P/N idle
Thread Starter
Drifting
Joined: Oct 2006
Posts: 1,724
Likes: 23
From: Minnesota
P/N idle
When my vehicle is in P/N it doesn't want to idle very well. It will idle fine for 5 mins,for example, then it will surge and over correct and drop low and continue to oscillate like that for a little while (sometimes it may stall). In gear it doesn't have this problem. The MAF and VE tables are dialed in pretty good. In gear idle the LTFTs are ~1% but in P/N the LTFTs can reach ~10% (maybe more). The idle overspeed and underspeed have been greatly reduced. And the idle timing is around 30*. And idle target speed is around 900rpm.
What tables should I look at to correct this P/N idle problem?
Mods:
Heads, cam, intake, TB, exhaust, supercharger.
228/232 .571/.573 112
What tables should I look at to correct this P/N idle problem?
Mods:
Heads, cam, intake, TB, exhaust, supercharger.
228/232 .571/.573 112
Pro
Joined: May 2014
Posts: 670
Likes: 46
From: Indiana
Sounds like an issue I had with my truck's 5.3 engine. You may have an air leak that comes or goes as the engine warms up.
Chevy came out with a service bulletin for the trucks which identified a poor design in their intake manifold gaskets. Over time when cold the gaskets would leak a small bit and then pretty suddenly as the temp came up seal up. Engine would go from a nice idle to nearly dying when the gaskets sealed because the idle air fuel trims had gone way negative trying to keep the idle at the programmed rpm level by nearly closing off the throttle body.
Chevy came out with a service bulletin for the trucks which identified a poor design in their intake manifold gaskets. Over time when cold the gaskets would leak a small bit and then pretty suddenly as the temp came up seal up. Engine would go from a nice idle to nearly dying when the gaskets sealed because the idle air fuel trims had gone way negative trying to keep the idle at the programmed rpm level by nearly closing off the throttle body.
Thread Starter
Drifting
Joined: Oct 2006
Posts: 1,724
Likes: 23
From: Minnesota
Sounds like an issue I had with my truck's 5.3 engine. You may have an air leak that comes or goes as the engine warms up.
Chevy came out with a service bulletin for the trucks which identified a poor design in their intake manifold gaskets. Over time when cold the gaskets would leak a small bit and then pretty suddenly as the temp came up seal up. Engine would go from a nice idle to nearly dying when the gaskets sealed because the idle air fuel trims had gone way negative trying to keep the idle at the programmed rpm level by nearly closing off the throttle body.
Chevy came out with a service bulletin for the trucks which identified a poor design in their intake manifold gaskets. Over time when cold the gaskets would leak a small bit and then pretty suddenly as the temp came up seal up. Engine would go from a nice idle to nearly dying when the gaskets sealed because the idle air fuel trims had gone way negative trying to keep the idle at the programmed rpm level by nearly closing off the throttle body.
The vehicle is an A4 and doesn't have any trouble idling in gear and the LTFTs are always low. However, in P/N it will idle fine for a while and then, out of nowhere the RPM will drop significantly causing it to drop and over correct and starting hunting for idle. Usually it will correct itself, or I can assist. Then it will idle fine for a while (5-10 mins for example), then out of nowhere the idle will drop.
It seems that when the motor is hot (and in P/N) the LTFTs are pretty high (+13ish), but when its just warming up they are low (+2ish). You can definitely smell unburned fuel. Cam isn't that big (228/232 .571/.572 112) so I don't know if this is normal. Also idle is set to be around 900rpm. I would think that even with high LTFTs it shouldn't be burning that rich as its just the amount of fuel necessary to keep it at stoich. Its not dumping black smoke out the pipes but you can definitely smell the exhaust.
It feels like its "loading up" on gas until it chokes itself and causes it to stumble and then go into "hunt for idle mode". Wide band gauge indicates 14-15 afr while it is idling.
Last edited by reactor2; Apr 20, 2015 at 10:03 PM.
Thread Starter
Drifting
Joined: Oct 2006
Posts: 1,724
Likes: 23
From: Minnesota
I'm not sure there is anything that can be done about the fuel smell on a cammed engine. The large amount of overlap between the intake and exhaust lobes is quite inefficient at idle speeds. Its not like it stings the eyes, but its definitely there.
Pro
Joined: May 2014
Posts: 670
Likes: 46
From: Indiana
I would think that even with high LTFTs it shouldn't be burning that rich as its just the amount of fuel necessary to keep it at stoich. Its not dumping black smoke out the pipes but you can definitely smell the exhaust.
It feels like its "loading up" on gas until it chokes itself and causes it to stumble and then go into "hunt for idle mode". Wide band gauge indicates 14-15 afr while it is idling.
It feels like its "loading up" on gas until it chokes itself and causes it to stumble and then go into "hunt for idle mode". Wide band gauge indicates 14-15 afr while it is idling.
Your wide band is telling you it isn't rich, but you think it is. Do you have faith in your wide band?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
5 Best & 5 Worst Corvette Daily Drivers
Joe Kucinski
The Headlights of Every Corvette Generation Explained
Joe Kucinski
5 Best & 5 Most Overrated Corvette Track Packages of All Time!
Joe Kucinski
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
When adding a larger cam you must increase the "desired airflow"--It has a table for both P/N and for in gear---If while in P/N you are having idle hunting you should add some desired airflow in P/N I would start by adding 15% to the whole table--If you add too much you'll get the reverse affect the idle will hang and take forever to drop down---If that happens simply start removing 5% at a time until it settles down
ALSO when cold your engine runs in "OPEN LOOP" mode--the fuel is commanded strictly by the open loop normal table---after the coolant reaches apprx 170* the ECM will switch to "closed loop" seems this is where you start having idle issues---
Make sure that after the ECM goes into closed loop that your LTFT fuel trims are at "0" or slightly negative---this has to be done 1st before your ever mess with the desired airflow----
ALSO when cold your engine runs in "OPEN LOOP" mode--the fuel is commanded strictly by the open loop normal table---after the coolant reaches apprx 170* the ECM will switch to "closed loop" seems this is where you start having idle issues---
Make sure that after the ECM goes into closed loop that your LTFT fuel trims are at "0" or slightly negative---this has to be done 1st before your ever mess with the desired airflow----