C5 with 90/90 setup - return to idle issues, idle hunts in neutral when in motion
 

I have an cam + bolt-on '03 C5Z. A year ago I had the car dyno and street tuned by Complete Street Performance who is a reputable tuner/shop here in PA and on this forum. Car ran good and made good numbers, they are good people, no complaints. Since then I replaced my stock LS6 intake manifold and ported stock throttle body with a TPIS modified 90mm LS6 intake manifold (TPIS cuts off the stock 78mm inlet and installs a 90mm inlet) and a ported LS2 throttle body.

Since installation, I've had some issues. If I start the car up and then give it a rev, the RPMs will drop way down to ~400 RPM and then jump up to ~1,200 RPM and eventually settle to where it should be. If I start the car and let it coast backwards out of my garage in neutral, the RPMs will bounce up and down and the car will typically stall. If I start the car up and drive it down the road and throw it in neutral the same thing occurs. Once the car has been ran for several minutes everything is great, no issues at all. However, if I shut the car off and then turn it back on the issues start again. Car runs great going through the gears, at WOT, etc.

After the install (a few months ago), I took the car back to the shop for a retune to accommodate the changes. Despite tuning the car on the dyno and street again, the issues remain. I dropped the car back off to the tuner a few weeks ago to have them sort out the tune since I'm obviously not happy with how the car runs. I got a call yesterday that the car is good to go; however, when I arrived to pick it up I noticed that it is no different, the issues remain. I left the car there so they can continue to tweak it.

To me this seems like an issue with the tune - especially since it's well known that tuning a 90+mm setup can be a bit tricky on the electronic throttle C5 Corvettes. I'm thinking it's an ETC scaler issue perhaps.

FWIW, I have done my due diligence and have ensured that I have no vacuum leaks, that the PCV routing is correct, I even temporarily swapped on a virgin non-ported LS2 throttle body to rule out the throttle body as the culprit all to no avail. The shop seems to think the issue is my intake manifold because they've never heard of it; however, many people have ran these manifolds and reported that they are easier to tune than a FAST and at the end of the day it's just a stock LS6 manifold with an LS2 90mm inlet on it. It shouldn't be much different than tuning a car with one of those Dorman 90mm manifolds.

What are your thoughts?

Did you update the scalar in the tune to reflect the larger throttle body?

The tune was performed by my tuner so I am not specifically sure what was or wasn't altered. But things like this is helpful, anything that can shed some light.

Yep, adjusted properly.

I'm working on sorting through it. Seems just to need a little more TLC to the throttle follower. Unfortunately when I had it out last the car was warmed up and the majority of this issue appears to be when cold, or when just started (both when the car isn't in its idle routine - but throttle follower, maybe cracker, routine). With it being mostly a cold start issue, its tough to test quickly as the car warms up. Plugging and chugging, seems like we're about there!

Good stuff Andrew! I appreciate the time and effort you are putting into the car. I wanted to see if I could get some insights from folks on here who have dealt with similar issues when going 90/90 on their C5 to see if I could perhaps aid in the process somehow since I feel bad that the car is being such a frustrating PITA.

As said if there's no vacuum leak it is entirely tune related. Sounds like the tuner is working through it. When you go to larger throttle bodies they can be flexible and take time to dial in.

Depends on the ls2 year. The scalars are 1/x at a certain point.

102 throttle:
1999 controller scalar is 0.0122
2007 controller scalar is 8192, but usually 7800 works better.

But assuming the tuner knew that much, its more likely in the torque follower or the scalar just needs to be manually tweaked.

Bringing this back up. My tuner got the car to a much better state back in October and for the first 2 weeks all seemed pretty well. However, as the months have passed and I've driven the car more I've noticed that it still needs work. Periodically the car will still freak out right after a warm start up and will idle hunt, fortunately it is far far less common and not as bad as before. The changes made to the tune to rectify most of the idle hunt issue have created a situation where the idles hangs here and there (example - I drove up my driveway the other day, stuck the car in neutral and the RPMs hung at 1,500+ RPM for several seconds while vehicle was stopped). I'm pretty frustrated. What are your thoughts? Any suggestions of someone who can sort this out within a couple hours of Philadelphia? Is it really this hard to make a 90mm setup work properly on a C5?

FWIW, I recently removed the intake manifold and throttle body assembly to ensure everything was sealed well, all connections tight, etc. All gaskets are fresh, all connections tight so no issues there.

Have your tuner look at the afterstart fuel enrichment table for warm start issues. For hanging idle, throttle follower and throttle cracker tables. You may also need to adjust the throttle blade set screw.

I'm no expert, but if the idle is hanging that high and won't come down, its possible that the higher rpm cells in the idle base airflow table need to be reduced. That should help the revs fall. Even when it doesn't hang, do you notice it takes longer to come back to idle than it did before the swap?

Additionally, if you have any interest in trying to play with it yourself, these videos were quite helpful.

Thank you, I know he messed with the follower and cracker tables which alleviated the vast majority of my idle hunt issues. Unfortunately, the adjustments created that hanging idle problem. I was told a hanging idle is better than no idle lol so as you can imagine I am looking for someone new. I just don't know who to go to when I went to someone who is very highly regarded and well known. Goes to show if you want something done right you probably need to do it yourself. I just don't have the time or desire to learn. I just want it to be fixed so I can move on.

Yes I do, it does take longer to come back to idle now than before the tune was tweaked further. If I stop the car completely at a stop light, the rpms stay at say 1,100 rpm or so for about 2 seconds before they drop to idle. They use to drop sooner prior to the adjustments.

Thank you, I will give those a look to be sure. I'm about 99.9% convinced this is a tuning issue because none of this was a problem until I swapped the manifold and my symptoms have improved as my tuner has performed additional tuning.

Hanging idle sometimes can only get "so good". If there is a throttle set screw you can drop it to consume less air which will help with a hanging idle. Otherwise, you can also look at the desired airflow table as it may be too high.

Oh ok I see, that's good to know. In that case I might just be blaming this as a tuning issue even though my tuner is realistically doing everything he can to make it right so that's not fair of me to blame it on that. I don't have a set screw since the C5 is drive by wire so I unfortunately can't mess with that. I will note the desired airflow table. I'm almost thinking that swapping back to the factory LS6 manifold and throttle may be my best course of action since it was never an issue with that setup. Whatever I gained going 90/90 was certainly not worth the associated bullshit, live and learn.

Was the scalar set for the 90mm electronic throttle? Or is it still the stock 78mm throttle scalar?

can you post up your desired idle airflow table?