QM600 cam in 2008 Z06 wants to come back to life after shutoff

[DSteck]

Quick rundown on the car:

QM600 cam (out of my car)
Ported/milled heads
FAST 102 manifold
headers

I had damn near this same setup on my car and it was fine. What happens is... when you shut the engine off, it'll rev down, but then it sounds like it hits again and comes back up, then shuts off. I could have sworn I saved a log, but apparently I didn't.

If you look at engine speed when you shut it off, it'll drop down from idle almost to zero, and then zombie back up to anywhere from 300 to 700, and then shut off. What I noticed was when you shut it off, ETC TPS % drops to 3%, and then opens to 20%. When it opens to 20%, that's what the engine revs back up. I have to assume there's just fuel puddled up (ZR1 injectors, so they may be hosing down the port wall more than ideal) and it's allowing it to diesel somewhat on shutdown. The static compression is somewhere around 12.5:1.

Am I just completely missing something? Is there anything to keep the throttle from coming back to 20% on shutdown?... It just doesn't make any sense that this is the only car that does this (have done plenty other Zs with equally big cams that have no issue). I'm even more puzzled because my car didn't do it. I know C5s have a park position... is there anything available for the E38 PCMs? Max Fail maybe?

[JoeyJoeJoe]

Sounds like dieseling with a carburated engine, except the injectors should be shut off, thus one rev up and die, rather than a few bucks then die. I dont have any clue as to the throttle opening(other than the throttle opening right as it is shutting down to try and keep a dieseling engine running).

Are the injectors turning off at shut down?
Does it always do it, hot and cold engine?
Have the other big cam cars had this much compression?

If you have done a few similar with no problems, I would look to mechanical.

[DSteck]

I've done cars with more compression that don't do it. I haven't tried it cold. If you stab the gas pedal and then shut it off, it won't do it.

[G8-4-speed]

Wild guess, but is it possible you left a ground off and something is pulling ground thru a power curcuit that is causing the issue. I have seen strange stuff happen when grounds were left off.

Either that, or do another upload and re-write the tune. I have seen VCM's get stuck in a glitch that goes away after a freash read and write.

[DSteck]

It's been reflashed a few times. I didn't do the install, but no grounds should have been disconnected to do everything except the one at the back of the cylinder head, and I know that one is hooked up.

[Sherminator]

I think you nailed it with the injector concern. I would try a the stock ones and see what happens. I have heard about the LS9 spray pattern concern as well. You don't need LS9's in a n/a application. Did you have those injectors on any other cars?

[DSteck]

I think you nailed it with the injector concern. I would try a the stock ones and see what happens. I have heard about the LS9 spray pattern concern as well. You don't need LS9's in a n/a application. Did you have those injectors on any other cars?

I have those injectors on multiple other cars, all making over 600whp NA. I am not comfortable running factory injectors at 95-97% duty cycle... The ZR1s are nice because they are a direct drop in, all the data is available, and they still fall under the 63.5 lb/hr limit in the PCM. I've not seen any lean spikes on tip in, which was the concern with the spray pattern. Other huge cam cars with high compression sometimes take a little longer to shut off, but this one literally almost shuts off completely, and then revs back up. It's wild.

[Sherminator]

Yes I hear you about the drop-in simplicity. I have seen this dieseling myself as well, just not as severe. How is the a/f at idle? Any chance you can post a scan of this?

[DSteck]

It's closed loop. It idles at stoich.

[DSteck]

Have another car doing the same thing. 2008 Z06, milled heads, custom grind huge cam, and some other stuff. It wants to come back to life after shutdown sometimes. If you floor it and then shut the engine off, it won't do it. If you shut it down and immediately hit the gas pedal, it'll take a long time to shut down. I am thoroughly convinced this has to do with the fact that the throttle position is dropping to 2-3% while the engine is shutting down, but then flicks back to 20%. If the delay could be increased on cutting throttle down that low, I think this issue would go away.

There's actually an 07 with the EXACT same setup, except a 102mm throttle body... and it does it too once in a while, but nowhere near as often as the car I originally mentioned and the other 2008 car (both 90mm stock throttle bodies).

What's weird is I haven't seen this happen to 06 cars.

[IDRIVEAG8GT]

Did someone screw up and advance the cam a few degrees?

[DSteck]

Did someone screw up and advance the cam a few degrees?

The car in the original post has my exact cam from my LS7, and my car never did it. I know it's not off a tooth, too. Both the QM600 and the custom cam in the last post have advance ground in though.

[IDRIVEAG8GT]

If it's not off a tooth on the cam than either

1. You have an excess amount of fuel pooling in EVERY cylinder. (This actually should be easily rendered by leaning out the lowest portion of the MAF sensor.)

2. You have an electrical problem from ignition switch to coils. (Reset the dwell time or even better drop all the timing out of it at the lowest point in the "dying" cycle.)

[IDRIVEAG8GT]

Yup I see it, make it drop the timing out. As soon as the car jumps back to life the timing goes from 8.5 to 13 degrees. Had that problem on a forklift here awhile back.

[DSteck]

Yup I see it, make it drop the timing out. As soon as the car jumps back to life the timing goes from 8.5 to 13 degrees. Had that problem on a forklift here awhile back.

I don't think there's an issue with the MAF calibration. The 13* of timing is coming from cranking spark. I can't take that out without impacting startup. I'd bet money the throttle opening has more to do with it than timing, because I can make it happen sooner by pushing the accel pedal down after shutdown.

[IDRIVEAG8GT]

It should not affect starts hardly at all especially with that compression ratio. I keep all the LS motors that I have built and tuned around 8 degrees for startup timing or less and generally they have a 10.5:1 CR. I noticed some of them detonate on startup with 13 degrees and don't start as well when it's warm... Try it, if you don't like it you can always go back. The only other thing I can think of is that the Crank Position Sensor is not taking an accurate reading. I read somewhere that they play a big role in shutdown to prevent dieseling. How you adjust it though, I have no idea.

[DSteck]

It should not affect starts hardly at all especially with that compression ratio. I keep all the LS motors that I have built and tuned around 8 degrees for startup timing or less and generally they have a 10.5:1 CR. I noticed some of them detonate on startup with 13 degrees and don't start as well when it's warm... Try it, if you don't like it you can always go back. The only other thing I can think of is that the Crank Position Sensor is not taking an accurate reading. I read somewhere that they play a big role in shutdown to prevent dieseling. How you adjust it though, I have no idea.

These cars aren't neccessarily dieseling though, just revving back up when the throttle opens back to it's default position.

Just in my experience, I've gotten cleaner starts with more timing like that. The cars start perfect.

[07 z71 silverado]

do these car's have a velocity ring for the fast 102 with the stock 90mm tb. I have heard of issues with idle and other things without having it vmaxmotorsports sells them. It might fix the problem but, it might not although it's probably a good idea to have it anyway though.

[DSteck]

One of the cars has a 102 throttle body and still does it sometimes, so I don't think the ring will do anything.

[IDRIVEAG8GT]

Vaccum Leak.