My high idle came back again a few days ago, 2500 rpm start up and stays that way for 20 - 30 minutes. Noticed that after a high idle for a while, I hear a great sucking sound then the idle drops back to normal........
Is there a set of valves or something in the breather box ( below the plenum where the two large hoses connect to the injector housings )
I'm guessing that maybe one of these valves is sticking open and allowing too much air into the injector housing ( ie vaccum leak ) but I don't know alot about the system.
The other question I would have is if the "Air Pump" system has a way to inject air into the plenum and could be causing too much air into the system.
It's not the first time Iv'e heard the "sucking sound" just as the idle returns to normal. I know it's not a static vacuum leak like a gasket or hose or something because after a while the idel settles down.
My high idle came back again a few days ago, 2500 rpm start up and stays that way for 20 - 30 minutes. Noticed that after a high idle for a while, I hear a great sucking sound then the idle drops back to normal........
Is there a set of valves or something in the breather box ( below the plenum where the two large hoses connect to the injector housings )
Joey:
Sorry, no valves under there. It's just a open space for the oil to separate from air under high RPM's. There a section on it in the Heart Of The Beast that describes how the engineers needed to cobble a system together with enough extra volume so that the oil would "de-foam" and return to a liquid. It's because the oil pan isn't very deep and the crankshaft whips the oil into a foam, especially at high RPMs. Aparrently in the first prototype engines blew oil all over the place, a big mess.
Quote:
The other question I would have is if the "Air Pump" system has a way to inject air into the plenum and could be causing too much air into the system.
It's not the first time Iv'e heard the "sucking sound" just as the idle returns to normal. I know it's not a static vacuum leak like a gasket or hose or something because after a while the idle settles down.
You may be onto something here, I'll have to dig out my Helms again and see how the emissions stuff all works. I seem to recall reading that the air pump does kick in on the initial start, but I think all it does is blow in extra air into the exhaust manifolds to help meet EPA cold start emissions regulations.
Sorry, no valves under there. It's just a open space for the oil to separate from air under high RPM's. There a section on it in the Heart Of The Beast that describes how the engineers needed to cobble a system together with enough extra volume so that the oil would "de-foam" and return to a liquid. It's because the oil pan isn't very deep and the crankshaft whips the oil into a foam, especially at high RPMs. Aparrently in the first prototype engines blew oil all over the place, a big mess.
You may be onto something here, I'll have to dig out my Helms again and see how the emissions stuff all works. I seem to recall reading that the air pump does kick in on the initial start, but I think all it does is blow in extra air into the exhaust manifolds to help meet EPA cold start emissions regulations.
TomC
'90ZR1 #792
Thanks Tom.
It's really weird, after the starter R&R the idle was fine at startup for over a week. It seems that when I do some work on the car ( plenum pull ) the car idles fine for several days at start up then it just starts with the high idle at startup for no apparent reason. Iv'e got two chips, happens with them both. Actually, I remember starting it one morning and it had the high idle, I shut it down and immidiatley swapped out the chip. When I started it, the idle was correct so I thought I had found the problem. It idled fine for the next 5 or 6 days just fine on startup, then....a few days later it the high idle at startup just came back again. Only happens on the first startup of the day and lasts for 20 - 30 minutes. If you can find something in your helms, let me know.
.....usually a sucking sound indicates an air (vac) leak, so the rpms would be expected to be elevated....but you say the elevated rpm's come before the sucking sound...right Joe?
Could the sucking sound be happening at the same time & the motor noise is too loud & you can't hear the sucking sound????
I hate to suggest this....it's a bit nuts but back with carbed motors we used to put a shop rag on the carb air horn to see if the motor would starve for air & die...if it didn't you knew to look for a vac leak...put a peice of stiff cardboard over the air horn & see if the motor loads up & wants to die??? If not then you have a vac leak...if it starts to die & not recover then the sound is something else.
Our A.I.R. pumps flow enough air that if there was a leak in that system maybe it would sound like something sucking???? I don't know but mine is very audible at start up....I hear the electric motor run from inside the car. Qualifier: I have a 90 so maybe there are no common denomitators to 91.
Sorry I don't have anything useful!
Tom
forgot this, TomC is right about the A.I.R. system, it blows!
Location: No more yankee my wankee, the Donger is tired!
Quote:
Originally Posted by tomtom72
I hate to suggest this....it's a bit nuts but back with carbed motors we used to put a shop rag on the carb air horn to see if the motor would starve for air & die...if it didn't you knew to look for a vac leak...put a peice of stiff cardboard over the air horn & see if the motor loads up & wants to die??? If not then you have a vac leak...if it starts to die & not recover then the sound is something else.
It's not nuts, and you'll have to use something sturdier than cardboard. WHat I have fund that works very well is vinyl/plastic covered 3 ring binders. Helps seal it up, and the motor should die within 4-6 seconds. I've had a car with injector O-ring issues that have continued to run with the airhorn blocked off.
It's not nuts, and you'll have to use something sturdier than cardboard. WHat I have fund that works very well is vinyl/plastic covered 3 ring binders. Helps seal it up, and the motor should die within 4-6 seconds. I've had a car with injector O-ring issues that have continued to run with the airhorn blocked off.
So if this were tried with cardboard, it might not actually have been an accurate test? I took a piece of cardboard and did that, but the suction was so much it would deform the cardboard and practically sucked the thing in! The motor didn't die, but rpms dropped to under 500. The cardboard deformed more and more so I wasn't sure it was really sealing that well. I'll have to dig up a binder.
On the drivers side, sort of below the throttle body, coming out of the plenum, there is a rubber block with two lines running out of it. The lines become loose in the rubber block and make that sucking sound. At least thats what it was on mine. Jeff is the man who I got mine from. I Hope this helps.
Location: No more yankee my wankee, the Donger is tired!
Quote:
Originally Posted by Aurora40
So if this were tried with cardboard, it might not actually have been an accurate test? I took a piece of cardboard and did that, but the suction was so much it would deform the cardboard and practically sucked the thing in! The motor didn't die, but rpms dropped to under 500. The cardboard deformed more and more so I wasn't sure it was really sealing that well. I'll have to dig up a binder.
Correct, as soon as it starts being sucked in, the cardboard will crease. This little amount will let air flow directly in. You need something that will provide a complete seal.
On the drivers side, sort of below the throttle body, coming out of the plenum, there is a rubber block with two lines running out of it. The lines become loose in the rubber block and make that sucking sound. At least thats what it was on mine. Jeff is the man who I got mine from. I Hope this helps.
On my 90 I had a bad sucking noise and it was at this rubber block it wasnt sealing anymore theres the hard PVC pipe going to it and a plastic one I just used rubber hose with clamps to fix that problem!
Corvette Store
High Idle and sucking sound........
