Hanging idle

[bobbycollier]

Time to pick your brains again... :) Recently my idle has begun to "hang." Here's the scenario... I pull up to a red light, step on the clutch, pull out of gear, and the RPM's get stuck at 1600 on the way down. It happens maybe every other red light. The only way to bring it back down is to ease off the clutch in gear while on the brake and then release around 1000 RPM. Note that idle is set at 800 RPM. What do you suspect the culprit is??

Thanks!
Bobby

[The Alchemist]

easy, two things:
throttle cracker in gear (a 3D table) & throttle cracker out of gear (a 2D Table).
Log the PIDs, cruise up to a stop and watch the resulting log.
The hang can sometimes be a result of both tables spilling over together as you hit the clutch. In the 3D table pay particular attention to the lower SPEEDS (note this is more speed dependant than rpm dependant). When you throttleoff the rpm drops to close to idle and stays there as the speeds rolls off to a gradual stop.
Many throttle cracker tables have a lot of variation throughout the years/makes/models so you have to have a play around to get what suits your setup.
Finally once the car is almost stopped and you clutch it the 2D table takes over. Sometimes a slight flair happens here too so simple reduce the values in the appropriate boxes.

Mike

[johnv]

If it was fine, and has just started doing this, it could be a vaccume leak, check the PCV hoses for splits.

[bobbycollier]

Thanks guys. I'll do some experimenting and see what works!
:cheers:

Bobby

PS: [Entertainment at my expense...] Actually I was just trying to do some experimenting. Looked like about 1/4 tank of gas and went for a short drive and next thing you know I'm parked on the side of the freeway running "lean." Stupid gas gauge is lying to me! Jogged home, got the truck and a gas can, and got the car started. Then sitting in the garage I'm watching the gas needle sway slowly up and down. What the F! So before I start replacing parts, is there anything besides the fuel level sender that would be the culprit?? And my speedo does weird stuff too. Sometimes reads zero up until 20-ish and then slowly climbs to the appropriate speed. I assume once again this would be the VSS sensor?? I think I need a fresh C6 and do a motor swap and toss the rest of mine!:doh2:

[joecar]

Check for loose/dirty grounds.

[bobbycollier]

Grounds for the sensors or the gauges?

[bobbycollier]

I hope you say sensor because no-way am i brave enough to take the dash apart!

[joecar]

Any grounds you can locate anywhere on vehicle.

If several gauges on your IPC are behaving strangely, then it could possibly be related to the IPC (do your PCM pids show ok...?)... what car do you have...? You should be able to remove the closeout panel from under the dash and you should be able to see the back of the IPC and any ground/cables/connectors... you may need the service manual (wire diagram, ground locations, other various details).

[bobbycollier]

I have a 1998 C5 Vette. Actually now that you mentioned it, my speedo VSS pid shows the same thing as the dash gauge (i.e. reported speed is lagging my actual speed). So in that context, I suppose it would be the sensor ground or the sensor itself that needs to be addressed?

Thanks, Bobby

[joecar]

Yes, in that case, check all PCM grounds.

[bobbycollier]

Hanging idle is FIXED! I made some tweaks to the Throttle Cracker as suggested and that resolved it.
:cheers:

My next problem was a wildly oscillating RPM from 400-1200. I was looking at my logs and my spark was all over the place (literally from 0 deg to 28 deg sitting at idle). I zero'd every correction table I could find but it was still swinging. So I forced the the HIGH OCTANE table to be the only source. Still a problem. Apparently the load of the supercharger causes big swings in g/cyl so I would go from a 0 deg cell to a 28 deg cell. I did some dramatic changes to the 0-1200 RPM range on the high octane spark table and now it is MUCH better. I still get an occassional RPM dip down to 400 and then back up to 8/900 where it should be. What would be the likely culprit for dipping too low at idle? Thanks again for all the great suggestions!

Bobby