Fast Idle Problem Continues
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
If your pcv fresh air supply is leaking, yes that's a problem. You should seal it up somehow, perhaps remove that swivel fitting and plug that hole and drill a new one, or maybe epoxy that one in place (though I would be afraid the epoxy would eventually crack).
You can't just unhook it and plug the hole, the intake manifold sucks air from the port behind the throttle body through the valley pan, and the hose you're looking at is the fresh air supply for the engine. If you leave that fresh air supply unhooked, the engine will be drawing air into itself that is unmetered by the MAF, thus throwing off your fueling and potentially your idle. You must provide metered fresh air to the engine for the system to work properly.
Perhaps you could buy a new fitting from Vararam.
It's possible this air leak right by the MAF is causing your problem. I'm a little dubious but it shouldn't leak, that is for sure.
You could test all of this by installing your stock air cleaner again and see if you still have the problem.
You can't just unhook it and plug the hole, the intake manifold sucks air from the port behind the throttle body through the valley pan, and the hose you're looking at is the fresh air supply for the engine. If you leave that fresh air supply unhooked, the engine will be drawing air into itself that is unmetered by the MAF, thus throwing off your fueling and potentially your idle. You must provide metered fresh air to the engine for the system to work properly.
Perhaps you could buy a new fitting from Vararam.
It's possible this air leak right by the MAF is causing your problem. I'm a little dubious but it shouldn't leak, that is for sure.
You could test all of this by installing your stock air cleaner again and see if you still have the problem.
How does the PCV system control air flow? I am used to a pcv valve that modulates air flow based on vacuum. How does corvette do this?
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
The valley cover pipe has an orifice that limits airflow and a baffle underneath that is supposed to limit oil getting into the intake. But it doesn't work that well so many people install a catch can online with the valley to intake port.
Why don't you just get a vacuum port and screw it into the vararam maf mount (they must sell one or perhaps a standard brass nipple will work). Then use a regular pcv hose to connect to the valve cover? That will eliminate the leak.
Why don't you just get a vacuum port and screw it into the vararam maf mount (they must sell one or perhaps a standard brass nipple will work). Then use a regular pcv hose to connect to the valve cover? That will eliminate the leak.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
Stopped the air leak on the pcv fitting by installing an o-ring. Did not change anything, still hanging idle. I just did a pressure test again by plugging the entrance to the maf body and pressurizing through the brake booster line. This time no leak at that pcv fitting. I sprayed soapy water over the areas I can see and no sign of leak. However, I can hear the sound of air moving through the intake manifold if I hold my ear near it. This would indicate that air is escaping somewhere, right? Only other explanations would be overlap in valves or leaking rings. The system is pressurizing because when I uncouple the compressor a considerable amount of air bleeds out from the line connected to the brake booster. I know someone with a smoke machine and that is my next step, since I cannot find any leak with my tools and I hear what sounds like air moving through the intake manifold.
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
A valve is always open when the engine is off.
You don't have a vacuum leak, it's not your problem.
Do you have any codes?
Can you borrow your buddy's MAF to test?
If not, can you read codes then unhook your battery and see what happens?
You don't have a vacuum leak, it's not your problem.
Do you have any codes?
Can you borrow your buddy's MAF to test?
If not, can you read codes then unhook your battery and see what happens?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
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
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
I understand that some valve or valves are going to be open, but if you have no overlap, you either have a intake valve open with exhaust closed or an exhaust valve open with intake closed. Either way, you don't get leak out the engine if the valve is seating properly.
I tried disconnecting the battery, then redoing the idle relearn procedure. Made no difference. Since the person with the smoke machine is the same one with the z06, I might be able to swap maf sensor with him if I go to his house.
I may try disconnecting the maf by removing the one wire, as described earlier in this thread.
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
There are no pending codes or set codes. The car runs fine, no loss of power, no hesitation.
I understand that some valve or valves are going to be open, but if you have no overlap, you either have a intake valve open with exhaust closed or an exhaust valve open with intake closed. Either way, you don't get leak out the engine if the valve is seating properly.
I tried disconnecting the battery, then redoing the idle relearn procedure. Made no difference. Since the person with the smoke machine is the same one with the z06, I might be able to swap maf sensor with him if I go to his house.
I may try disconnecting the maf by removing the one wire, as described earlier in this thread.
I understand that some valve or valves are going to be open, but if you have no overlap, you either have a intake valve open with exhaust closed or an exhaust valve open with intake closed. Either way, you don't get leak out the engine if the valve is seating properly.
I tried disconnecting the battery, then redoing the idle relearn procedure. Made no difference. Since the person with the smoke machine is the same one with the z06, I might be able to swap maf sensor with him if I go to his house.
I may try disconnecting the maf by removing the one wire, as described earlier in this thread.
Does your cruise control work?
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
But wouldn't that just pressurize the cylinder if the exhaust valve is closed?
I can log air flow rate from MAF and intake manifold absolute pressure with Dash Command. Can this data be used to determine proper operation of MAF?
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
I was wondering if your clutch switch went bad making the car think the clutch is partially depressed.
If you can log maf HZ while idling normally, then while moving slowly while the clutch is pushed in, it should read higher while the engine is idling higher as more air is going by. So if that's what you get that's what I would expect to get, hz goes up as the engine rpm goes up and flows more air.
If you can log maf HZ while idling normally, then while moving slowly while the clutch is pushed in, it should read higher while the engine is idling higher as more air is going by. So if that's what you get that's what I would expect to get, hz goes up as the engine rpm goes up and flows more air.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
Another datalogging effort. This time I had engine idling, car stopped in neutral with brake on. Let off brake and car coasts downhill. Allowed car to accelerate to 12 km/hr then starts slowing down without braking till car stops. Logged engine rpm, control module voltage, maf frequency, and speed. Here is what I got:
rpm module V MAF Hz VSS km/h
660 13.945 1971 0
1036 14.019 2647 4
1109 13.969 2668 6
1251 13.969 2735 7
1276 14.044 2754 8
1308 13.969 2784 10
1462 14.069 2872 12
1514 14.069 2864 12
1543 14.044 2867 12
1550 14.044 2905 12
1453 14.044 2872 9
1457 14.069 2862 5
781 14.094 2580 0
680 13.87 2074 0
663 13.945 2012 0
What I see is that maf frequency increases with increasing engine speed, which is what it should do. Anything else?
rpm module V MAF Hz VSS km/h
660 13.945 1971 0
1036 14.019 2647 4
1109 13.969 2668 6
1251 13.969 2735 7
1276 14.044 2754 8
1308 13.969 2784 10
1462 14.069 2872 12
1514 14.069 2864 12
1543 14.044 2867 12
1550 14.044 2905 12
1453 14.044 2872 9
1457 14.069 2862 5
781 14.094 2580 0
680 13.87 2074 0
663 13.945 2012 0
What I see is that maf frequency increases with increasing engine speed, which is what it should do. Anything else?
Race Director
Joined: May 2004
Posts: 16,667
Likes: 1,209
From: Raleigh, NC
St. Jude Donor '15
I agree with Joe.. doesn't sound like that is your problem. You'll always get a little bit of leakage through the engine, at the very least from the cylinders where the intake valves are open (rings will leak some). Totally normal.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
So now where do I turn? I doubt those maf readings help any.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 1,190
Likes: 68
From: Woodland Hills California
0 615 0 14.9 22
0.2s 630 0 16.1 22
0.6 848 6 18.4 22
0.7 1093 4 18.4 22
0.8 1093 3 18.4 22
1.0 980 2 18.8 22
1.1 788 0 18.4 22
2.7s 659 0 14.5 22
What I notice is that the throttle plate is moving without any pedal input.
Also measured fuel trims:
RPM STFT1 LTFT1 STFT2 LTFT2
637 -1.6% -4.7% -7% -6.2%
695 -2.3 -4.7 1.6 -6.2
1768 -0.8 -4.7 -0.8 -6.2
1993 -1.6 -4.7 -3.1 -6.2
2405 -0.8 -5.5 0.8 -8.6
Race Director
Joined: May 2004
Posts: 16,667
Likes: 1,209
From: Raleigh, NC
St. Jude Donor '15
The throttle moving around without any input to the pedal is normal...
The computer controls it (among other things) to control idle RPM.
Idle RPM can be different for different speeds, gears, neutral vs. in gear, etc. so without seeing the tune I can't say what's normal or not. I was hoping I'd see something obvious but don't.. That TPS value at idle is a little lower than mine ever was if memory serves, but I don't know if it's enough to matter
The computer controls it (among other things) to control idle RPM.
Idle RPM can be different for different speeds, gears, neutral vs. in gear, etc. so without seeing the tune I can't say what's normal or not. I was hoping I'd see something obvious but don't.. That TPS value at idle is a little lower than mine ever was if memory serves, but I don't know if it's enough to matter
Tech Contributor
Joined: Jul 2002
Posts: 14,950
Likes: 264
From: St. Louis, MO
St. Jude Donor '08
This is perplexing. With a stock tune this shouldn't be happening. With an aftermarket tune, it's common and easy to fix.
It started with the throttle body. Now you have a new one and it still does it.
If it was me, I'd put the stock air cleaner back on and see what happens.
It started with the throttle body. Now you have a new one and it still does it.
If it was me, I'd put the stock air cleaner back on and see what happens.