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Discussion Starter · #1 ·
Hey guys. Been trying to track down the cause of this random misfire. It comes and goes and it seems like once the engine gets warm it is more prevalent. Alot of people have been suggesting the coil pack but I was able to get my hands on a digital multimeter and unless I did something wrong readings were normal although I've also heard they can still be bad with a normal ohm reading. I've tried to cover all the basic/ simple things that could possibly be the cause like spark plugs..vacuum leaks..checked and cleaned the injectors and MAF sensor. I've noticed the air filter is not very clean but it isn't severely bad. Think that's about it.. I started out with p0171 code also but I got that fixed..just can't get rid of the dreaded p0300. Any advice is appreciated..what am I missing here?
 

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How many miles on the car?….on the plugs?
Do you have the tools to do a compression check?
Big warning…that misfire can kill your catalytic converter in just a few miles if not taken care of very quickly
 

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Discussion Starter · #3 ·
How many miles on the car?….on the plugs?
Do you have the tools to do a compression check?
Big warning…that misfire can kill your catalytic converter in just a few miles if not taken care of very quickly
Just over 150,000. The plugs I replaced when I first started noticing the problem which was going on 3 weeks ago. Took them out yesterday just to do a visual inspection and I mean..they seemed ok other than one of them was a sort of lime green color on the tip which I'm not sure if that has anything to do with anything. (When I say tip I mean the part that goes onto the boot) We have been trying not to drive it as much as possible but right now it is our only vehicle. Luckily I live 5 minutes from where I work but still I am worried about further damage obviously. One thing I can add is I checked the fuel pressure with a digital pressure gauge I got from work and it does not seem to want to hold pressure. Drops pretty dramatically when I increased RPMs and also when I turned off the engine it slowly loses pressure. I mean like all the way down below 20psi. So I'm guessing maybe fuel pump? I've read that they are pretty problematic on this particular car. And by the way I do have a friend who is supposed to be bringing by one of those gouges to do a compression check.
 

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Discussion Starter · #4 ·
I even took a video of the fuel pressure test if that would help at all..but basically it sits at around 55psi until you increase rpms then it will slowly decline for as long as you hold down the pedal. And like I said when you turn it off it slowly drops into the teens..I've read could possibly be fuel pressure regulator also but just not sure.. also checked the fuel filter and although it was a little dirty it wasn't clogged or anything. Would've went ahead and replaced it but I was broke at the time and couldn't afford it.
 

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Ok @Sanchez
We must
get this misfire under control quickly or your going to be hit with a catalytic converter problem and an extra $200 exhaust bill very quickly.
Now a few motherhood statements,
Your 2006 L61 is very sensitive to its ignition system
That means USE STOCK GM iridium spark plugs…..they are good for over 100k miles
The ignition module is a dual coil wasted spark ignition system, ( Look it up) it’s known to wear and crapmout (~$100)
So right off the bat we know we must use genuine GM Plugs and must have a non-damages ignition module.
The ignition module is an electrical potted HV dual coil pack. Once a single arc happens within the coil pack, it is toast and nothing can fix it. And then if the plugs are out of specifications, it’s much harder on the coil pack feeding the problem.

now you say your engine has 150k miles. No problem…what is the compression in the system?
Do a quick compression check..,,beg, borrow, or rent a compression gauge setup from your local auto parts house. Pull all 4 plugs and get the DRY compression reading on all 4 cylinders.
This should give you over %90 status on your engines health.
I would assume if you get this missfire under control, one could expect another 20-50k trouble free miles out of your Cobalt
 

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Discussion Starter · #7 ·
Well thanks for the advice. And yes I've been scouring the internet and have seen alot about the ICM being troublesome. So am I correct that with a multimeter I can check for sure if it is good or bad? I found what I think to be decent video on YouTube showing how to do it but I'm usually at the house by myself working on it and I needed someone to turn the key over while I read the results on the meter so I guess I need to do that..get GM spark plugs which I didn't last time..got the cheap ones..and get this compression test done asap. I'll be in touch soon as I can get these things done. But what about the fuel pressure situation? I thought it was supposed to hold pressure better than what it is..I won't retype it but I mentioned it in the posts before. But anyways thanks for the help my friend I do appreciate it!
 

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Discussion Starter · #8 ·
I'm not sure if it would help but I have an OBD2 scanner so I have the freeze frame data available if maybe it would help with a diagnosis. I'm not super experienced as far as interpretation of the data.
 

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I would recommend you get a set of ACDelco OEM plugs for your car ASAP a full set of 4 and use a good antisense on the spark plug threads…don’t get any on the electrodes.
2 get a compression check. Borrow a compression gauge from your local auto parts store such as AZone, laryCurrley&Moe, or whomever you have close and get that compression reading.
ONLY then can an educated diagnostic be made…aside from that…everything else is just spitting into the wind and wasting time.
Once you have the plugs, and have the compression reading, install the new plugs and report your findings…….good luck

PS. Did you take time to look up wasted spark? And tell me if you really want to spend an extra $200-$500 on new cats because you waited another week and 100 miles to fix this misfire.
 

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Discussion Starter · #10 ·
Absolutely not on spending the 200/500 on the cat..lol. problem is I'm financially strained at the moment because of the holiday last week I didn't get alot of hours at work. But actually the spark plugs I bought were in fact AC Delco..getting that compression check done later on today. Have a buddy that is going to help me out with a compression gauge. I'll let you know how it goes. Thanks again for your time!
 

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First things first
Your car has 150 k miles on it.
What is the compression reading dry in all 4 cylinders. Then dump 1 CAP full of oil into the cylinders and do the compression reading again, one cylinder at a time. Using the exact same compression gauge. Then report your readings.
Without this simple measurement ….your waiting time, energy, money, and….
 

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Hey guys. Been trying to track down the cause of this random misfire. It comes and goes and it seems like once the engine gets warm it is more prevalent. Alot of people have been suggesting the coil pack but I was able to get my hands on a digital multimeter and unless I did something wrong readings were normal although I've also heard they can still be bad with a normal ohm reading. I've tried to cover all the basic/ simple things that could possibly be the cause like spark plugs..vacuum leaks..checked and cleaned the injectors and MAF sensor. I've noticed the air filter is not very clean but it isn't severely bad. Think that's about it.. I started out with p0171 code also but I got that fixed..just can't get rid of the dreaded p0300. Any advice is appreciated..what am I missing here?
If your fuel pressure is dropping below spec look no farther until you get it up to spec. Check or replace fuel filter. If still low pressure, check regulator. Most likely A bad pump, if filter is not restricted.
 

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Hey guys. Been trying to track down the cause of this random misfire. It comes and goes and it seems like once the engine gets warm it is more prevalent. Alot of people have been suggesting the coil pack but I was able to get my hands on a digital multimeter and unless I did something wrong readings were normal although I've also heard they can still be bad with a normal ohm reading. I've tried to cover all the basic/ simple things that could possibly be the cause like spark plugs..vacuum leaks..checked and cleaned the injectors and MAF sensor. I've noticed the air filter is not very clean but it isn't severely bad. Think that's about it.. I started out with p0171 code also but I got that fixed..just can't get rid of the dreaded p0300. Any advice is appreciated..what am I missing here?
I had this same issue on #3...81,500 miles on a 2007 cobalt. Did all I could do...plugs. Replaced fuel injector harness as there was a break in the circuit. BUT...when did pressure check on #3...none! Just put in new engine a few months ago. ERR that hurt. Only thing I have now is P0442...small EVAP leak. Have replaced purge valve, still get code.. Find no other issues, but think mechanics may have pinched a line along the way on this engine install.
 

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Discussion Starter · #16 ·
Hey tbird..I've been looking in to how to read freeze frame data on my OBD2 diagnostic tool and I came across the topic of short and long term fuel trim values. I noticed that both short and long term fuel trim values on my cobalt are at zero percent. From what I can gather online this indicates that an O2 sensor isn't working. When I very first started diagnosing this problem I went to advance auto and they used their scan tool and came up with p0300 and p0171. The guy told me the p0171 was most likely my o2 sensor. So I bought a new one and put it in. (This was the upstream sensor close to the manifold) But I cheaped out and got one that you had to splice into the existing plug on the car. I know I got the wiring right because I checked it many times before I did it. But I don't know..this has me second guessing myself or maybe it could be another o2 sensor. I'm not sure if the car has only 2 of them or what bit I know the other is obviously downstream close to the catalytic converter. Anyway..the new o2 sensor did nothing to fix my problem and I ended up noticing that the flex pipe connecting the exhaust to the manifold (right close to the new o2 sensor) had a huge hole in it. So I got that fixed and that got rid of p0171. The car ran fine for a day or so until we got on the highway and then p0300 showed itself again. But anyway I just wanted to know your thoughts on the fuel trim value situation..am I even on the right track here? I've read that they definitely should not both be at zero percent. And I know you've mentioned a compression check several times and I've been waiting around for a guy at work to bring me a gauge but I guess I'm going to just have to go rent one. I ended up taking my old thermostat out and checked it by putting it in boiling water and then cold..it opens and closes just fine..and I tried a new maf sensor and that didn't help either. I was able to return it and get my money back at least though. So that's where I'm at..know it's been a while since I've responded I've been really busy.
 

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Discussion Starter · #19 ·
Man no store around me has a friggin compression gauge..pitiful. But I decided to take another educated guess and it paid off this time.. Bought a new coil pack and that did the trick. Car is running better than ever. Thanks for the advice though I appreciate it.
 
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