C_spencerr

So we'll start from the beginning. As I'm cruising at about 70mph steadily, car seemed fine. Then went to give it some gas in 6th to pass a car and well, the car felt different as if it was struggling. Listened to the car, it had a misfire. Pulled over, turned it off and let it cool down. Start it back up, cylinder 4 wasn't firing and the idle was surging from 1000-1200 to about 2000 every few seconds. Thought maybe it was a vacuum leak for the idle problem, but nope. Got it towed to a shop near by that specifically works with Corvettes and Camaros. Oil pressure is great (30-35 psi @ idle), no CEL or any codes, nothing other than the idle surge and misfire. Checked for spark first at that cylinder and it was getting spark no problem, but soon after car went into "reduced engine power" mode. Didn't think much of it yet since I figure with starting the car to check for spark it was just sensing that the plug wire wasn't connected. So, we plugged the wire back onto the plug, and then decided to pulled passenger side valve cover. Low and behold, broken rocker. Luckily, I had an extra one from when one of the rockers broke sometime ago in the past and it happened to be the same side and it was just a simple unbolt broken rocker, replaced with new. So we replaced the rocker, tighty everything back up, and good to go right? NOT.

After starting the car once rocker was replaced, now the car is still in limp mode or as the car says on the dash, "reduced engine power" along with something with traction control. The idle isnt jumping around anymore so no surging. But also, get this.... now oil pressure gauge is reading at 10 psi @ idle. Also, CEL is throwing a couple codes.

U0001
P0107
P0641

We retraced our steps to make sure everything was plugged in, everything was seated alright, tried disconnecting the battery to reset things, nothing. Only thing we touched was what was needed to get the valve cover off to replace that rocker. I'm at a loss for words right now, I don't even know where to begin. I just don't get how the car was misfiring/idle surging with perfect oil pressure with a broken rocker, and then with the new rocker installed, makes the car run worse in limp mode along with low oil pressure, throwing codes, etc.

Any thoughts? Going to dig into it more on Monday but any help would be appreciated.

AORoads

What does the shop that works on Corv. and Camaro say? I'll think out loud and ask, any extra pieces come unglued during the rocker breakage? Did you write that this is the second time on the same side where it broke? Is this a completely stock engine, or what are the other modifications to the engine, if any? Have you owned the car since new, and what year is it, and miles on the car?

kedar

P0641 is 5 volt reference circuit. It is a string of five or six sensors. One of the sensors is the cam position sensor that is at like the one o'clock position above the dampener (I mention this because of your rocker issue).

I chased the solution to this code in my car for months. Replaced all sensors on the circuit to no avail. Was a short in the transmission wiring harness. The transmission wiring harness has a metal bracket zip tied to it that attaches to the back of the block, my wiring harness abraded against the bracket and started shorting out wires.

kedar

The oil pressure sensor is also on the 5 volt reference circuit. Intake manifold must be removed to access. It is at the back of the block on top of the engine. You can also check the wiring that snakes around the back of the valve covers. You could be shorting something out. The five volt reference wire is gray.

C_spencerr

Sorry, little background info on the car. It's a 2005 z51 with over 90k miles on the chasis, and less than 10k miles on the motor rebuild when most of the modifications were installed. D1sc procharger, heads, cam, fast 102 intake, fly cut pistons, longtubes, catless x pipe, 1000cc injectors, meth injection, etc. Running on an speed density tune @ 6.5 psi making 634/554 on 91 pump with meth injection.

So far when we looked at it last night, it got late and we couldn't retrace what was wrong and called it a night hence why we'll mess it with more on Monday since it is Easter today. Made sure all the pieces (just the one big piece that broke off) was cleaned out to avoid anything getting inside the motor.

Ill take a look look at all those sensors. I find it hard to believe the car really has low oil pressure since it went from ~35 psi @ idle to ~10 psi @ idle in a matter of seconds from just turning the car off, replacing the rocker, and turning it back on. So hopefully it's a sensor that shorted out or something. I didn't get a chance to look at the oil pressure when the message of reduced engine power popped up right after we checked for spark before we even knew about the broken rocker, but I'm willing to bet it has something to do with that. I checked the oil, still full, just recently did an oil change not even a 1000 miles ago. No shavings or anything, still smells and looks new.

C_spencerr

So what did you do to actually fixed it? Did you replace the whole harness or just a wire or? I'm going to throw it on the lift as well as take the manifold off tomorrow and look around and see what we find.

AORoads

Good posts by kedar.

HOXXOH

This may sound weird, but I'm going to throw it out there anyway, because I don't think inside the box all the time.

When you mentioned checking for spark on #4, it sounds like you pulled the plug wire and then re-attached it.

Remembering that the problem started with an acceleration event and that's the time when iridium tips seem to break off plugs for some reason. if the tip got stuck between the valve head and seat, then the stem would fail to return to the top position, which leaves the pushrod and rocker in a partial loose condition and could cause the rocker damage.

I'd suggest pulling and inspecting that plug (maybe all the others too) and if it's not perfect, getting an internal look with a borescope for a bent valve/damaged seat/cracked piston. If you see anything wrong, the head's coming off.

I assume you checked the pushrod when you replaced the rocker, but if not, you should do it now. Take note of the wiring you move, in the event you may have damaged it during the rocker replacement time.

C_spencerr

Before I even found out about the rocker damage, I actually replaced all the plugs and wires before checking for spark. They needed changing anyways. Only plug on cylinder 4 was fouled out, per the picture above. I'm running the NGK TR6 so not iridium's.

I inspected the push rod, the valve spring, the tip of the valve, they all look perfect.

C_spencerr

UPDATE:

So started the car up again which started up perfectly fine, and saw 0 oil pressure at idle. Quickly shut the car off. After looking through everything, checking every sensor and following wires and grounds.... found the culprit. Guessing after checking spark the other day, it must of shorted out the oil pressure sending unit sensor, one of the sensors that's in the circuit throwing the p0641 code as Kedar said above. So took the intake off, swapped out the sensor, put everything back together and tada. 35psi @ idle again, not throwing anymore codes, and no more "reduced engine power" mode or any other crazy messages on the DIC. It's crazy how one sensor can throw the car off so much but luckily it was just a sensor and nothing worse. Hope this helps anyone in the future and thanks to everyone that tried to give me advice and guidance!

MYCoupe

Glad it turned out to be a rather simple "fix" and that the engine really hadn't suffered any damage.

R_W

I love stories with happy endings

kedar

Glad you ended up finding your problem. For me, after diagnosing the actual problem, the quick solution was to run a jumper wire from the front of the car near the computer to the rear. My problem was easier to diagnose after the nine volt wire shorted and I lost the ability to get into reverse (reverse lockout solenoid on a manual) and the shocks (f55). The ultimate solution was to drop the transmission and torque tube and replace the rear transmission harness. I hope that some day I'll save someone a big headache with p0641. Have not heard of anyone else's rear transmission harness shorting out like mine.