4Driver4

So my fixer upper saga continues. I bought the car with low compression on #6 and did a fairly thorough rebuild (pistons on up). I fired the car last weekend, and found that it would idle inconsistently, and would throw codes for open circuit to the heaters on both primary (sensor 1) O2 sensors. Of course half the idiot lights were lit as well. Hoping a drive might shake out some of the issues, I put a bunch of miles on the car Sunday. It drove great, and all the issues appeared to have resolved themselves...all but the stalling and O2 codes. Today I replaced the sensors, and the codes are gone. However, the stalling persists. Here's the deal:

If I start the car and allow it idle, it's fine. Cold, warm, AC on or off, whatever; fire it up and it idles fine all day. You can gently rev to 4k rpms and it drops back to happy idle. But if you stab the throttle or really rev it up, when you let it drop to idle it will start misfiring on one or more cylinders on bank 2 (4-5-6) before stalling. Sometimes it struggles along at lumpy idle before the misfires overcome it and it stalls. Restart, and happy idle returns until you hit the gas again.

Any direction? I have Durametric and can provide any info needed to help with the diagnosis.

Petza914

Since the cores are only on one bank, items specific to that bank are where I'd start. Vacuum leak on that side of the plastic intake manifold or that side of the plenum after the throttle body. Could also be the cam position sensor for that side. Are you getting any other codes other than the misfire codes?

4Driver4

Just the misfire codes, and only after revving the engine. If I just start the car and let it idle, it will run all day. Stab the throttle, and when RPMs drop below 2k on the way to idle the misfires start. Seconds later the car stalls. Restart and happy idle resumes.

I was thinking vacuum was unlikely because the misfires are local to one back, and because idle is so stable until revved.

4Driver4

Update:
Swapped the varo-cam solenoid from right to left and the misfires went with it (now 1-2-3) . Looks like I may have my root cause! <fingers crossed>

SoCal C2S

SoCal C2S

4Driver4

Great minds think alike?
Or am I giving us too much credit?

SoCal C2S

Doug H

Petza914

Yup, if the misfires moved, likely the cam position sensor we mentioned. Good thinking swapping it to the other side.

Wayne Smith

Well done on the diagnosis. Sounds like you've got a fix 8)

SoCal C2S

Petza914

Strange timing (no pun intended), but drove my silver 997 to lunch today, pulled up to a red light and got the Visit Workshop message once the car dropped to idle. Still seemed to run fine for the rest of that drive and on the way home. Once I get home, I hook up the durametric and see misfires on 4 & 6 with a Bank 2 Idle out of range message too. Clear the fault codes and get the message again, but this time with 4, 5, & 6 and a misfire checksum error. Track misfires by cylinder # and they're all on bank 2 and frequent. Look at camshaft deviation and they're quite a bit different with Bank 2 showing all zeros (very unlikely and probably getting no data from the sensor), so I think I had a CPS go bad today too and haven't moved it to the other bank as part of the diagnosis since my 928 is on the lift right now for a wiring project to clean up how the aftermarket stuff is wired & fused, but ordered a couple CPSs from FCP Euro since they're not too expensive, and figure I might as well replace both at the same time since they likely have a similar lifespan, but I'll do one first and see if the misfires go away, and if so, then do the second one - don't want to introduce 2 variables at the same time.

4Driver4

Closing the loop on this:
New solenoid was installed today and I've put about 40 blissful miles on the car. Next up is getting all the readiness codes to set so I can pass emissions. Also, I have about 300 miles on the break in oil, so I'm going to be changing that very soon.

Thanks to all who weighed in!

sober_owl

i was always told and have practice to start engine idle and then change oil right away since most of the stuff will "break in" on 1st start.. then with new oil drive car a few hundred more miles then change oil again..