Hey everyone. I've posted in Corvette pages on FB and asked local car pages, this is my last ditch effort. I picked up an 01 Z51 a few weeks ago. Mod list:
- MP112 magnacharged
- green top Injectors
- Pacesetter LTs
- Pacesetter catless X-pipe
- Borla stinger rear section
- Vararam intake
- MSD coils
- Taylor 10.4 wires
- NGK Tr6 plugs

About 2 weeks ago I experienced a flashing engine light after a hard pull. The only code to come up was P0300 (random misfire.) After using a Tech2, cylinder 8 was the main culprit. We swapped coil 2 and 8, thinking it was the coil. After test driving, there was no misfire.

As I drive it to work every day, the engine light will occasionally come on and make the car run very rough/sluggish. However, even while driving occasionally, the light will shut off on it's own and smooth right back out. I havent had it happen while it's cold, it seems to only be when warm. Shutting the car off will sometimes resolve it, other times not. Sometimes sitting it off and turning it right back on makes the light come on if it wasnt prior.

I took out the plug wires to check the resistances on them and noticed the plugs were barely finger tight and could be wiggled. I put in a fresh set if NGK tr6s and took it for a test drive. Ran fine for 10 minutes, including a couple hard pulls. The light then came on and cylinders 2 and 7 were the only misfires with over 2,000 misfires each in a 10 minute drive. The other cylinders had zero. After getting home, I swapped coil 2 and 4 to see if the problem followed the coil. After swapping, this is what happened (see image.) Almost every cylinder was misfiring.

This tells me it is probably not the coil, as it seems to jump from cylinder to cylinder. I am desperate for help and have heard every possible cause from plugs to coils to crank sensors to cam sensors to blown out cam lobes. As I said, the p0300 is the ONLY code and sometimes it runs like a top. So to me, this says it should not be something as serious as those mentioned? A crank relearn was performed with no effect.

I am open to any suggestions.

 

should check the injector plugs ???(our 02 Z had the same code and all it was the # 1 injector was not plugged in tight ??)

 

Injector plugs are tight, just went out and checked.

 

at the drivers side rear, the harness can rub against a bracket back there, known issue ????

 

Can you explain that a little more and ill check?

 

 

Sometimes a misfiring cylinder can cause others to misfire since they are detected from the crank sensor. All we can do is offer suggestions as to what may the cause. Without specialized equipment like a scope all you can do is try swapping parts...the coils would be the last thing I would change. What do your long terms fuel trims look like ??...an injector misfire would cause your long terms to be pretty high...if you haven’t swapped injectors I’d try that too...as a quick method to rule out anything mechanical you can do a “clear flood crank” by holding the gas pedal to the floor and try starting the car...listen if the cars cranking sounds even...if it sounds uneven you may have to do a compression test...I don’t think I see your location in your profile...this really helps you...a member a few weeks ago had some issues and all I knew that he was in my state..,he actually lives in my town...you say it happens when warm...warm as when it goes into closed loop or the engine has been running for 5-10 minutes ??

 

I had a hard to diagnose P0300 code for a while. Would get a blinking CEL. Did all of those same repairs and fixes (plugs, coils, injectors, fuel rail, wires) and finally took it to a mechanic. They diagnosed 2 broken valve springs and a damaged valve. Once those were repaired, car ran smooth with no more problems.

 

I recently had the same issue but mine was favoring cylinder 7 but all cylinder were effected. My problem was 2 fold. 1) the wires on the spark plugs had small tear and there was a trace of singe where there was a random spark that was part one of the misfire. Obviously, we changed that wire. Later we replaced all of the as an added measure of reassurance.

2) the second thing we found was that I had a few loose ground wires. Nothing serious but loose enough that on a hard pull (as you describe) with high RPM the wires would rattle enough to cause a misfire. We didnt know which word it was because we checked ALL of them and tightened each a bit. I suspect your issues are similar. THAT resolved my problem but we also changed all the coils after that happened as an added measure of reassurance again because the car had 76k miles on it.

Good luck,

C5Z06CE

 

Any chance there is a thread or link to where I can locate all of the grounds?

 

It mainly happens after 10-15 minutes after driving. The other day I drove home from work (~20 minutes) with no issues. After getting home I hoped back in about 5 minutes later and had the flashing engine light and rough idle immediately after starting it.

I have also updated my profile.

 

Very basic but gives you a idea where to look, google c5 corvette ground locations, there's a few links with close up pics. google search works better than the forums search

 

My mechanic did my grounds while I was at a meeting. Here is what I found online.

 

did you check the wire harness on the drivers side? Ithe back of the harness cuts into the metal bracket..those wires are for the injectors, coils..

 

when you put the new plugs in what gap did you set them at?... check both ends of the wires and make sure they have solid connection with the plug and coil ends and also check the resistance, I have seen msd wires measure over 2k ohms right out of the box which is ridiculous for a wire that's so short... I know you don't have msd wires but still a good thing to check and rule out... the msd coils are also known to be overpriced junk, maybe not what you want to hear but you bought the car that way so it is what it is... if it were me I would find a known good set of oem coils and ditch the msd's

 

Well i have been chasing misfires and nutty fuel trims..no codes. So i pulled my intake and i found this gaskets on all the ports were gone..the intake to the head was the seal..and here is after the new gaskets! Idel is smooth car now has power it was running rich..

 

Now that I'm looking at the misfires counts more closely I agree about some harness chaffing at the back of the drivers side fuel rail...seems like cylinders 1,3,5,7 have the most misfire counts with cylinder 6 thrown in for good measure...what do your long term fuel trims look like on either bank ??????????????????

 

Seems like you have checked both ignition and fuel without any luck
Have you ever preformed a "crank angle re learn procedure" ?
Anytime you swap cams or have the engine apart this is a must !! Anyone with a TECH II or similar device can do this as well as HP Tuners or EFILIVE-----It's worth a try as this sensor makes sure the crank is at TDC----

 

Supposedly the engine has not been touched, but we did to a crank relearn with no luck. I will be purchasing new coils to replace the MSDs and see if it helps.

 

I know that there have been many failurea of the early MSD coils it could be your problem---
PS the stock GM coils are very powerful and have been used even on boosted engines--A good trial and error would be to change back to the stock coils----