ChevyCobb

Idle will intermittently hang while coasting to a stop at 1500rpm and sometimes will climb up to that or even higher then fall to correct idle as soon as I come to a stop. Does it mostly while driving slow(traffic, parking lots, etc). Idles fine as long as im not moving. Makes it very annoying to drive.

It will also rise on its own while coasting. This will happen clutch in or out. This began after ported MSD and ported stock throttle body(which I think is the culprit).

Mods before this happened: cam, wcch heads, headers, halltech cai

Video showing it rising while coasting...this is just coasting downhill clutch in and out. No throttle input from me and only letting off the brake.

If I can get my hands on a stock TB, I will swap out to see, but is there any other way to track this down? I thought vacuum leak, but the way it does this while coasting makes me think otherwise...Never while stationary.

rio95

Tuning needed.

erufle

I don't know because I'm not there but if your cam street hot to racing, usually one with a lot of duration, manifold vacuum is going to vary, especially at low rpm. That is the simplest thing to fix. . . . Raise the idle when you have it tuned. Give your tuner the cam specs and I'm sure he will rely on his experience to tell you if that's really the problem or if you need to track a vacuum leak or the like. Have you listened to a dragster? The idle is set higher than you think . But I can't tell if that's the problem from here. Could be you're driving through a swarm of locusts for all I know

ChevyCobb

I tried to go back to the tuner for it but of course when I did that, it wasn't doing at that time so he saw nothing abnormal about the tune and couldn't get it to replicate. Its an intermittent issue at the moment.

Turbokraft

Since the problem is intermittent and not present during the visit to your tuner it would seem something is wrong with a sensor or wiring to a sensor. The tune doesn't change intermittently other timing corrections for knock (unlikely at idle) and minor fuel trim corrections. Perhaps you could data log some sensors during the symptom, these might include ECT, IAT, APP, TPS and present those to your tuner or rectify the problem yourself if something is very obvious. Say, for example while your coasting the APP indicates a throttle command of 1-2% with your foot off the accelerator.

Michael_D

Note the ECT when it acts up, and when it behaves. Min air flow rates change with ECT change.

ChevyCobb

Only way I have to log something is through the torque app, but maybe I can go for a short drive later and see what happens using it and try to schedule something with the tuner again.

Michael_D

Just look at your cooling temp gauge. Note the temp whenever you have these issues come up. If it acts up only when temp is say, 120 or less, then you can asume the min air flow table (in the tune file) needs adjustment.

ChevyCobb

Will do, though I'm certain it does it when higher too at "normal operating temp"

Screamin Z

had a similar issue once. Turned out the vac line on the back of the intake came off. But it did it all the time.

ChevyCobb

Can confirm not temp dependent, though it seemed to do it more often at a higher temp(190+)...just not consistently

Michael_D

Weird.... You could start ruling out the mechanical things, like mentioned above by Screamin Z, your vacuum line to the exh. Look for other leaks too. It still could be tuning, but at this point, a log file would be real helpful to determine that.

rjacobs

mine does the same thing. Its in the tuning.

Ive been back to the tuner 3 times to work on it, and like you, i cant reproduce it when with the tuner. Although it has gotten better and is usually just a slight return to idle hang where as at first it was bouncing due to idle falling so fast it would miss the target and the throttle body would crack open and bounce it to 1500 or so rpm.

I have a NW102 throttle body though.

Twinscrew

Sounds like min airflow table in the tune needs work.

MTPZ06

Would be interesting to see if an untouched OE TB makes a difference. Who ported the TB?

ChevyCobb

Mamo ported the TB at same time as the intake.

MTPZ06

Probably not the case then...he knows what he's doing.

ChevyCobb

I sent him a message to see exactly what it's ported to. Tuner may just not know what to set air at with it

Michael_D

The I.D. of a ported TB does not get changed. Can't change that unless you also change the blade....... When you go to a larger (or smaller I suppose) TB, you figure out what the minimum cross sectional area is, minus blade area, within the TB. That figure then gets input in the TB scaler table - done. The ECM then references this table value for other air flow functions. The minimum air flow table is one that references the TB scaler value. The min air flow table does take a few tweaks to get set just right. This table has RPM along the X axis, and engine coolant temperature along the Y axis, which is why I asked if you are seeing this issue pop up at different ECT ranges. It acts similarly as a choke would when ECT is low.

Did you look at the vacuum line connector that goes to the muffler exh valve solenoid? I’d rule that out as a potential issue prior to going to the tuner.

ChevyCobb

Damn I sure wish I knew more about tuning than I do.
I assume that vacuum line is connected because I was still able to make them open and close manually. I was messing with them trying to get a consistent response with the idle.