Spark Plug Diagnosis P0306 ‘05 Tacoma 4.0L

Joined
Sep 19, 2012
Messages
607
Location
Athens, GA
Truck (2005 Tacoma 4.0L) threw a P0306 code yesterday which is a cylinder misfire on cylinder 6. Lots of talk on the web about potential fixes, one of which being the dreaded head gasket.

I pulled the #6 and #3 spark plugs. Pretty noticeable difference between the two. The coil also had some oil on the body which seems odd. Seems like some oil fouling on the plug too.

Any thoughts based on the appearance of the plugs? Truck is new to me but plugs have about 30k miles on them.

Cyl 6 on left:

046E3C65-A94F-4DB6-83AC-C82E6E941F06.jpeg


Cyl 6 on right:

DCE5710B-B6E6-47E2-B36A-382D81E1E104.jpeg
 
Joined
Nov 30, 2015
Messages
150
Location
North GA
Whats the mileage on the truck? I see you replaced them about 30k ago, before that when is the last time you changed them?

The 4.0 head gaskets generally start with rough running and misfires on cold start but go away once warmed up, have you experienced this?

With oil on the coil Id clean it, fix the leak (its leaking into the tube) and see what happens. If its wet with oil It can cause a misfire.
 
Joined
Sep 19, 2012
Messages
607
Location
Athens, GA
Whats the mileage on the truck? I see you replaced them about 30k ago, before that when is the last time you changed them?

The 4.0 head gaskets generally start with rough running and misfires on cold start but go away once warmed up, have you experienced this?

With oil on the coil Id clean it, fix the leak (its leaking into the tube) and see what happens. If its wet with oil It can cause a misfire.
Thanks for the feedback. Mileage is 215k. Previous owner was religious with maintenance with plugs every 30k. The code just popped the other morning which I guess is the coolest it’s been in awhile. Before that no rough running symptoms. Once warm it still shuttered bad especially at low rpm which seems like the coil but the web suggests other potentials as well.

I ordered a new coil for cyl 6 and all new plugs. It’s odd it had oil on the coil - almost seems like a leaky valve cover gasket as oil would have to come out the cylinder if it was the head gasket to get there. My read on the spark plug is that somethings going on in that cylinder but it’s minor at this point.

I’ll report back once I swap the new parts in.

Thanks again!
 
Joined
Aug 5, 2011
Messages
1,873
Location
Chapel Hill ,TN
@tgreco
Hold the engine at a very high rpm for a solid 15 minutes. This will turn any minor issue into a major issue which will point you to the problem. Now please keep in mind, this type of diagnostics can be very costly so extreme caution is highly advised.
Good Luck Bro
 
Joined
Sep 19, 2012
Messages
607
Location
Athens, GA
@tgreco
Hold the engine at a very high rpm for a solid 15 minutes. This will turn any minor issue into a major issue which will point you to the problem. Now please keep in mind, this type of diagnostics can be very costly so extreme caution is highly advised.
Good Luck Bro
As usual, an extremely insightful approach to the fix! Sounds about like riding up the hill at windrock in the wheeler - sounds like nothing is wrong with it despite losing a quart of oil after every trip!
 
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