P0300 code only

[H2Oboy]

So I know there are a few threads with this topic and i have read many of them here and other places but they all seem to have a specific cylinder causing the issue. Unfortunately for me I just can't figure this one out and have been working on this for longer than i care to admit.

Without further ado here's my story:
2002 Trailblazer LS 4x4 with 136,000 miles

About the beginning of April my wife said the TB is running weird so i drove it and is idled rough and stumbled through acceleration. I figured bad plugs or coil. I decided to replace all of the plugs. Seemed to fix the problem for a day. went to the auto parts store and had codes read. Said the #6 cylinder was the culprit. replaced the #6 coil. Still not fixed. I was headed out of town so said screw it and took the TB to a local shop they said i put crap plugs in it and should put AC Delco in. I put the new plugs and new #4 coil in and I no longer get the P0306 code but rather a P0300 code. and still runs the same.

While feeling the exhaust and listening to the miss fire it seems very consistent and constant (makes me wonder why i can't get a cylinder #)

I took off and cleaned the throttle body
Checked for broken wires- # 6 coil plug has been replaced but looks like quality work
added bottle of Techron fuel injector cleaner
cleared the codes(disconnected the battery), drove for and hour and was fine. The next day right back to rough idle and stumbling through acceleration.

Did some more reading:
checked the intake bolts for tightness. one bolt was loose(very) between 5 and 6 cylinder.
no water or evidence of water in any of the coil wells when replaced the plugs. Other places mention cowling leaks onto engine/ coil
one plug did have a little oil on the threads. Didn't think much of it at the time (#4 if i remember right its been almost a month when i did that!)
drove it again today and still stuck.

Did more reading:
Did i foul up my CAT with driving on a miss fire and now a blocked CAT is my issue not ignition???

So this is where I'm at
1) Don't know what to do.
2) Buy another coil and start switching them out - hopefully find a bad one
3) Bad CAT
4) Replace O2 sensors for luck...
5) Plugged/bad Injector??? FYI fuel filter was replaced about a year ago
6) Dying fuel pump

Going to Advance and start on #2 and have the code read AGAIN and hope something shows up. Almost forgot SES flashes from time to time but does not stay lite up.

In general the Trailblazer is in great shape first real problem since I've owned it (about 80,000 miles).
Sorry this is so long but i know even small details can help

Any pointers would be most helpful and i will keep everyone up with any new developments.

 

[Sparky]

When I was dealing with something similar I pulled all the coils. Found one (I think it was #5) had some surface rust on the electrode where the spring pushes against. Have to remove the boot to see it. Maybe start there.In my case I just used some sandpaper to shine up the electrode and spring, and it has been fine ever since.

 

[The_Roadie]

A clogged cat can be troubleshot by removing the upper O2 sensor and taking it for a spin.

Next thing I'd do is check compression and vacuum.

With a real scan tool, you or an experienced mechanic can check the true O2 sensor readings, long and short term fuel trims, and many things that might get you to a better diagnosis.

Not all problems are DIY unless you have a couple thousand $$ worth of diagnostic equipment and some experience.

 

[KNBlazer]

What I would do while the engine is idling poorly, I would disconnect the coil's plug one at a time until finding the one that doesn't cause an effect on the idle.... if that doesn't work, then more envolved diagnostics as Roadie suggested, will be required....another thing to consider, sometimes when replacing the #1 coil, it doesn't seat right because of the harness that's over it, you have to play with it...

 

[H2Oboy]

Thanks for the suggestions. I'll start with removing one coil at a time then O2 sensors. I hope that gets me a result other wise its not looking good for the home team...

As for a compression check I've never done one but it doesn't sound too difficult, is this something i can rent? I will check local auto parts store too but just was on my mind.

 

[KNBlazer]

Thanks for the suggestions. I'll start with removing one coil at a time then O2 sensors. I hope that gets me a result other wise its not looking good for the home team...

As for a compression check I've never done one but it doesn't sound too difficult, is this something i can rent? I will check local auto parts store too but just was on my mind.

not the coil itself, unplug the connector one at a time, then plug it back in....

 

[H2Oboy]

yeah i did that. once i read doing that it made way more sense then the whole coil.

each plug i pulled made a noticeable difference. on the the O2 sensor

 

[KNBlazer]

yeah i did that. once i read doing that it made way more sense then the whole coil.

each plug i pulled made a noticeable difference. on the the O2 sensor

I take it, it also made a difference in engine sound?

 

[KNBlazer]

If you've never replaced your o2 sensor, might be something to consider, not sure though if it will cure your misfire... you could google it and see if there is a connection between the two.... Use AcDelco, it's hit or miss if a particular truck will accept an aftermarket electronic replacement.... I bought mine from Amazon for like $45.00

 

[H2Oboy]

i doubt it has ever been replaced and after struggling with it i was considering just replacing it while it was out. letting it "soak" for a few then to try again. i guess a great time to look the cost options.

 

[H2Oboy]

pulled the O2 sensor and still rough... but i did notice two things.

First: i put my hand down by the exhaust to feel it again ( like other times) but this time i still had my rubber glove on and it felt like it was sucking it up. i then took a piece of paper towel and it infact was. Bad valve???

Maybe not

Second: like other times when working on it it ran just fine. purred like a kitten. it did this while i was at the exhaust and pushed the paper towel out right away.

So i'm wondering. i read on some post they suspected a piece of carbon to have broken free and keeping the valve from seating completely (exhaust i would assume) their suggestion was to pull the plug and fill the cylinder with seafoam and let sit, remove seafoam and hope it broke up the deposit. I haven't run seafoam through a vacuum hose yet. I would do this first. before filling the cylinder-especially since i don't know which is the culprit.

Roadie mention checking Vacuum has anyone had any experience with common areas for vacuum leaks? places to start?

for now i'm going to direct inject seafoam and see if this helps, also while at the store check into a compression tester rental.

 

[bobdec]

P0300 is multiple cylinders misfiring, SES solid on is misfires less than 1% of the time (emission problem) , SES blinking is misfires greater than 5% of time (CAT damaging). Since you already changed the fuel filter I'd borrow a fuel pump tester (lender tools) at a local parts store and test fuel pressure before changing the fuel pump. Personally, I'd throw out the O2 after 135K miles and put in a new Delco or Densco , soaking may fix a fouled O2, but it could be switching slow or wrong. Do you have an android phone or table) ? The Torque App and a $20.00 ELM Bluetooth to ALDL adapter will give you very basic scan tool capabilities. Enough to look at and reset DTC's, monitor fuel trims, O2 voltage switching and vacuum.
It it's misfiring due to lack of fuel then your CAT would not be damaged. Howevr if it's misfiring due to plug not firing then raw gas or dirty burn is entering the cat and can plug it up. That would be evident by inspecting the plugs, all s/b white colored.

 

[H2Oboy]

Most of the time i get no engine light. If i do it blinks for a few seconds then off.

I do plan on replacing the O2 sensor. By soaking it i meant some WD40 to get it out.

I do have a kindle fire.... may need to look into that app

Honestly i'm ready to take it back to the shop that originally looked at it. They offered to look at it for "free" if the new plugs didn't fix the issue. I know my wife wants her TB back.

 

[The_Roadie]

I gave my wife a Kindle Fire for Christmas. She lasted 3 days with it. I gave it 4 hours of fiddling before I returned it for being locked out of the Google Apps store. Closed ecosystems suck. Shun them.

 

[Texan]

For possible vacuum leak, read the post about "What's this hose?" If you have a SWB (not EXT), this nozzle on the
intake manifold should have a rubber cover. I have not learned how to provide a link with the new system, yet.

 

[H2Oboy]

Got it back together with new upstream O2 sensor and Seafomed it. Took it for a test drive and after the initial Seafoam sputter and smoke ran just fine with no hint of miss fire. I'm nowhere close to 100% certain this problem is gone as i have had this result before just for the miss fire to come back a day later. so for now i'll cross my fingers.

 

[H2Oboy]

Texan
i looked at the "Whats this hose" thread and i do not have a cover for the rear heater hose. i know there hasn't been one on there for as long as i've owned the TB without any noticeable affects. I'll see about getting a plug for it anyway. Thanks for the info

 

[Texan]

Just a suggestion. My 03 does have a cover on it. Check it for vacuum leak, and if it
does not, don't worry about it. It seems some do and some don't.

 

[H2Oboy]

IT'S BACK !!

and worse than ever. Wife had code read when it happened and i just checked them and got a lot of them this time. Some old 4x4 stuff, ABS stuff i need to look into and FINALLY a P0305 code. I think this is a current code not a history code from me pulling the plug to the coils as this is the only one that is a specific cylinder. Also got a knock sensor code p0327 not going to worry about this until the coil is replaced and can try it out.

Finger crossed!

 

[H2Oboy]

Oh any thoughts on BWD coils from advance auto. already have two this will make three.

Hell kinda thinking about getting the rest now.

 

['03EXT]

With 156,000 miles , I have gone through 4 coils, all the new ones are Borg Warner ( bwd) , I think my first OE coil took a dump at around 70,000, and none of the BWD coils have failed as of yet, being the pessimist I am ,I have 2 more new BWD's waiting in my garage for the 2 remaining Πcoils to fail

 

[H2Oboy]

FIXED! (yep I said it)

it's been a few days now and still running great! I ended up replacing all the coils this last go around just cause it seemed to be time as they were dropping like flies. There was a very noticeable difference in performance right away from the other times which makes me feel good about replacing the remaining coils.

Lesson learned: Don't keep resetting the code history. I think I was shooting myself in the foot and not letting the car tell me what is wrong. Basically i would only allow it enough time to give me the P0300 code not the specific cylinder code. (or they were all just going out and # 5 finally crapped out, we'll never know) Hope this may help someone in the future.