Pls Help Diagnose Misfire - Chevy Impala Forums
Chevy Impala 8th Gen Discussion W-Body 2006-2013 (2014+ Limited). Discuss all Chevy Impala 8th Generation Performance and Technical Discussion here.

User Tag List

 12Likes
Reply
 
LinkBack Thread Tools Display Modes
post #1 of (permalink) Old 04-25-2019 Thread Starter
Junior Member
 
DonH's Avatar
 
Join Date: Oct 2018
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Thanks: 0
Thanked 0 Times in 0 Posts
Pls Help Diagnose Misfire

2008 3.5L Flex Fuel Impala. P0303 misfire. Watching mode 6 data, I can see that it is worse at idle, very little or no miss at higher RPM. Car has 144k mi, owned it only for about 6 months, prior history not known. Misfire started a few weeks ago. Had an old school mechanic look at it and he was stumped and wanted to tear into the heads, thinking it could be a weak valve spring or related. He is old school and didn't put a scope on it. I hate to throw parts at it not knowing exactly what it is. He said he did leakdown on #3 and it was ok.

So, I just replaced spark plugs and will post pics shortly. All plugs look well worn, could be original, gaps way over .040. Plug #3 has major wear/burn on electrode. #5 also has similar wear. Still misfire.

My hunch is the #3 injector. How can I test injectors easily. Does it make sense an injector would fail at idle but operate fine at higher rpm?
DonH is offline  
Sponsored Links
Advertisement
 
post #2 of (permalink) Old 04-25-2019 Thread Starter
Junior Member
 
DonH's Avatar
 
Join Date: Oct 2018
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Thanks: 0
Thanked 0 Times in 0 Posts
Here are pics of the plugs. Both old and new are AC Delco 41-100 Iridium
Attached Thumbnails
Click image for larger version

Name:	2008 3.5 Allsixplugs2.jpg
Views:	25
Size:	420.0 KB
ID:	145441   Click image for larger version

Name:	2008 3.5 wornplugs3.jpg
Views:	30
Size:	257.7 KB
ID:	145443  
DonH is offline  
post #3 of (permalink) Old 04-26-2019
Senior Member
 
1970cs's Avatar
 
Join Date: Dec 2015
Location: Michigan
Posts: 346
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 39 Post(s)
Thanks: 10
Thanked 51 Times in 41 Posts
Send a message via MSN to 1970cs
Here is some ideas.
https://www.engine-codes.com/p0303.html

Pat
1970cs is offline  
Sponsored Links
Advertisement
 
post #4 of (permalink) Old 04-26-2019
Senior Member
 
1999 White C5 Coupe's Avatar
 
Join Date: Jul 2014
Posts: 1,328
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 193 Post(s)
Thanks: 107
Thanked 225 Times in 202 Posts
Does the misfire cause a flashing check-engine light? Or, does it turn on the light?

I would use two rounds of Techron Fuel System cleaner, according to label directions. Techron makes the cleaner under license for GM, which is sold as GM Fuel System cleaner.

The cleaner will help the fuel injector tips, if clogged or dirty.
1999 White C5 Coupe is offline  
post #5 of (permalink) Old 06-01-2019 Thread Starter
Junior Member
 
DonH's Avatar
 
Join Date: Oct 2018
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Thanks: 0
Thanked 0 Times in 0 Posts
Thanks for the replies.. Just an update, a second mechanic thinks injector, but didn't really do any tests other than listen to what I've had done before. I did run one bottle of Techron through the gas in addition to replacing plugs. Misfire occurs at idle only sometimes and does turn on the CEL (not flashing) but it also goes off sometimes. You can see the misfires on #3 while watching mode 6 scan data. At higher rpm (say 1000) there is no misfire showing in mode 6 data.

Could it be a dirty injector that misfires only at idle? I might run another bottle of Techron through it. If I do have to get down to the injectors, I might just replace them all, but its such a pain I'd like to confirm that is the problem before I tackle it. JUst an fyi, this is a FLEX fuel engine (Vin K) so I suspect the prior owner ran E85.

On a side note, does anyone know if the ECM is programmed differently for Flex vs nonFlex fuels? or is it just internal engine parts that are different?

Thanks all!!
DonH is offline  
post #6 of (permalink) Old 06-01-2019
Senior Member
 
Bart77's Avatar
 
Join Date: Sep 2012
Location: New Garus, Wisconsin
Posts: 611
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 37 Post(s)
Thanks: 22
Thanked 199 Times in 147 Posts
Quote:
Originally Posted by DonH View Post
Thanks for the replies.. Just an update, a second mechanic thinks injector, but didn't really do any tests other than listen to what I've had done before. I did run one bottle of Techron through the gas in addition to replacing plugs. Misfire occurs at idle only sometimes and does turn on the CEL (not flashing) but it also goes off sometimes. You can see the misfires on #3 while watching mode 6 scan data. At higher rpm (say 1000) there is no misfire showing in mode 6 data.



Could it be a dirty injector that misfires only at idle? I might run another bottle of Techron through it. If I do have to get down to the injectors, I might just replace them all, but its such a pain I'd like to confirm that is the problem before I tackle it. JUst an fyi, this is a FLEX fuel engine (Vin K) so I suspect the prior owner ran E85.



On a side note, does anyone know if the ECM is programmed differently for Flex vs nonFlex fuels? or is it just internal engine parts that are different?



Thanks all!!
The difference between flex and the non-flex fueled engines are that the flex fuel engines have wider injectors to allow for more fuel flow and ECM calibrations to see the ethanol content in the fuel and adjust the spark timing and fuel delivery accordingly. Everything else from the fuel tank, pump and engine internals are all the same. If the previous owner did run E85 it would not be responsible for the issues you are having.
I have a '08 3.5l flex and I run E85 and swap back to E10-E15 all the time with no issues. I've got 179k on her now and I haven't even done a tune up yet! I've had the car since 43k miles.

For your issue with idle misfires it could be a partially clogged/dirty or sticking injector as long as spark is good and compression is good. A couple of ways to check it is doing a cylinder balance test or using AFIT, active fuel injector tester (most GM dealers should have one). This tool will test fuel pressure, injector resistance and flow rate to find an injector that isn't flowing properly.

Sent from my SM-G930R4 using Tapatalk
plano-doug, sheila and rjayl like this.

Last edited by Bart77; 06-01-2019 at 07:02 PM.
Bart77 is offline  
post #7 of (permalink) Old 06-01-2019 Thread Starter
Junior Member
 
DonH's Avatar
 
Join Date: Oct 2018
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by Bart77 View Post
The difference between flex and the non-flex fueled engines are that the flex fuel engines have wider injectors to allow for more fuel flow and ECM calibrations to see the ethanol content in the fuel and adjust the spark timing and fuel delivery accordingly. Everything else from the fuel tank, pump and engine internals are all the same.
Bart77... So on this model of Flex engine, is there a Fuel Sensor in the fuel line someplace to determine if E85 is in the tank?? I don't seem to see it, but on the later GM Flex vehicles there is one. If none, how does the ECM know its E85 in the tank?

Thanks

Last edited by DonH; 06-01-2019 at 08:48 PM. Reason: typo
DonH is offline  
post #8 of (permalink) Old 06-02-2019
Senior Member
 
Bart77's Avatar
 
Join Date: Sep 2012
Location: New Garus, Wisconsin
Posts: 611
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 37 Post(s)
Thanks: 22
Thanked 199 Times in 147 Posts
There is no sensor. This model uses a less instant method of determining the ethanol content. Instead of a flex fuel sensor it uses the oxygen sensors and a special algorithm to make the determination of ethanol content. The ECM only starts making this ethanol calculation after the fuel tank has been filled with at least 3 gallons of fuel. After this it takes at least 7 miles of driving, during the drive cycle after the fill up, to figure out the ethanol content. Until the ECM figures it out the engine may run rich or lean for a while after swapping fuels.

Sent from my SM-G930R4 using Tapatalk
plano-doug, sheila and DonH like this.
Bart77 is offline  
post #9 of (permalink) Old 06-02-2019 Thread Starter
Junior Member
 
DonH's Avatar
 
Join Date: Oct 2018
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Thanks: 0
Thanked 0 Times in 0 Posts
Thanks. It is great to have someone with your knowledge on this forum.

Followup question. I saw online that there might be a pid that reports what the ecm is seeing for the alcohol ratio. I haven' checked my torque app yet. Is that true?
DonH is offline  
post #10 of (permalink) Old 06-02-2019
Senior Member
 
Bart77's Avatar
 
Join Date: Sep 2012
Location: New Garus, Wisconsin
Posts: 611
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 37 Post(s)
Thanks: 22
Thanked 199 Times in 147 Posts
Quote:
Originally Posted by DonH View Post
Thanks. It is great to have someone with your knowledge on this forum.

Followup question. I saw online that there might be a pid that reports what the ecm is seeing for the alcohol ratio. I haven' checked my torque app yet. Is that true?
Yes there is. On a Tech2 it would show the ethanol percentage, if I remember correctly, it is in the ECM menu then either the module setup or special functions menu then ethanol reset menu. For those out there with a non-E85 compatible vehicle you can see the percentage as well. You just need to build the vehicle as a K motor vs a N motor in the Tech2.

Sent from my SM-G930R4 using Tapatalk
sheila likes this.
Bart77 is offline  
post #11 of (permalink) Old 06-07-2019
Member
 
sidemouse's Avatar
 
Join Date: Feb 2019
Posts: 76
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 14 Post(s)
Thanks: 8
Thanked 6 Times in 6 Posts
Unfortunately all I have to go by is the 2005-2007 Impala 3.5L so I hope we're on the same page...
Because before doing much of anything else I would see if the spark plug wire is at fault...
However at some point past 2007 Chevy stopped using wires...

Either how, the number THREE cylinder would be the CENTER BACK one.
Just in case you didn't know this, I do presume that much hasn't changed, I also wouldn't rule out that just one spark plug has gone bad...
Might even be as simple an issue as a LOOSE spark plug wire, I'd definitely make sure with the below diagram that it's on the plug and the coilpack good and snug...
Basically the firing order on the V6 (and I'm pretty sure this hasn't changed) goes:
Attached Images
 
sidemouse is offline  
post #12 of (permalink) Old 06-08-2019
Senior Member
 
hatzie's Avatar
 
Join Date: Nov 2009
Location: Baker River Valley, NH
Posts: 921
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 149 Post(s)
Thanks: 80
Thanked 146 Times in 128 Posts
Notice the 1 & 6 plugs are not exhibiting ground or spark electrode erosion and appear to have a disc bonded to the ground electrode. New plugs will have this disc in place.
Plugs 3 & 5 have significant ground terminal erosion and no disc. Someone might've attempted to gap the plugs and dislodged the disc.
Plug 2 has spark electrode erosion and corresponding wide gap. Not sure what happened here... time and miles likely.

Looks like you've simply reached the end of the useful service life on your spark plugs.

Get six AC Delco 41-110 Iridium spark plugs and install them. Or six 41-962 Platinum... I believe the 08 came with Iridium spark plugs. Don't overthink things... Use the same spark plug metal type that your engine came with from GM. Iridium causes spark issues in the T400 trucks and the Gen III LS motors in the T800 trucks.
DO NOT GAP the plugs. They are factory gapped... the iridium or platinum disc can separate if you monkey with the ground electrode.

Get either Delphi XS10544 or Denso 6716258 wire sets for your 3.5L. AC Delco wires are made in China now. Delphi and Denso actually still make a quality product and Delphi is likely what she was wearing when she rolled off the line.

Wipe a dab of non-conductive silicone Di-electric grease inside the end of the coil and plug wire boots.
I usually put a thin layer of never seize in just the plug threads with a plumbers acid brush. This is very conductive so you absolutely do not want to get this on the ceramic or the not-threaded metal bits so be careful with it.

2009 Impala SS
"Speed never killed anyone. Suddenly becoming stationary... that's what gets you." Jeremy Clarkson

Last edited by hatzie; 06-08-2019 at 12:13 PM.
hatzie is offline  
post #13 of (permalink) Old 06-17-2019 Thread Starter
Junior Member
 
DonH's Avatar
 
Join Date: Oct 2018
Posts: 7
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Thanks: 0
Thanked 0 Times in 0 Posts
Thank you Sidemouse and Hatzie. As noted in the original post, I did replace the plugs with the same iridium plugs. Existing plugs were original. I did swap the #3 wire with another and still saw #3 misfire on mode 6 data, only at idle. At 1000rpm or greater it does not misfire. Go figure.

Now trying Techron and will try BK44 to try and clean injectors. Hopefully it helps. CEL comes on and goes off, depending on how much idling is done. No flashing CEL. Also might try spraying some water around the coil and wires to see if I can determine if there is some arcing. I could throw new wires at it, but resistance was normal and swapping didn't help. Also thought of trying a new coil, but spark looked strong. Just got a scope, so I might do some testing to see if something looks unusual.

I like Bart77's idea of the GM AFIT (active Fuel Injector Tester) to test injectors. Not easy to get to injectors, so I'm hoping to determine misfire cause before throwing new injectors at it.

Thanks all and keep the ideas coming.

Last edited by DonH; 06-17-2019 at 03:30 PM. Reason: addl info
DonH is offline  
post #14 of (permalink) Old 06-18-2019
Junior Member
 
dtsimpala's Avatar
 
Join Date: Oct 2017
Location: South Carolina
Posts: 29
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 11 Post(s)
Thanks: 4
Thanked 3 Times in 3 Posts
I've had the same issues with my 2012LT ~178K miles. You can see my thread history which is me flailing about trying to pin down what was going on. The only thing I know for a fact is that on my vehicle replacing the EVAP purge solenoid regularly seems to help and fiddling with the front PCV valve thing seems to help. Also, I can reproduce the misfire if I have half a tank of gas, and I make a hard left turn from a dead stop.

Let me know if you ever nail down whatever it is that causes our engines to miss.
dtsimpala is offline  
post #15 of (permalink) Old 06-18-2019
Senior Member
 
sheila's Avatar
 
Join Date: Sep 2013
Posts: 1,576
Mentioned: 4 Post(s)
Tagged: 0 Thread(s)
Quoted: 163 Post(s)
Thanks: 311
Thanked 300 Times in 235 Posts
^ What/where is this front pcv valve thing you speak of? And are you talking about the underhood purge valve or the vent valve located near the fuel filler pipe?
sheila is offline  
Sponsored Links
Advertisement
 
Reply

Related Threads
Thread Thread Starter Forum Replies Last Post
Misfire after replacing oil pressure sensor on 07 LTZ Bobo1516 Autocare and Maintenance 0 03-03-2019 04:06 PM
2000 3.8 series 2 misfire. I'm in need of some input please screwlegg Chevy Impala 7th Gen Discussion 3 12-07-2018 10:47 AM
Intermittent misfire and lean fuel jdeville 9th Gen Engine Performance 0 04-30-2018 12:22 AM
Misfire #4 cylinder Bigbubble95 Chevy Impala 8th Gen Discussion 8 01-03-2018 03:51 AM


Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevy Impala Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome