a multimeter and it should ohm out at 1.7 or 1.8 ohms, anything else is bad. Also x2 on the question of what brand of plugs were used?

So...it would not be worth the extra $$$ for all new coils and just replace the ones that are actually bad (if any) but clean all of them and replace all boots/springs? I assume they are either working or not, and do not gradually lose functionality?

I used Motorcraft SP-515 plugs.

Some people do new coils I don't recommend it unless they ohm bad. I bought a new coil, swapped it out. Cleaned up the old OEM coil and put new boots and springs on it. I then put the OEM back on and the miss was gone. I did have a bad coil here recently when I was changing my boots and springs. When I put the multimeter on it I was only getting 1.3 ohms so I put my spare coil in place of that one.


You used the right plugs, there is still always the possibility of a cracked plug or even the slight chance of a bad plug.

I will start with testing the coils/replacing boots springs and see what I get.

Should the rear O2 sensor on that bank stabilize somewhat once the issue has been corrected (providing cat is not toast)?