Auxilliary air valve function in supercharges Ljet
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From: Park Ridge, IL (near Chicago)
Auxilliary air valve function in supercharged Ljet
Hi guys,
Since my car is coming out of hibernation, my brain is trying to do the same. I have noticed that my cold idle warm up system behaves in a somewhat exaggerated manner, and I wondered if this is related to my supercharger.
I set my fully warm idle speed around 800 RPM, but when started from cold, the car starts out around 1800 RPM then settles down to normal idle after about 3-5 minutes. I understand that the auxilliary air valve or AAV is essentially an extra idle bypass valve, that is controlled by its bimetallic strip and engine block temperature. I have examined my valve and it opens and closes properly, but on my car it seems to really, really increase idle speed. It's almost to the point that if I drive off before the idle has dropped to normal, I can hardly hold the car in place if I shift into gear. 84 US Automatic.
I was wondering if this exaggerated situation (if it is really abnormal) might be because of positive intake pressure from the SC. If so, maybe a flow restrictor in the in the AAV plumbing might moderate the RPM increase. I'm thinking about a 1" bushing with a 1/4" hole in it, placed inside the tubing. Exact size determined by trial and error.
Any comments?
Thanks,
Dave
Since my car is coming out of hibernation, my brain is trying to do the same. I have noticed that my cold idle warm up system behaves in a somewhat exaggerated manner, and I wondered if this is related to my supercharger.
I set my fully warm idle speed around 800 RPM, but when started from cold, the car starts out around 1800 RPM then settles down to normal idle after about 3-5 minutes. I understand that the auxilliary air valve or AAV is essentially an extra idle bypass valve, that is controlled by its bimetallic strip and engine block temperature. I have examined my valve and it opens and closes properly, but on my car it seems to really, really increase idle speed. It's almost to the point that if I drive off before the idle has dropped to normal, I can hardly hold the car in place if I shift into gear. 84 US Automatic.
I was wondering if this exaggerated situation (if it is really abnormal) might be because of positive intake pressure from the SC. If so, maybe a flow restrictor in the in the AAV plumbing might moderate the RPM increase. I'm thinking about a 1" bushing with a 1/4" hole in it, placed inside the tubing. Exact size determined by trial and error.
Any comments?
Thanks,
Dave
Last edited by j.kenzie@sbcglobal.net; Mar 7, 2020 at 12:35 AM.
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Check to see if the problem is actually related to the AAV and its plumbing by disconnecting the valve electrically prior to a cold start. If it still idles high or really if it doesn't stall, there's air coming from somewhere else. It's a simple quick-and-dirty test that might narrow your search some. Right now, all we know is that there's air from somewhere that causes the fast cold idle.
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Thanks, Bob. It's been a while, but I I think the valve is open when unpowered. Then when powered, the electrical heating of the bimetallic strip, or the engine heat closes the valve. That being said, there is probably a way to turn on the power to the valve and make it close, in order to perform your test. Good idea.
Thanks,
Dave
Thanks,
Dave
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Hi Dr. Bob,
Actually, looking at the wiring diagrams, it appears that if I were to jumper the fuel pump relay for say 5 minutes, that would energize the heating element in the AAV and should warm the bimetallic strip and cause the valve to close. If I got a normal warm state idle speed, then I would know that the open AAV is causing too much air intake and too much idle speed increase. I will give that a try.
Thanks,
Dave
Actually, looking at the wiring diagrams, it appears that if I were to jumper the fuel pump relay for say 5 minutes, that would energize the heating element in the AAV and should warm the bimetallic strip and cause the valve to close. If I got a normal warm state idle speed, then I would know that the open AAV is causing too much air intake and too much idle speed increase. I will give that a try.
Thanks,
Dave
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How much boost are you making at idle? 0-1 pound won't really affect idle that much but it sounds like you have at least a few pounds with Carl's stage 2.
That will definitely raise you idle as you are forcing more air through the AAV.
You could buy another AAV and bend the bi metallic strip inside so it only is open partially when the engine is cold so less air gets by.
That will definitely raise you idle as you are forcing more air through the AAV.
You could buy another AAV and bend the bi metallic strip inside so it only is open partially when the engine is cold so less air gets by.
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You've had this installed for a while now, this sounds like a new issue or has it always done this?
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Hi Blake,
That's kind of what I was thinking with some kind of flow restriction. I still want some flow for the cold start system to work, but not too much. By the way it's kind of a vicious cycle, because as the idle goes up to 1800 to 2000 RPM during warm up, the boost keeps it high until the valve closes. I'm thinking maybe I can just dampen this system a little by reducing AAV flow.
Thanks,
Dave
That's kind of what I was thinking with some kind of flow restriction. I still want some flow for the cold start system to work, but not too much. By the way it's kind of a vicious cycle, because as the idle goes up to 1800 to 2000 RPM during warm up, the boost keeps it high until the valve closes. I'm thinking maybe I can just dampen this system a little by reducing AAV flow.
Thanks,
Dave
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From: Park Ridge, IL (near Chicago)
You shouldn't have positive pressure at idle unless your BOV isn't working. Back when my 81 was still L-Jet, even at 17psi of max boost I never had idle issues even when relatively chilly outside.
You've had this installed for a while now, this sounds like a new issue or has it always done this?
You've had this installed for a while now, this sounds like a new issue or has it always done this?
I've had this for a while. I think it dates to a time when I fixed some intake leaks that limited boost. I have a boost gauge, but I think it measures after the throttle plate. You're right though, I can hear air escaping near the BOV so there shouldn't be much pressure. Either way something is letting too much air past the throttle plate during warm up, then gradually returning flow to normal. Presumably this is the AAV just acting a little strongly?
Thanks,
Dave
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From: Park Ridge, IL (near Chicago)
Thanks,
Dave
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Dave, you are correct. At idle, the BOV should b open, and there is very little to no positive pressure in the intake system. Diagnose this as if the car was not supercharged. Does your engine have the secondary idle speed valve strapped onto the engine cross-brace?
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Dave, my '79 exhibits exactly the same behavior - also right at about 1,800 RPM and I have my fully warmed up idle set at 800 too. Mine has done it since the supercharger build and I never even ran the car bafore that (bought it and shipped it straight to Carl) so have no frame of reference for how NA cars of the same vintage behave. If you come up with something workable that limits the cold start RPMs to 1,200 or so, let me know. I'm inclined not to mess with mine since it starts every time and always exhibits the same behavior.
The line that controls my BOV comes from the front of the spider intake. Under boost, the valve is forced closed so the pressure goes through the intake and under vacuum it's open so the supercharger air diverts through it. With the car at idle, even an elevated idle you shouldn't be anywhere near positive resistance (boost) on the Vacuum/Boost gauge so the BOV should be open and bypassing most of the air.
For the guys with non supercharged CIS cars, what's your just started idle behavior (RPMs)?
The line that controls my BOV comes from the front of the spider intake. Under boost, the valve is forced closed so the pressure goes through the intake and under vacuum it's open so the supercharger air diverts through it. With the car at idle, even an elevated idle you shouldn't be anywhere near positive resistance (boost) on the Vacuum/Boost gauge so the BOV should be open and bypassing most of the air.
For the guys with non supercharged CIS cars, what's your just started idle behavior (RPMs)?
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Thanks Pete. What you and Eric say sounds correct. I should not have much if any boost at idle, even 1800RPM warm-up idle.
I did the AAV jumper test:
The AAV is energized by the fuel pump power, so I jumpered my fuel pump relay for 5 minutes to simulate engine warm-up period. Indeed, the car then started easily, and idled right at 800 RPM. This tells me that the additional intake air is coming through the AAV, and if I want to reduce that super high idle, I could modify the AAV intake mechanism. I thought the easiest, most reversible way would be to put a flow reducer inside the AAV piping. I will measure, but I thought I'd use a 3/4" copper pipe cap and drill maybe a 1/4" hole in it before clamping it inside the rubber hose. I will then have some flow when cold to run my cold start valve but maybe not enough to put the car to 2000 RPM. BTW that 2000 RPM will put the car into a hunting idle as the idle TPS activates fuel shutoff. Therefor with this high idle, I have to disconnect my idle TPS at the throttle body.
Thanks,
Dave
I did the AAV jumper test:
The AAV is energized by the fuel pump power, so I jumpered my fuel pump relay for 5 minutes to simulate engine warm-up period. Indeed, the car then started easily, and idled right at 800 RPM. This tells me that the additional intake air is coming through the AAV, and if I want to reduce that super high idle, I could modify the AAV intake mechanism. I thought the easiest, most reversible way would be to put a flow reducer inside the AAV piping. I will measure, but I thought I'd use a 3/4" copper pipe cap and drill maybe a 1/4" hole in it before clamping it inside the rubber hose. I will then have some flow when cold to run my cold start valve but maybe not enough to put the car to 2000 RPM. BTW that 2000 RPM will put the car into a hunting idle as the idle TPS activates fuel shutoff. Therefor with this high idle, I have to disconnect my idle TPS at the throttle body.
Thanks,
Dave
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Thanks Pete. What you and Eric say sounds correct. I should not have much if any boost at idle, even 1800RPM warm-up idle.
I did the AAV jumper test:
The AAV is energized by the fuel pump power, so I jumpered my fuel pump relay for 5 minutes to simulate engine warm-up period. Indeed, the car then started easily, and idled right at 800 RPM. This tells me that the additional intake air is coming through the AAV, and if I want to reduce that super high idle, I could modify the AAV intake mechanism. I thought the easiest, most reversible way would be to put a flow reducer inside the AAV piping. I will measure, but I thought I'd use a 3/4" copper pipe cap and drill maybe a 1/4" hole in it before clamping it inside the rubber hose. I will then have some flow when cold to run my cold start valve but maybe not enough to put the car to 2000 RPM. BTW that 2000 RPM will put the car into a hunting idle as the idle TPS activates fuel shutoff. Therefor with this high idle, I have to disconnect my idle TPS at the throttle body.
Thanks,
Dave
I did the AAV jumper test:
The AAV is energized by the fuel pump power, so I jumpered my fuel pump relay for 5 minutes to simulate engine warm-up period. Indeed, the car then started easily, and idled right at 800 RPM. This tells me that the additional intake air is coming through the AAV, and if I want to reduce that super high idle, I could modify the AAV intake mechanism. I thought the easiest, most reversible way would be to put a flow reducer inside the AAV piping. I will measure, but I thought I'd use a 3/4" copper pipe cap and drill maybe a 1/4" hole in it before clamping it inside the rubber hose. I will then have some flow when cold to run my cold start valve but maybe not enough to put the car to 2000 RPM. BTW that 2000 RPM will put the car into a hunting idle as the idle TPS activates fuel shutoff. Therefor with this high idle, I have to disconnect my idle TPS at the throttle body.
Thanks,
Dave