Hey guys,

Car is an 06, Auto, with 90K on the clock. Mods are a custom tune, LG LT Headers, and No CATS.

I had pep boys do a simple install of plugs on my vette because I couldnt get my fat hands between the primaries to get a few of the plugs started by hand. Long story short, and multiple trips to pep boys the last 4 days, I have a P0300 code.

Plugs were: AC Delco
New plugs are: Bosch +4 Platinum (4458)
Wires: AC Delco (not changed during the spark plug replacement)

Car doesnt idle harsh, occasional misfire can be felt if im on a hill at a stop light, misfire occurs at say HWY speed let off gas, hit it about 1/2 to WOT throttle and the engine shakes violently. Everytime this happens. Cruising with little throttle no shaking.

Ive searched the forum extensively and found various "causes".
1. Clogged CAT - not my case I have none
2. Bad plugs (Pep boys swears these should work, but cliam should have stuck with OEM
3. Bad wires
4. Battery dying
5. Exhaust manifold leak.

I checked the wires and the first cylinder wire has a rating of ~250ohm and the rest are in the ~750ish range. I read somewhere the wires are from 250-750 ohm, so is that 1st wire bad?

Plugs should work according to Bosch, and are in the right heat range. Battery is fine considering voltage. dont know how to check for an ehaust manifold leak.

Have no way to vaccum check. Could the 1 wire reading 250 ohm be bad, I always assumed it had to read 0 if it was bad, and thus somehow give a P0300 code without say giving me a P0301 (cylinder 1) code?

Thoughts? Im at my wits end with pep boys to say the least.

 

0300 is misfire. You changed the plugs. Bad plug, damaged plug wire (yanked off by wire and not boot), cracked plug, not connected at coil.

Pep boys? I think that is the source of your problem. LG LTs? Did the Pepboys move the wiring around? Did they put the O2 wiring on to the headers and burn them?


Get a scan (EFILIve or the like) and see what cylinder is giving the code.

Elmer

 

If all else fails, wait until tonight and put the car in the garage. Start it up, turn off all lights in the garage, make it dark in there and look at the engine while it runs. If you see the aurora borealeus in there, you will locate your issue.

 

Thanks Elmer, I checked the plugs last night both the O2 sensors and they look to be fine considering the wiring harness, the harness isnt touching the headers. They didnt remove the LG LTs to install the plugs, their tech has small hands. As does the wiring harness on each engine bank which sits just above the heads, which i assume handles the coil packs. Plug wires are in good condition minus the issue with the OHM rating (250 ohm) on one of the wires.

 

Bosch Platinum Plus 4 might be your problem...I put a set in my 2011 GMC Sierra 5.3 (similar engine) and it set codes. I removed the plugs and replaced with the originals and it was fixed. Just my experience.....

 

UPDATE:

Installed new AC Delco wires and did as Eboggs suggested to run the car in a pitch black garage. Didnt see any sparks flying then again the metal boots are covering the wire and plug. So I assume that its fine I didn't take the metal boots off and reinstall the wires when doing this check?

Ran the car home as hard as I could trying to replicate the terrible shaking under load. I didnt feel a single miss, no codes (then again the CEL didnt always activate when there was a miss i could feel. If I come back tomorrow with more issues, we know the plugs are next for replacement.

Thanks for the help guys!

 

I suggested that you check the wires for sparking in the dark. That would show is you had a loose connection or cracked wire. Changing the wires pretty much eliminates the bad wire.

I also suggested that you get a scan of the car to see what cylinder is giving the code.

You might have fixed the issue with the wire change. It could have been a bad wire or loose connection at the plug or coil. If the code didn't pop up and the car is running smooth, what's to fix? Sounds like the problem is gone. You are a great mechanic!!!

Elmer

 

Sorry I have not read all of the posts on this thread but:

GET RID OF BOSCH PLUGS ON A VETTE!!!

I had the SAME scenario with my car. I was backfiring like mad. The reason turned out to be the new Bosch platinum +4 plugs that I installed.

Replaced them with the old (used plugs that I removed) AC Delco's and the car went right back to normal. I ruined my expensive cats before I figured out what the problem was. Hopefully it is not to late for you.

 

Ok so now I read all the posts. Yup, totally agree. I lived in Twentynine Palms, CA when this happened to me. I gave away those Bosch plugs to some dude with an import for free.

 

I like this post.

San

 

I just had this happen on my 09 base c6. Took the fuel rail covers off and found a wire off the coil pack. Gunna order some new wires just to be on the safe side

 

A few years back in my wife's 4.2L Trailblazer I did a plug change and got a misfire code. After doing a lot of troubleshooting I reluctantly bought a set of delco plugs and put them in. That took care of my misfire issue too. I would have never guessed that would have done it.

 

My po300 was the torque tube Its a automatic manuals are differant

 

Date of original post was in 2012.

 

My torque tube seems to have a little play. What in yours caused the random missfire code?
Thanks

 

the rubber connectors went bad causing a vibration that made crank sensor to be erratic at idle the torque tube turns even in park on a auto !!!

 

That is 100% the issue I'm having. I have a little play that feels like a ujoint on a car. I checked the torque tube and can move the flywheel side a 1/2" without the back moving. I was wondering if that caused my issue. Its mainly rear cylinders at idle. Good thing is I already have new plugs, wires and fresh cleaned injectors due to this. Looks like I will be dropping the drive train.

Thanks

 

do you have a clunk sound when you move it into gear ?

 

p0300 is a random misfire count. misfire dtc’s are generated from the crankshaft position sensor. Doesn’t mean the sensor is bad, but sometimes a sensor relearn is needed. also, this dtc can be from certain drivetrain vibrations.

you also need to find out which cylinders are misfiring. The individual cylinder misfire count has to exceed a certain threshold to give a specific cyl misfire. the p0300 will sum misfires from several cylinders and when that sum exceeds the threshold the dtc is shown. Do you have a check engine light too? if not then vibration solving, and CPS re-learn might help.

So you can change plugs, wires, coils , injectors and never solve the problem. That’s called firing the parts cannon.

 

Did this resolve your issue ??