Surging
Thread Starter
Advanced
Joined: Jan 2006
Posts: 60
Likes: 0
From: NY NY
Surging
To All:
Thoughts on why after warm up, my 86 auto surges (at idle between 500 - 700 rpm) If i gently hold accelerator at 900rpm - no issues.
Under acceleration - NP
During warm up, all is behaving normally.
I did the usual checks on vacc lines, etc.
I have done some simple mods, but never had this isuue until
..............Yes, here it is...
I put a wet Nitrous NOS , simple after TB plate, really no mystery as instructions are straight fwd.
I reviewed wiring, connections, vacc lines, etc.
Having the NOx on or off (at idle there is no fdifference.
When using the NOS, no issues
W/O using the NOs no issues
I guess this falls under the driveability heading
I can't help but think I'm overlooking something basic
Note - I build serious HP SB & BB for a major aftermarket retailer, but they are always carburetored. I mention this only b/c I don't know everything (obviously) but this NOx and going thru it step by step, but recent development has me miffed. I am thinking about disconnecting the NOx and going thru it one step at a time, but was hoping someone may have a thought.
Thnx
Mike
Thoughts on why after warm up, my 86 auto surges (at idle between 500 - 700 rpm) If i gently hold accelerator at 900rpm - no issues.
Under acceleration - NP
During warm up, all is behaving normally.
I did the usual checks on vacc lines, etc.
I have done some simple mods, but never had this isuue until
..............Yes, here it is...
I put a wet Nitrous NOS , simple after TB plate, really no mystery as instructions are straight fwd.
I reviewed wiring, connections, vacc lines, etc.
Having the NOx on or off (at idle there is no fdifference.
When using the NOS, no issues
W/O using the NOs no issues
I guess this falls under the driveability heading
I can't help but think I'm overlooking something basic
Note - I build serious HP SB & BB for a major aftermarket retailer, but they are always carburetored. I mention this only b/c I don't know everything (obviously) but this NOx and going thru it step by step, but recent development has me miffed. I am thinking about disconnecting the NOx and going thru it one step at a time, but was hoping someone may have a thought.
Thnx
Mike
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
Your car engine has sensors that are constantly being monitored by the ECM. The ECM is programmed to give you an idle based upon the coolant temperature....assuming a constant MAF, TPS, MAT and coolant temp readings......and you do not have a vac leak. The ECM uses the IAC (idle air control) motor to achieve the desired RPM. This little stepper motor simply opens an air valve in the throttle body to give the desired idle RPM. Something is causing variation in one or more of the above sensors to create an erratic idle. Without a scan device, you are simply rolling dice to find your problem. The engine sensors are not cheap and you have to find the problem child. I wish this was easy and simple but....... Get an engine scan 
Thread Starter
Advanced
Joined: Jan 2006
Posts: 60
Likes: 0
From: NY NY
Surging
Originally Posted by RRT vette
Any codes? How long since you check/replaced the O2 sensor?
No codes, O2 sensors less then 1000 miles ago - good thought though, as (if I have this right) O2 sensors on line after op temp is up - just could be the ticket - Thnx
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
I doubt it's the 02, my bets still on a dirty IAC if your sure you have no vac. leaks. Also what year is this? I ask only so I can tell if you have a CSV. Of course scanning it would tell alot.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
This is a 86 vette with a TPI, correct?
Things to check for surging:
1. loose or leaking air duct between MAF and throttle body.
2. ignition timing... should be at 6* BTDC with EST disconnected.
3. fuel filter.
4. vacuum lines for kinks or leaks.
5. spark plugs for cracks, wear, burned electrodes, improper gap, heavy deposits and check condition of cap, rotor, and plug wires for unwanted splits.
6. EGR should not be functional at idle...check to see. Also check EGR filter.
7. pull the O2 sensor and check for silicon contamination. Could be from use of improper RTV sealant.
The IAC is not listed here but is another item I would pull and clean along with the TB. The CVS will not come into play here. It is only active during crank mode and times-out after 8 seconds (if engine is <95*f) as the engine temp rises.
Rick, I know you have mentioned this in the past. I believe you stated the IAC and CVS somehow have something to do with each other. I have looked through my 86 FSM and don't see anything about that. From looking at my car they don't come anywhere close to each other. The thermal switch is just below the IAC, but it's in the intake manifold with coolant. If you can clarify I would like to know. Thanks
Things to check for surging:
1. loose or leaking air duct between MAF and throttle body.
2. ignition timing... should be at 6* BTDC with EST disconnected.
3. fuel filter.
4. vacuum lines for kinks or leaks.
5. spark plugs for cracks, wear, burned electrodes, improper gap, heavy deposits and check condition of cap, rotor, and plug wires for unwanted splits.
6. EGR should not be functional at idle...check to see. Also check EGR filter.
7. pull the O2 sensor and check for silicon contamination. Could be from use of improper RTV sealant.
The IAC is not listed here but is another item I would pull and clean along with the TB. The CVS will not come into play here. It is only active during crank mode and times-out after 8 seconds (if engine is <95*f) as the engine temp rises.
Rick, I know you have mentioned this in the past. I believe you stated the IAC and CVS somehow have something to do with each other. I have looked through my 86 FSM and don't see anything about that. From looking at my car they don't come anywhere close to each other. The thermal switch is just below the IAC, but it's in the intake manifold with coolant. If you can clarify I would like to know. Thanks
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Hard to say without knowing more (particularly all of the mods to your motor including whether or not it still has an air pump or a/c), but I'm going to assume that everything is clean and that the sensors are working (absent a trouble code they usually are) and agree that the nitrous install is where you should look first as it may be sucking in air. It might be coming in around the fitting - or if you removed anything, there's always the possibility of a gasket leak. In any event, if the air is unmeasured by the MAF, the ECM doesn't really know what to do about it, but when it sees a lean condition from the O2 sensor (which it will when it goes closed loop), it adds some fuel - which presto given the air from the leak, raises the idle speed - and then, because there's a targeted speed burned into the chip, the IAC slams shut to get that speed back to where the ECM thinks it should be. I'd start by spraying some throttle body cleaner or aiming some propane around the NOS fitting. Otherwise, scanning it will probably confirm the lean condition or show an unusually low IAC # in open loop and you'll need to start looking at anything that may be letting in some extra air.