High Idle - Vacuum leak or tune?
Thread Starter
Safety Car
Joined: May 2007
Posts: 4,812
Likes: 46
From: Taylorsville North Carolina
High Idle - Vacuum leak or tune?
Having trouble diagnosing my high idle. New motor has about 3k mi and has run smooth for past 7 mos. But last week, it started surging to around 2k rpms when I depressed clutch to stop at red light. In about 7 seconds, it will return to normal idle. If I am coasting up to a red light, it will stay around 2k rpms until I stop, then 5-7 seconds later, it will return to normal idle. It only does it when car is moving. I have sprayed every hose, TB connector, intake manifold etc with tb clearner but nothing. It only kicks into high idle when car is moving - even if in gear. There are no codes. Search showed nothing except for coolant sensor but mine appears to read correctly.
Thoughts?
Thoughts?
Melting Slicks
Joined: Nov 2009
Posts: 2,922
Likes: 219
From: West Branch, MI
You might try spraying or replacing the MAF sensor. If dirty or faulty, your computer could be picking up false air signals, which introduces more fuel and higher rpms. Without any codes, I think I'd try that first.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Is your engine MODDED or STOCK? I have the same issue and mine is heavily modded. Mine is a TUNING issue.
Theres an IDLE Air table and a off idle air table in the PCM. If one is say,, set at 50 and the other is set at 80, it can bounce between the two. My tuning guy says he can fix it!
Theres an IDLE Air table and a off idle air table in the PCM. If one is say,, set at 50 and the other is set at 80, it can bounce between the two. My tuning guy says he can fix it!
Thread Starter
Safety Car
Joined: May 2007
Posts: 4,812
Likes: 46
From: Taylorsville North Carolina
Sorry - forgot to include signature. Car is heavily modded and has run great for the past 7 mos. Sending tuner a vid of the issue. Hopefully they will have a clue. Not sure how a tune can go bad.
I did hook my Bluetooth scanner up to car. Intake manifold pressure doesn't seem to fluctuate abnormally so I don't think it is a vacuum leak. O2 sensors are all within voltage range so I don't think it is that. The MAF only shows lbs but I don't know what that should be reading, so I will research that.
One odd thing I noticed was the intake air temp was saying 98* when the outside temp is only 79* (weather channel verified and also what int display said. Not sure if that would have an affect or not. Will ask tuner.
My other vehicle is in the shop so I have to drive it like it is.
Here is a vid of what it does. It usually takes it about 5 seconds longer to go back to normal idle. You can see it go to 2400 rpms when I depress the clutch.
I did hook my Bluetooth scanner up to car. Intake manifold pressure doesn't seem to fluctuate abnormally so I don't think it is a vacuum leak. O2 sensors are all within voltage range so I don't think it is that. The MAF only shows lbs but I don't know what that should be reading, so I will research that.
One odd thing I noticed was the intake air temp was saying 98* when the outside temp is only 79* (weather channel verified and also what int display said. Not sure if that would have an affect or not. Will ask tuner.
My other vehicle is in the shop so I have to drive it like it is.
Here is a vid of what it does. It usually takes it about 5 seconds longer to go back to normal idle. You can see it go to 2400 rpms when I depress the clutch.
Advanced
Joined: Feb 2012
Posts: 56
Likes: 2
I had a similar problem, stock c5, fast idle in gear, warm to hot OUTSIDE temp. Turned out to be the air intake temp sensor, located right before the throttle body in the rubber accordion sleeve, Thanks to Bill Curlee, she is running normal again.
again.
again.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
If the engine is COLD and the car is not in DIRECT SUN (heating up the engine compartment) the two should pretty much match.
The real secret is,,, does the actual sensor OUTPUT increase or decrease in a linear scale as temp increases or decreases.
Use an inferred temp reader and compare the temp of the sensor bulb to what the outside temp and or the air tube/MAF body temp is.
QUESTION... Are your primary 02 Sensor outputs properly fluctuating high to low, low to high ect...???
Should be going from 2mv to 850 mv (somewhere around there) Any reading that hangs out near the middle (450mv) is a poor/weak sensor reading.
I would also find the OXYSEN Fuse and measure the voltage on that fuse test points to battery or chassis ground and see if you have full battery voltage on the fuse test points.. If the O2 sensor heaters are not fully functioning, the sensors will not work properly.
The real secret is,,, does the actual sensor OUTPUT increase or decrease in a linear scale as temp increases or decreases.
Use an inferred temp reader and compare the temp of the sensor bulb to what the outside temp and or the air tube/MAF body temp is.
QUESTION... Are your primary 02 Sensor outputs properly fluctuating high to low, low to high ect...???
Should be going from 2mv to 850 mv (somewhere around there) Any reading that hangs out near the middle (450mv) is a poor/weak sensor reading.
I would also find the OXYSEN Fuse and measure the voltage on that fuse test points to battery or chassis ground and see if you have full battery voltage on the fuse test points.. If the O2 sensor heaters are not fully functioning, the sensors will not work properly.
Last edited by Bill Curlee; Jul 12, 2016 at 04:02 PM.
Thread Starter
Safety Car
Joined: May 2007
Posts: 4,812
Likes: 46
From: Taylorsville North Carolina
If the engine is COLD and the car is not in DIRECT SUN (heating up the engine compartment) the two should pretty much match.
The real secret is,,, does the actual sensor OUTPUT increase or decrease in a linear scale as temp increases or decreases.
Use an inferred temp reader and compare the temp of the sensor bulb to what the outside temp and or the air tube/MAF body temp is.
QUESTION... Are your primary 02 Sensor outputs properly fluctuating high to low, low to high ect...???
Should be going from 2mv to 850 mv (somewhere around there) Any reading that hangs out near the middle (450mv) is a poor/weak sensor reading.
I would also find the OXYSEN Fuse and measure the voltage on that fuse test points to battery or chassis ground and see if you have full battery voltage on the fuse test points.. If the O2 sensor heaters are not fully functioning, the sensors will not work properly.
The real secret is,,, does the actual sensor OUTPUT increase or decrease in a linear scale as temp increases or decreases.
Use an inferred temp reader and compare the temp of the sensor bulb to what the outside temp and or the air tube/MAF body temp is.
QUESTION... Are your primary 02 Sensor outputs properly fluctuating high to low, low to high ect...???
Should be going from 2mv to 850 mv (somewhere around there) Any reading that hangs out near the middle (450mv) is a poor/weak sensor reading.
I would also find the OXYSEN Fuse and measure the voltage on that fuse test points to battery or chassis ground and see if you have full battery voltage on the fuse test points.. If the O2 sensor heaters are not fully functioning, the sensors will not work properly.
Much thanks Bill. Ready to get this behind me.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Now we might be getting somewhere. I watched the O2 voltage on the way home. #1 fluctuated between 385 and 925. #2 stayed at 455 the whole time. It never moved, even when the high idle went back to normal. So now what - new O2 sensors? Car is 15 yrs old (build date 9/01). There are no rear sensors, as they are tuned out. Anyone know the part number for the ones I need?
Much thanks Bill. Ready to get this behind me.
Much thanks Bill. Ready to get this behind me.
You MUST DO THIS! Grab you volt meter and find the OXY SEN (oxygen sensor heater circuit fuse) Measure the voltage at the TEST POINTS on top of the fuse to ground. The ignition switch must be ON to see voltage on that fuse!
Please report your findings. You should see full battery voltage on BOTH test points to ground. If the voltage is noticeably less than actual battery voltage, that's a bad thing and the O2 Sensor will not function properly or live long!
Here is the heater circuit schematic that you need to make sense of the readings. IF, you have improper heater circuit voltages, the IGNITION SWITCH is normally the issue as it supplies the battery voltage to that circuit. The contacts in the switch get burned and corroded.
I strongly recommend replacing BOTH primary O2 sensors as a pair! Your car will THANK YOU for it! Make sure that the connectors and wiring are well clear of the exhaust piping and not damaged!
Bill
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
WOOPS,,,,,,,,,,, Forgot the schematic! 
Note where the voltage comes from. HOT in ON (The ignition switch)
If the voltage is good, use the meter OHM Meter to check the ground pin "C" continuity on the sensor connector to chassis ground. You have to have a good ground on pin "C" to complete the circuit:
Note where the voltage comes from. HOT in ON (The ignition switch)
If the voltage is good, use the meter OHM Meter to check the ground pin "C" continuity on the sensor connector to chassis ground. You have to have a good ground on pin "C" to complete the circuit:
Last edited by Bill Curlee; Jul 13, 2016 at 11:51 AM.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
JUST A PIONT OF INTEREST!
The reason that you are seeing the 455 mv reading when the sensor is bad is, the PCM feeds the sensor a 450mv reference voltage. The sensor its self produces a voltage when it sees a RICH AFR or a LEAN AFR. When the AFR is RICH it produces a POSITIVE voltage and ADDs to the 450 mv reff voltage. When it sees a LEAN AFR, it produces a NEGATIVE voltage and subtracts from the 450mv reff voltage.
If the sensor is BAD, it can NOT produce that neg or pos voltage (doesn't sense the oxygen or lack of oxygen inside the exhaust pipe) and only sees the 450mv reff voltage.
Bill
The reason that you are seeing the 455 mv reading when the sensor is bad is, the PCM feeds the sensor a 450mv reference voltage. The sensor its self produces a voltage when it sees a RICH AFR or a LEAN AFR. When the AFR is RICH it produces a POSITIVE voltage and ADDs to the 450 mv reff voltage. When it sees a LEAN AFR, it produces a NEGATIVE voltage and subtracts from the 450mv reff voltage.
If the sensor is BAD, it can NOT produce that neg or pos voltage (doesn't sense the oxygen or lack of oxygen inside the exhaust pipe) and only sees the 450mv reff voltage.
Bill
Last edited by Bill Curlee; Jul 13, 2016 at 11:58 AM.
Thread Starter
Safety Car
Joined: May 2007
Posts: 4,812
Likes: 46
From: Taylorsville North Carolina
UPDATE -
It was not the O2 sensor. The one reading 455mv is Sensor 2, which I have deleted and tune out because I have headers. More info though:
Re torqued FAST bolts. A little loose but not enough to make a difference. Coasted down a hill, clutch in, clutch out - doesn't matter. TC on/off doesn't matter. If car is moving, it stays at high idle. Once car is stopped, in 5-7 seconds is goes back to normal idle. I just lifted it onto jack stands, started it and put it in gear. Once rear wheels are moving, it stays at high idle once clutch is depressed and stays until brakes stop the wheels. It is too late at nite now, but at least I can look for a vacuum leak now with it in high idle! Checked again and there are no codes. Seems like someone should have run across this before. I have a thread going in the Tune and Scan section in case it was a tuning issue but they are befuddled also. Looks like I will end up taking back to tuner - 1 hr down there to drop it off, hour back, hour down there to pick it up and an hour home.
It was not the O2 sensor. The one reading 455mv is Sensor 2, which I have deleted and tune out because I have headers. More info though:
Re torqued FAST bolts. A little loose but not enough to make a difference. Coasted down a hill, clutch in, clutch out - doesn't matter. TC on/off doesn't matter. If car is moving, it stays at high idle. Once car is stopped, in 5-7 seconds is goes back to normal idle. I just lifted it onto jack stands, started it and put it in gear. Once rear wheels are moving, it stays at high idle once clutch is depressed and stays until brakes stop the wheels. It is too late at nite now, but at least I can look for a vacuum leak now with it in high idle! Checked again and there are no codes. Seems like someone should have run across this before. I have a thread going in the Tune and Scan section in case it was a tuning issue but they are befuddled also. Looks like I will end up taking back to tuner - 1 hr down there to drop it off, hour back, hour down there to pick it up and an hour home.
Racer
Joined: Feb 2015
Posts: 299
Likes: 33
From: Chambersburg PA
My vote is for a tuning issue, when I had my car tuned (cam&headers) not heavily modded, the tuner talked about a cruise effect, where car at coast mode can hang a high ilde before rpms would drop. He says there is a fine line between stable idle (which he says is the hardest thing to tune on modded engines) and hanging idles. My car when rolling to a stop, off gas, clutch in, coming to stop the idle drops to 1100 rpm and stays there for maybe max 5sec then settles at 850rpm. And he was looking at that on our roadtest after the tune completed, he was happy, but also stated it was a fine line between right and not so right. Why yours changed after months of being ok I'm surely not qualified to say, but your tuner should be able to resolve it.
Thread Starter
Safety Car
Joined: May 2007
Posts: 4,812
Likes: 46
From: Taylorsville North Carolina
Just to clean this up for future searches, my FAST intake had a crack in the very back where it is underneath the cowl. Not wanting to wait for repair, I repl with new FAST and all is good. Could have saved myself some money by taking CTD's suggestion in another thread and pulling intake myself.
