NEED HELP - P0305

Hi guys, I have been reading this forum for years and I had to finally create an account as I am needing help diagnosing an issue. I have a 2005 Toyota Tacoma 6 cylinder with 288,000 miles. Truck has always ran like a champ. I recently got some rough idling when starting the truck early in the morning or after a cold start and a constant CEL throwing a cylinder 5 misfire code. I have been dealing with this issue for months now. I know of the potential head gasket issues on these trucks so I had a block test ran and it passed! So then I had the spark plugs changed, coil on cylinder 5 changed, and I also changed the fuel injector for cylinder 5. Truck ran much better on startup but the CEL was thrown again with the same code. I am completely lost now and have no idea what is causing this. Any help is greatly appreciated. Thank you!!

 

What do you mean block tested? Did you have a compression test done? Or a leak down test done? Did you have your coolant tested for exhaust gases? Need more info. If you changed the plugs, injector, and coil I would say its a head gasket.

 

Hey thanks for the response. My mechanic told me he did a block test and it passed. I believe this was the test to see if the coolant had exhaust gases present.

 

You may try checking the a/f/r air fuel ratio sensors(the upper o2 sensors) and see if they are any good. Your mech should be able to hook up a scan tool a read the o2 readings and go from there

 

Likely headgasket, I had a taco last month that only leaked coolant into the cylinders after long heat drive and pressurize. I could see the coolant entering the cylinder with my boroscope.

It never leaked cold, but it would leak warm and cause the misfire when cold start.

 

A/f sensors wont cause a misfire. You could unplug them and the engine will run fine.

It could still be a head gasket, those block testers do a pretty good job but they won't always pick up the exhaust gases, especially if it's a very small leak. How is the coolant level? is it going down slowly over time?

Sometimes it will take a couple tests to get it to fail. I've had customers where we were chasing a slow coolant leak, it passed the first couple block tests and then I had them come back a few days later and bam it failed

Maybe it is an intake gasket leak. Those rubber O-rings can get hard and crispy over time. Next time the engine is cold and the problem is happening try spraying some brake cleaner around the intake back by cylinder number 5, see if the engine bogs down. Check the fuel trims are they close to zero or are they high?

 

Block testers work if there is a gap you can fit a credit card between, for tiny leaks they suck. I had a neon with a bad head and I knew it had a bad head and the block test “passed”

 

Thanks for these responses guys I really appreciate. Coolant level stays pretty constant I don’t really lose much. Sometime it gets low but after a couple months. I just recently also changed all the hoses before I changed the fuel injectors.

 

The reason the head gasket is still suspect is because that number 5 cylinder is the notorious one for failure.

You could also try sort of a backyard running compression test / vacuum test by doing this..

When the misfire is happening, gently hold a piece of paper over the exhaust pipe and watch to see if the paper tries to get sucked back up the tailpipe. If everything is good valve wise the air flowing out of the exhaust should be one drection only out the tailpipe. If the paper is sometimes trying to get sucked back up the tailpipe that could be a valve that is sticking open or floating.

 

Oh wow ok I’ll try that - thanks!! I’m hoping I can get to the bottom of this

 

I hope that makes sense. LOL I'm talking into my phone while walking the dog! It tried to make One Direction all caps like the name of the band

 

Hahaha that’s hilarious. Yea it makes sense. Thanks!!! Before I changed the injector it was running a lot worse but now that I changed it, it starts up better but still threw a code and has slight vibration on startup

 

That startup flutter could be that cylinder choking on some droplets of coolant that ooze its way in there when the truck is parked.

good luck

 

Would it be possible for a valve to intermittently stick open? Chasing down a similar problem with similar symptoms. When I get a rough idle I can have partial vacuum at the tailpipe. Could I experience the same thing if I had an injector problem?

Have checked plugs, coils, wires, compression and everything looks good so far.

Thanks!

 

The "paper test" is only for finding mechanical issues, like valves opening/closing properly. Yes an intake or exhaust valve van stick intermittently, thats what the test is really helpful for. Think of it as a sort of running compression test.



A bad fuel injector will not affect the flow of air through the engine (remember an engine is simply an air pump) and would not show up with this test. You could disable the fuel and spark systems completely and the flow of air through the cylinder heads would/should still be the same

 

Well crap. Can you think of any other diagnostics to pinpoint a root cause and location or do I need to pull the heads at this point? What types of things would cause and intermittent sticking of a valve? The misfire/roughness happens more frequently when it is cold but is not limited to startup and it is certainly not consistent which is why I have been focused on electrical.

Working on a 1996 2WD 6 Cylinder (not technically the correct thread but similar behavior).

I thought something was funny based on hearing a sort of "gasping" from the intake side during rough idle.

 

A bad valve guide and/or weak spring can cause an intermittent stick

Have you tried the paper test? Gotta do it while the problem is happening!

 

Update on my end - gave the truck to a very trusted mechanic back in my hometown and he performed another block test and it failed. He suspects the coolant was leaking into the Head gasket. Currently getting replaced.

 

I did the paper test again on the way home from work and filmed and I think this is the result you described.

https://youtu.be/WeuNuFJ52z4

Misfire/rough idle is not consistent. More frequently upon startup when cold but on my 6 mile drive home it will start with a rough idle during warmup, then may clear up after a few miles of driving, and then return by the time I get home (or maybe not). I rarely drive this truck for more than 10 miles at a time.

Always get a P0303 even after swapping coils and plugs. Also did a standard compression test.

 

Umm.. you gotta use both hands and kind of cover the tail pipe outlet with the paper, like try to seal it better. Try using a credit card or a paper plate, something stiffer.

Its hard to describe, but what you're looking for is (for a split second) the paper will stick to the pipe and hang there because the exhaust is actually sucking back up the pipe