In April of this year I bought an '01 6M vert with 130k+ miles. I took it to an out of state forum member's shop who did a lot of overdue maintenance on it --- too much to list unless needed to help diagnose. They also installed an AA supercharger for me. Everything was great.

Now within the last couple of months the car has a misfire or rough idle. It has the following symptoms:

After warming up, just idling in the driveway or at a stop light it misses/runs rough. Not enough to stall the car but it's not smooth. Driving through my neighborhood trying to keep a steady 30-35 mph speed it doesn't "ride" smoothly. What I mean is when I alternate between slight gas pedal pressure and easing that slight pressure to keep a steady speed the ride is very jerky and erratic. It's much more noticeable at lower speeds and lower gears but at times I can notice the behavior at higher speeds/gears.

I took it to a local shop recommended on these forums to get a dyno tune and to have them fix the overall roughness. The plugs and wires are all brand new. The valve springs were changed as a result of being on the dyno and seeing a big dip/sharp incline at a high rpm but the tune itself could not get rid of the rough idle/running. I haven't gotten it back yet but what they believe to be the problem is the following:
"One problem is the pcv system is incorrect for the car. The pcv was altered during the supercharger install. This is allowing unmetered crankcase air to enter the intake manifold. This causes fouling of the plugs and o2 sensors."

Anyone else run into this kind of issue?

 

As soon as they changed the plugs it should have been fine, IF they honestly say it's fouling the plugs and O2's, so that isn't the trouble since you still have the issue.

It is pretty basic if you have a rough idle, i.e. either a vacuum leak, bad coil, bad plug, bad spark plug wire, or injector. Knock these out one by one and you will cure your issue.

 

They did a "smoke" test, I assume to look for vacuum leaks, that didn't find anything. The pcv system diagnosis is the most recent thought. They originally thought it was the plugs as the plugs that were in there were the ones that came with the AA kit. This local shop doesn't like those plugs and recommended the new ones believing it would fix the rough idle. Unfortunately it did not.

Under moderate to heavy gas, the car takes off like a bat outta hell like it should since adding the SC. I mentioned the possibility of it being an injector issue but they like the pcv system for the problem.

 

Is the crankcase vented from the passenger side valve cover to a small air filter? Normally the engine oil cap is modified to run a hose to the engine intake filter, but I didn't want any crankcase air to enter my intake charge air. I pulled the hose out of that engine filter and installed a small filter separate from the engine filter so NO crankcase air is used for the engine.

I wouldn't even think the PCV is the trouble but who knows? If they think it is, at least they can fix it for you with very little trouble.

 

Superchargers require a hotter plug. I'd think the installer used the correct plugs. Now another shop knows better? It should still run ok though. Without seeing the pcv system its hard to say but many c5's have oil in the bottom of the intake as a result of how the pcv vents. I think a catch can would help.

Did you have upgrade work done on the fuel system?

To me it sounds like an issue with the tune more than anything else.

 

This other shop is a performance shop as well and just preferred a different plug made for SC systems as well and since the original plugs were fouled they had to be changed in any event. The original shop did upgrade the fuel system. I don't remember exactly what was done as my documentation is with the car currently.

 

Info from mechanic about what has been done to find the misfire:

Misfiring was mostly on cylinders 1, 2, 7 and 8. Oddly enough, all of these cylinders are in sequence together in the firing order. So, it was not related to one cylinder or one bank of cylinders. Misfiring has 3 major components.
1. Compression

2. Fueling

3. Ignition

This even goes back to the last time you were here. Pulled the spark plugs and they were very black and of the wrong material. Most shops will install the NGK TR6 Copper Plus because they are cheap. I use the factory iridium spark plugs from the Corvette ZR1. The coil packs and ignition system were designed for Iridium and not Copper. Iridium last 5 times longer and has lower resistance. Copper plugs runs the coil packs harder and can lead to charging system faults. We also performed an engine compression test and leak down test. The compression test passed and the highest cylinder was 190 psi and the lowest was 175 psi. The allowable range is 15% (28.5 psi) and yours passed with 8%. Great for a 133k Mile LS1. The leak down test check for valves that might not be seated and any other small leaks. The engine is pumped to 100 psi as the standard. Then, we take a measurement of the leak down based on zero leak down holding the entire 100 psi. No engine does that and the accepted range is 20% loss or 20 psi. The gauge has green, yellow and red zones and yours fell well into the green zone. So, the shows the engine is in good mechanical order.

Next was to move on to fueling. First thing I usually do is to put the car into open loop. This temporarily disables the Oxygen Sensors and the misfiring was eliminated. Put the car on the lift and removed the front oxygen sensors and these are the closed loop feedback sensor for the fueling calculations. Upon inspection, these were very black and potentially fouling out. Those were replaced with AC Delco Oxygen Sensors. The retest showed the misfiring was better but, still present. As another test, we tested the engine back pressure with a manometer. This test checks for clogged catalytic converters and you have almost zero backpressure and the catalytic converters passed.
Another improper component was the PCV System and was installed incorrectly. The PCV system pulls fresh air from the intake tract and enters the crank case. The dirty air exits and gets consumed by the engine for an EPA Closed Emission System. Since the supercharger was installed, the PCV was incorrect. The supercharger system requires the fresh air input to be moved to the oil fill cap and the 3/8” hose then leads to the air filter. Filtered air enters the system at the oil cap. This particular Corvette had an upgraded valley cover that did not come with this car. GM uses 3 different valley covers.
1. 2001-2003 LS6

2. 2004 LS6

3. 2005-2013 LS2/LS3 and LS7

These valley covers are very different and we did not know which one you have. In order to determine which version you have, we had to remove the intake manifold. We removed the valley cover and you have the 2001-2003 LS6 Valley Cover. This one has the oil/air separator on the bottom side, has provisions for knock sensors and does not have an internal PCV valve/orifice. I ordered a new PCV hose and PCV Valve from GM and now this correct. Not having the correct system causes idling issues and unmetered dirty air entering the system. The PCV regulates flow based on engine vacuum. Without the valve, these is always full vacuum on the crankcase. The intake manifold was cleaned and cleaned all of the sealing surfaces. Prior to removing the intake manifold, we also smoke tested the intake tract and found no leaks.

The engine continues to misfire and we will continue on. The next fueling component is the MAF Sensor. We have a spare in the shop and once it was replaced, it still misfires. We even thought that maybe the supercharger system was dragging down the engine and as the next step, separate the intercooler discharge pipe from the MAF Sensor and start the engine. Letting the engine reach full operating temp, the misfiring continues.

Moving on to ignition, we have spark coil pack rails at the shop and replaced both banks. Oddly enough, still misfiring. We used a test light to check ignition spark and injector electrical pulses. All coil packs and injectors tested good for signals from the PCM. Once of the last resort items is the PCM. We have an extra PCM at the shop and replaced yours with a different PCM and loaded your current file. Still misfiring and we reinstalled all of your original components. You can see we are running out of ideas. Each time a major component was replaced, I performance a crank relearn and to no avail. We disconnected the Engine Camshaft Sensor and that had no effect and engine will run without it. It determines which stroke the engine is on, intake or exhaust. That was reconnected and back to the misfiring diagnosis.

As a last ditch effort, we replaced the fuel injectors. The car is much smoother and we may have found the culprit. We were getting low on fuel and filled up with Shell 93. Drove the car around and seems much better and even twisted it up to 6,000 rpm and no issues. Came back to the shop and loaded it on the dyno. After some tuning, it made 553 rwhp on 10.5 psi of boost. Last time we were on the dyno, the car will completely cut out at 4,500 rpm. That could have been due to a couple of issues, weak valve springs and bad injectors. The engine pulled all the way to 6,000 rpm.