The case of the mysterious misfire...

khill

Original poster
Member
Jan 7, 2012
86
My 2004, 4.2 I6, Trailblazer LT with 135,000 miles has been acting peculiar for the past year. It has a tendency to misfire occasionally and gives either a P0300 (random misfire) or P0302 (cyl2 misfire) separately or together from time to time. This can happen for days in a row or once a month. It also ONLY occurs when I leave the truck in idle, never occurring in gear or at speed. It has recently cropped up again for the past few days and I now have adequate time to tackle it once and for all!

Steps taken:
-Replace plugs, all brand new as of 6 months ago. No effect.
-Move coil pack. This resulted in a P0300 again but never another code that would isolate a cylinder.
-Replace coil pack. I figured I'd try it anyways since the P0302 never came back. No effect.
/////Didn't have a problem for some time, figured it was ok. Problem came back and went away a few times over the next few months. Some point in here I took it to the dealer and they looked at it and wanted thousands of dollars to fix a "small timing issue" without providing additional details. I said screw you (in nicer words)./////
-Replace o2 sensor, it was time to anyways. This resulted in a P0171 (system lean) and persistent high idle of 900. I cleared the code and it went back to normal. No effect.
-Replaced camshaft position actuator solenoid. There was oil in the connector and a damaged screen inside. No effect.
/////At this point, I have not gotten a code after a few hours idling, perhaps the gremlin got scared temporarily again or maybe the issue is really fixed. However, now the engine tries to die while at idle, with no code showing up./////

Current situation. After a few hours idling, there are no codes however the truck tries to die, rpms jump down and back up, every few minutes while idling in park or neutral.

Anyone see anything interesting that indicates the problem? I don't mind throwing a few hundred dollars at things that really should be replaced every decade anyways, but I would like a definitive solution eventually, especially before the problem becomes worse.

A little more information: the truck tends to misfire more often when idling on an uneven surface. Like when parked on the side of the road there is the 10-15 degree slant sideways, 30 minutes here would almost always guarantee a misfire code. The misfires were also never what I would consider serious. There would be a slight wobble in the engine that could be felt in the car, but it was only momentary and only felt every few seconds. It is noticeable however, as the vehicle is usually so smooth that when working correctly, I can forget it's running if there's music playing. Also, there is no liquid of any kind in the spark plug chambers.


Thanks in advance!
khill
 

djthumper

Administrator
Nov 20, 2011
14,955
North Las Vegas
With all that you have done... What plugs did you put in when you replaced them?
 

fadyasha

Member
Dec 21, 2011
1,134
I know people have said this isn't related but I had this problem in the past and turned out my Thermostat had gone bad and was stuck. With that water kept flowing through never allowing my TB to go up to operating temperature like it should. With a weak coil pack I kept getting a misfire every once in a while when in Idle. I changed the thermostat and it went away.

Eventually my coil pack failed me and got a new one which got rid of the problem thankfully. You might wanna make sure you're operating temp is normal.
 

khill

Original poster
Member
Jan 7, 2012
86
fadyasha said:
I know people have said this isn't related but I had this problem in the past and turned out my Thermostat had gone bad and was stuck. With that water kept flowing through never allowing my TB to go up to operating temperature like it should. With a weak coil pack I kept getting a misfire every once in a while when in Idle. I changed the thermostat and it went away.

Eventually my coil pack failed me and got a new one which got rid of the problem thankfully. You might wanna make sure you're operating temp is normal.

Operating temp is normal, between 190 and 200 when warm. T-Stat was changed only 12k miles ago. Monitored with scangaugeII and live-data OBD scanner. Also swapped in a new coil pack with no results.
 

djthumper

Administrator
Nov 20, 2011
14,955
North Las Vegas
do you have any way to check the actual temperature? Fady could be on the right track since he went through it.
 

khill

Original poster
Member
Jan 7, 2012
86
djthumper said:
do you have any way to check the actual temperature? Fady could be on the right track since he went through it.

You mean like a thermometer under the hood? I suppose I could tomorrow... What're the odds that the computer is sending bad information? When the thermostat went bad before, the computer did indicate an accurate temperature.
 

fadyasha

Member
Dec 21, 2011
1,134
khill said:
You mean like a thermometer under the hood? I suppose I could tomorrow... What're the odds that the computer is sending bad information? When the thermostat went bad before, the computer did indicate an accurate temperature.

I hate it when you do everything right but still have the problem. Drives you crazy! I'm guessing if the scanner gave out right temp then you're fine enough with that. Only other thing I can think of is a weak spark plug.

I once put in new spark plugs and one failed me almost immediately after starting her up. That was different though, my check engine was blinking and getting a rough Idle. Turned her off, installed one of the old plugs where the misfire was happening and problem solved.

Throttle clean, coil pack new, T-stat new, Spark plugs new, water levels are fine and hoses are running water smoothly in and out... I think i'm missing something, it's still early here so let me drink my coffee and will get back to you :blinkhuh:
 

khill

Original poster
Member
Jan 7, 2012
86
fadyasha said:
Only other thing I can think of is a weak spark plug.

That... I haven't thought of yet. I do have an extra spark plug. I'll try that tomorrow... I'm gonna be slightly pissed if that's it. I got that friggin spark plug just in case and it's been in my glove box ever since.......
 

djthumper

Administrator
Nov 20, 2011
14,955
North Las Vegas
khill said:
You mean like a thermometer under the hood? I suppose I could tomorrow... What're the odds that the computer is sending bad information? When the thermostat went bad before, the computer did indicate an accurate temperature.

no like an OBDII that will do real time monitoring of the engine temp
 

djthumper

Administrator
Nov 20, 2011
14,955
North Las Vegas
khill said:
Operating temp is normal, between 190 and 200 when warm. T-Stat was changed only 12k miles ago. Monitored with scangaugeII and live-data OBD scanner. Also swapped in a new coil pack with no results.

khill said:
Yeah, I have two. Both say the same thing, acceptable temps...

Sorry, I had missed that post just before I asked. I must have been reading through things when you posted.
 

fadyasha

Member
Dec 21, 2011
1,134
khill said:
That... I haven't thought of yet. I do have an extra spark plug. I'll try that tomorrow... I'm gonna be slightly pissed if that's it. I got that friggin spark plug just in case and it's been in my glove box ever since.......

Good luck! i'm sure others will give their bit soon enough!:thumbsup:
 

Mark20

Member
Dec 6, 2011
1,630
I'm no expert but engine coolant temperature sensor may need replacing. When I did my T-stat I replaced that too since I was in the neighborhood.

Just watch the amount of teflon tape the factory put on. Too much on mine such that it leaked requiring me to pull the alternator again, pull the sensor and rewrap. 2.5 wraps is more than enough.
 

khill

Original poster
Member
Jan 7, 2012
86
Replaced the spark plug to no avail. It hasn't tried to die on me but it hasn't gotten any smoother on idle. On another note, I did have one of the biggest "OH $#!T, !@#$%^&*(*&^%$#" moments of my vehicle maintenance life when I heard the following snap:

View attachment 17809

View attachment 17810

Thankfully there was a burr sticking up I was able to fish it out with, took ten minutes but I was praising every deity I could think of that I didn't have to drill that thing out...

View attachment 17811

That's what I get for using a generic replacement coil from napa... Thankfully, also, I still had my original coil pack to put back in there. No performance difference at all still...


Regardless, I have some supersparks on order so maybe, just maybe, getting rid of those darned springs in the coil packs will help my idle. Here's to hoping. This is one of those problems you can never really consider fixed because it might still be there, just sleeping for the time being... Grr...


Thanks for all the suggestions so far guys.


On another note, how much crap is supposed to be on the inside of the cylinder? When I had the plug out I took a peek and it was black in there. I would have compared it with another cylinder but didn't want to do any more discombobulating out in the cold. Wish I had a garage... I'll be doing another seafoaming this week, maybe that'll help.

On another another note, apparently disassembly is not a word spell-checker recognizes, but discombobulating is :undecided:
 

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Busterbrown

Member
Dec 4, 2011
253
khill said:
That's what I get for using a generic replacement coil from napa... Thankfully, also, I still had my original coil pack to put back in there. No performance difference at all still...

Glad you were able to get that treaded stud out. It's been suggested only to use AC Delco coil packs. I just got done replacing my #4 ignition coil as it was showing an occasional misfire on my OBD2 tool. It was running fine at speed with no misfires detected. But if it idled for more than 5 minutes, these misfires would show up. So, I went out and purchased the recommended coil pack last week. Installed it and monitored my Torque application. Hadn't seen a misfire again until Friday when I let the truck warm up and idle on it's own. Sure enough, the misfires still exist but arent' throwing any codes this time.

So, I need to investigate further what could be the culprit. The truck's idle isn't bad, as I just feel a "hickup" once in a while. So I'm really curious as to what's causing yours.
 

khill

Original poster
Member
Jan 7, 2012
86
Buster, sounds like we're having the same exact issue now... I'm hoping that the supersparks will arrive this week and magically fix the problem. But after everything I've done to this engine, I'm starting to think the problem could be a little deeper inside.


Has anyone heard of timing chain wear causing these kinds of problems? What are the odds that the chain or sprockets may have worn enough in 150k miles / 8 years to cause a minor timing error? I mean, it sounds like a long shot, but I've already taken out all the easy targets. Gotta start thinking further out there I guess and this comes to mind.
 

djthumper

Administrator
Nov 20, 2011
14,955
North Las Vegas
Yeah, I hate chasing down those things that you "fix" and then they come back. Especially with everything that you have done so far.
 

fadyasha

Member
Dec 21, 2011
1,134
You may wanna do an engine flush since the plugs are new. You'd do it before changing your oil usually and cleans out alot of the stuff inside.

This may be another long shot but have you checked your exhaust system? When was the last time you replaced any barrels or the Cat? Anyhow if you're getting a misfire code then I'd exclude that probably.

Last thing about the coils replacements may have problems. try sticking to OEM cuz as you probably know TBs are sensitive when it comes to that.
 

khill

Original poster
Member
Jan 7, 2012
86
The oil only has about 2500 miles on it, rather not do it again so soon...

All the exhaust is stock as far as I know.

Yes I do know that our trucks are very picky when it comes to OEM stuff in the engine, that was more of a let's try this, temp sort of thing. Didn't help at all anyways.
 

khill

Original poster
Member
Jan 7, 2012
86
Got my supersparkz today and popped them in. Also did a full can of seafoam spray right after. No difference, 15 minutes idling and threw a p0300, 5 more minutes and got the p0302. Going to give the PCM a good reset now and hope that does something. I'm fairly sure it won't but what's the problem in trying right?

So, new plugs, changed coil, new cpas, lots o seafoam, new oil, supersparkz, clean tb... Any more suggestions guys?

I'm almost inclined to think there is wear in the lifters or cam system or something, maybe in the piston rings... I suppose the next thing will be a compression test. Gotta find a decent mechanic in Corvallis I guess.....
 

fadyasha

Member
Dec 21, 2011
1,134
khill said:
Got my supersparkz today and popped them in. Also did a full can of seafoam spray right after. No difference, 15 minutes idling and threw a p0300, 5 more minutes and got the p0302. Going to give the PCM a good reset now and hope that does something. I'm fairly sure it won't but what's the problem in trying right?

So, new plugs, changed coil, new cpas, lots o seafoam, new oil, supersparkz, clean tb... Any more suggestions guys?

I'm almost inclined to think there is wear in the lifters or cam system or something, maybe in the piston rings... I suppose the next thing will be a compression test. Gotta find a decent mechanic in Corvallis I guess.....

Wow u must have the strangest problem ever! What else could it be! Bad air filter and engine not getting enough suction? Your engine not getting enough fuel? Maybe fuel filter is clogged up or one of the hoses? Strange!
 

khill

Original poster
Member
Jan 7, 2012
86
Bad air filter and engine not getting enough suction? Your engine not getting enough fuel? Maybe fuel filter is clogged up or one of the hoses?

Air intake is good.
Engine gets plenty of fuel.


Keep em coming folks.


On another note, is there any downside to using too much seafoam? I'm thinking along the lines if there is carbon buildup or something sticking in there that doing multiple runs of seafoam will help clean it out. Another shot in the dark but hey, what's the downside besides cost right? And 30 bucks for 4 cans of seafoam is nothing compared to having the stealership or a mechanic tear it apart. If they want some big bucks to fix this one I may as well just get a new engine. I friggin like my trailblazer....

Another thing on the seafoam, where is the best place to inject it into the air intake? I know the usual route is the brake booster line but I've heard the rumor that only gets the rear cylinders. Should I use the TB or maybe use the booster line and the test port near the front as well? Ideas?

My engine is gonna be squeaky clean by the time I'm through with it, I hope. I remember the first time putting it through the booster line, I smoked out the whole block...


Thanks all.
Khill
 

dcrews

Member
Dec 4, 2011
4
2 things:

1) swap the #2 and #1 fuel injectors, and see if the problem follows.

2) I had a similar problem on my 04 Bravada, and after doing EVERYTHING I could think of, including #1, I replaced the PCM with one from PCMforless, and it solved the problem. No idea how or why, but it fixed it 100%
 

MichEnvoyGuy

Member
Dec 3, 2011
522
I would say do a compression test next.

There have been reports of owners with 2004+ T360's with I6's, of them dropping a valve or having a valve not seat properly causing an intermittent or solid misfire.

Hoping thats not your case, but a compression test could rule it out.
 

hdodd1

Member
Dec 3, 2011
25
I had a miss at idle and after new plugs, new coil, seafoam and fuel additives. It was the fuel injector. The engine ran great at speed but would miss at idle. Fuel injectors on the 4.2 are not easy to get to:frown: Good Luck on your problem.
 

khill

Original poster
Member
Jan 7, 2012
86
That's encouraging, thanks!


On another note, tonight I had an odd experience with seafoam. And slightly worrying as well... I was running it through the booster vac line and got to about the 1/3 mark when the engine started to chug, probably added too fast. I stopped adding and went to turn off the engine. Problem is, after the engine was off, the engine continued to chug... for about 20 seconds.... Thoughts? Cuz I have none that are really that good. I'll start it back up in about 30 minutes and let you know how it goes but if you have any ideas on what may come, I'd appreciate it.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
The run-on is not unusual if you over loaded the engine with fuel (in this case Seafoam).
 

khill

Original poster
Member
Jan 7, 2012
86
Thanks for that wooluf. Good to know.


In the end, the car simply started as normal (with white seafoam exhaust of course). I have a strong feeling that one of three things is now happening: either my mass seafoaming is curing my problem, or it is masking my problem, or the problem just went away for the time being. I just sat in the parking lot for the last half hour at idle and my TB is running smooth as silk with only a few snags... No "bumps" like the misfires used to be, just the occasional stronger wobble. It's possible that the problem is a gummed up injector or valve and the seafoam is actually curing it! If that's the case I will be simply ecstatic. I know from looking in the cylinder when I changed the plug that I need to decarbon my engine, but without dropping the cat, all I really want to do is just can after can of seafoam. So far I have run a total of 1.5 cans through the intake system and probably another 1.5 through the fuel tank (with another two in a full tank now). Here's to hoping! :undecided:

Anyone know where to get seafoam for cheaper than 8$/bottle? I know it's a relatively small price to pay for fixing my problems but still...
 

MichEnvoyGuy

Member
Dec 3, 2011
522
Just out of curiosity, when you say you were experiencing this misfire - was your SES light flashing while it was occurring? I know you had a few DTC's stored but Im just curious.
 

khill

Original poster
Member
Jan 7, 2012
86
Hm... I swear I thought I updated this...

Anyways, after about 80$ of seafoam I can't tell that there's a problem anymore! Again this may just be masking it or it may have settled down only to come back a month from now. Either way, seems to be problem solved with mucho seafoaming, through the vac line, throttle body, and fuel tank. Here's to hoping it stays fixed! :wootwoot:
 

fadyasha

Member
Dec 21, 2011
1,134
khill said:
Hm... I swear I thought I updated this...

Anyways, after about 80$ of seafoam I can't tell that there's a problem anymore! Again this may just be masking it or it may have settled down only to come back a month from now. Either way, seems to be problem solved with mucho seafoaming, through the vac line, throttle body, and fuel tank. Here's to hoping it stays fixed! :wootwoot:

yay! that's great news! End of the month i'm installing my supersparks, new sparkplugs and doing a seafoam! the Idle is extremely good but u can still feel the motor not being at easeeeee. something is bothering it lol! Glad you got it fixed and let's hope it stays fixed! :mad: :raspberry:
 

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