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Hello everyone. I’m new to the forum as I just bought my c6 ls3 and love it. The problem I have is a po308 code I can’t get rid of new plugs, coils, wires , injectors map, air filter. I had other codes but worked those out. The car idles rough and seems lacking on power ad surges at steady speed. Vacuum test read 17hm. Pulling the coil wire makes no difference in the way it runs. Like a dead cylinder. Any help would be appreciated.
May be a “mechanical” issue...broken valve spring, valves etc...rent yourself a compression gauge from an auto parts store and see what you have !!!...you can do a “cranking compression test” by listening to the engine crank...should sound the same on every crank...you do it by doing a “clear flood crank”...gas pedal to the floor and crank the engine over !!....and as far as the new injector did you check for power and that the PCM is grounding it...can be tested with a 12 volt test light connected to battery positive or a “noid” light !!
I pulled the valve cover and everything was tight and looked good and clean. I highly suspect the coil or injector wiring. How can I test that? Also plan on doing a comp check.
Don't know if you did this to check for spark just pull the plug wire off the coil pack and see if the spark jumps between the 2...now if it is intermittent you would really need a scope to check that out...to check the injector first unplug it and see if you have system voltage on the pink wire...now take a 12 volt test light connected to battery POSITIVE...the injector circuit is "ground side switched"...means the PCM is grounding the injector circuit...now with the engine running probe the other injector wire...if the PCM is grounding that circuit the test light will DIMLY blink...might be easier to see when it's a little dark outside !!...when I scope the injector circuit I look at both injector voltage and current waveforms (see pics)
...it tells a lot but without specialized equipment you'll only be changing or swapping parts !!...and BTW my LS1 draws about 21 in/hg of vacuum at idle...17 seems a little low....and yes I would still do a compression test and try the clear flood crank and listen for cranking inconsistencies !!
I will try your test on the injector. And double check the coil. The plug was lightly coated but still worked fine on another cylinder. I do know if you pull the coil wire when running it makes no change where it does on the others. The coil wire has power coming in
Ok, so when you pulled the plug wire and saw no change in RPM that cylinder is not contributing...just make sure that coil pack spark has somewhere to go (ground) when you pull the plug wire...I use my 12 volt test light connected to battery negative when I pull the plug wire to check. a good spark coming out of the coil pack will jump a good 1/2 to 3/4 of an inch !!....good luck in your testing !!
Ok. Checked compressional #8. It was 180. Tested injector connector also the one next to it. Both were flashing as you said. Pulled plug wire from coil and had a good blue arc.
Ok. Checked compressional #8. It was 180. Tested injector connector also the one next to it. Both were flashing as you said. Pulled plug wire from coil and had a good blue arc.
MMM ???...How did you ascertain you had a P0308 ??...you have a scan tool ??....if you do and you can tell me what your Long term Fuel Trims I can probably tell you if it is fuel or ignition related and look at your misfire counters. Even if I found a bad injector I would still want to confirm that the new one was indeed working correctly...try swapping the new injector with another...with the tooling you'll just have to do some "swaptronics" !!
When I first bought the car the stc and cel was on. I had it put on a scanner and found po171,po174 and po308. After some ridding I found a bad brake booster. That took care of the first 2. Po308 is kicking my can!! I did buy a scanner. That’s got me where I am now. I know nothing about trims but recorded these... shrtft 22-99.2%
21- .8,2.3,3.1%
12- 99.2%
11- .8-3.0-7.0%
1- 0%
2- 0%
longft.1-6.3%
2- 9.4%
Fuel system1 ol. Fault
2- ol fault
map 12.0inHg
cat temp11-901
21- 989
load 25-27%
A vacuum leak will cause a P0171/0174... getting to know your scan tool and what some of the numbers mean will help you immensely in diagnosing...does it show misfires ??...and BTW what kind of scan tool is it ??...now is the car fully warmed up here ??...the first five items I don't have any idea what they mean !!...I see your LTFT on bank 1 is 6.3%...that's OK...anything over 10% you worry....bank 2 (cylinder 8) is 9.4 %....I think it may be an ignition misfire but looking at fuel trims to determine types of misfires is still "out for discussion"... some may say with unburned fuel in the combustion chamber it will be a "rich" condition...wrong...remember your O2 sensors read the oxygen content and not the fuel !!...I also see in both banks you are in "open loop"...has the car been running for a few minutes ??...my car goes into "closed loop" in about 90 seconds or so...if it has been running for longer that may have to be looked at. Here is a good video to get you spooled up on fuel trims...crack open a beer and watch !!...this stuff has to be learned and not by turning wrenches !!...fuel trims is like a Doctor looking at your "vitals" !!
I knew there was a TSB concerning rough idle and misfires...see enclosed PDF...covers cars from 2000-2017...maybe try GM's Top End Cleaner...I did mine this spring even though I had no symptoms...I used 2 cans...sometimes there is a carbon buildup on the back of the valves especially intakes...basically it acts like a sponge for the injected fuel into that cylinder...I believe I read that the rear cylinders are more prone sometimes due to intake airflow concerns...since yours is a rear cylinder I'd feed a can into the brake booster vacuum line and the other into the PCV vacuum line...I'd suggest this because you checked and/or replaced everything that may cause a single cylinder misfire.
I watched one half of the video you sent. Very good info to have and a better understanding of how these cars think. I’ve also read through the bulletin and think at this point it’s worth a try. I’m anxious to get this resolved and enjoy the car. I’ll let you know how it turns out. Thanks for your time
Yes, you must "know your enemy" as the saying goes. Like I said see if your scan tools shows misfires and see if they are still counting up...if your issue is intermittent it's a little harder to nail down. You can pick up some top end cleaner on Amazon...let us know how it turns out and glad you learned something...diagnosis is a whole different ballgame !!
As soon as I read the bulletin I ordered 2 cans on eBay. Should be here in a few days.still haven’t watched the rest of the video but I will tonight. And amen to learning your enemy!!! I’ve always done my own repairs. But never had to work on a car that was smarter than me. Lol. The education continues !!!
Super !!....NO car is smarter than you...all the engine's computer knows are 1's and 0's !!!...and I'm sure you can count higher than 1...LOL !!....not promising this will be a fix but it won't hurt trying it !!...what scan tool do you have ??
Received 2 cans of engine cleaner today. I ran both through the brake booster vac hose. After the first can the engine died and was very hard to start. I ran the second can and started seeing black **** from the exhaust. Again the engine died. And wouldn’t restart. Pulled a plug and it was coated in black sludge. Guess I’ll clean them all and see if there’s any change
Received 2 cans of engine cleaner today. I ran both through the brake booster vac hose. After the first can the engine died and was very hard to start. I ran the second can and started seeing black **** from the exhaust. Again the engine died. And wouldn’t restart. Pulled a plug and it was coated in black sludge. Guess I’ll clean them all and see if there’s any change
You have to raise the RPM a little and like I said do 1 can at brake booster and other can at PCV...I changed my plugs when I did my top end cleaning...if you're seeing black **** the back of your valves must be pretty bad...now you may have a good spark at idle but you may be having issues at higher rpm...scoping the secondary ignition is really the way to go !!...good luck and keep us advised !!....you can consider doing a can at the front end too...it's up to you !!
The coil connector has 4 wires. Is it possible for there to be a break somewhere ? How would you check continuity in these. Where do they end. At the ecu? What do these wires do?
Po308 help!
