[Qik1320]

My 70 chevelle is a ls6 454 4spd 4;10 gear car . I love the cowl hood and i have hooked everything up as close to original on the hood as I could , The cowl induction door closes on start-up but doesnt open up after that while driving it. On Sunday I lined it up down at the industrial park . Hammered it down all 4 gears up to 6 grand on the tac then shut it down. No open door. Any ideas on this. Thanks Dave

 

[mr 4 speed]

Dave,your secondaries might not be fully opening...

 

[Qik1320]

I forgot to mention I am running a 4779 750 cfm holley mech choke mech secondaries , but I will check to see if they are opening . Thanks, Dave

 

[LS_5]

I'd try replacing the in-line Flow Control Valve first (mounted in the rubber hose about 8" below the flapper door). They seem to go bad fairly regularly, it seems. Just like alot of aftermarket stuff.

Ground Up carries them:

http://www.ss396.com/mm5/merchant.m...13&Category_Code=ENGINEDETAILINGCOWLINDUCTION

 

[chevelless1]

if the valves fail, they fail in the open position

 

[LS_5]

Mine went slowly, as described, before it failed completely.

 

[mr 4 speed]

I don't run the flow control valve on mine..nor did did I on my last 70 SS that I had for 10 years

 

[DZAUTO]

Do some simple troubleshooting. Tooooooooooooo easy-----------
First, does the outer door open when the engine is shut off? It should.
The vacuum pod which actuates the outer door is simply a rubber diaphragm with a spring pressing against it. If there are no holes or cracks in the diaphragm, then when manifold vacuum is applied to the pod, the diaphragm overides the tension of the spring pushing against it and then pulls the rod which is connected to the outer door. When manifold vacuum drops low, or completely goes away, the spring inside the pod pushes against the diaphragm and the rod pushes open the outer door. When a vacuum is applied to the pod, there should NOT be any leak. So, disconnect the vac hose from the engine, apply a vacuum and see if the pod will remain closed.
Part of the vac system for the outer door includes a check valve spliced in the vac supply hose to the pod. This check valve allows vacuum to flow freely ONLY IN ONE direction. Therefore, if operating properly, the check valve should flow easily one way and have a significant resistance to flow the other way. You can use a vacuum pump/gauge to check the flow in both directions, but the simplest way to check it is simply to just suck on each end of the valve. There should be a noticeable difference when sucking on each end. The side of the check valve which has a noticeable resistance to vacuum goes TOWARD manifold vacuum.
Now, lets stop right here.
There are TWO types of engine vacuum: manifold and ported (or venturi vacuum if you prefer).
Manifold vacuum is ANY vacuum source BELOW the throttle plates of the carb.
Ported (or venturi) vacuum is ABOVE the throttle plates.
During hard acceleration (or a heavy load condition), manifold vacuum will drop drastically, or even go to zero. Whereas, ported vacuum INCREASES with hard acceleration or a heavy power load. Therefore, if you have the vac hole for the outer door pod connected to a ported vac source-----------------IT WILL NOT OPEN DURING HEAVY ACCELERATION. IT MUST BE CONNECTED TO MANIFOLD VACUUM!!!! From the factory, it was connected to a fitting in the rear intake runner.
The outer door for a CI hood functions better with a manual tranny car than it does with an auto tranny car. The reason for this is because with a manual tranny, when you depress the throttle, the load is fixed (if that is a good explanation), as opposed to an auto tranny because if you accelerate hard enough with an auto tranny, it will down shift which allows the rpm to increase, which decreases the load on the engine and manifold vacuum jumps up and the vac pos closes the outer door. Does that make sense to everyone?
For example, if you are driving 30mph with an auto tranny, push the gas pedal down, the tranny immediately down shifts and manifold remains relatively high and the outer door stays closed, or doesn't open much. With a manual tranny car, when you accelerate, and leave the tranny in 4th gear, the manifold vac drops WAY down and will not increase until the load on the engine decreases. Thus, the outer door opens faster and stays open longer-----------------------if everything is properly connected and functioning properly. :thumbsup:

The inner door is opened/closed by an electric solenoid, and that solenoid is activated by an elec switch on the accelerator pedal when the pedal is pushed to the floor. Thus, the inner door is either fully open, or fully closed------------no inbetween for it.

 

[Qik1320]

Tom , Thanks for explaining the operation in detail. I will follow all the steps and see how I make out. It is fed on manifold vacuum right where the brake booster connects.The door does close on start up and open when the car shuts off. I tested the pod and it holds pressure But I am not running the check valve, I was told I dont need it. I will contact ground up for the part. I wonder if timing would have anything to do with it. I am running 12 degrees after

 

[DZAUTO]

I wonder if timing would have anything to do with it. I am running 12 degrees after

Nah.
It probably stays shut when running because you don't have the check valve. Although, even without the check valve, under hard acceleration, especially under a load, it SHOULD open up.
The check valve allows the door to pop open quickly, and then close sloooooooooooooowly. :thumbsup:

This is SIMILAR to how the outer door should work. Mine closes slower than it should with a factory or repo valve because I have a slightly different check valve in mine. But, you get the idea.

 

[Qik1320]

Thanks again Tom . Above and beyond ! Mine closes shut and doesnt open again . But now I see how its supposed to work.

 

[Big James 4XL]

Should work without the check valve. Be sure the vacumn line is not being crimped between the hood and cowl when the hood is closed.

Mine works fine with no check valve, it's wide open in this picture from CB08, just like it should be.

 

[DZAUTO]

Should work without the check valve. Be sure the vacumn line is not being crimped between the hood and cowl when the hood is closed.

Mine works fine with no check valve, it's wide open in this picture from CB08, just like it should be.

And James is 100% correct on all points. But without the check valve, it will close very quickly when manifold vacuum builds back up instead of closing slowly.
I just personally like the quick opening, slooooooooooow closing as it was intended from the factory.

 

[fastkawasaki454]

I know this thread is a month old, but can someone elxplain to me the difference between the inner and outer doors? Im assuming the outer is the one you can actually see open, but I never knew there was an inner door.

 

[novaderrik]

i just want to know who at GM thought it was a good idea to put a cowl flapper door on mass produced cars that stays open when the engine is off? these cars were designed to be driven in all climates and left outside in all conditions- i can just see brushing 6" of freshly fallen heavy wet snow off the hood of the car while half awake at 6 am and taking the flapper door off with the broom. and then think of all the snow that gets packed inside there, and all the leaves and junk that could wind up in there while parked when the car lives a totally outdoor existence.
i'm talking about the cars back when they were just another driver- and not the pampered pieces of automotive art they have become.

 

[Alwhite00]

This is SIMILAR to how the outer door should work. Mine closes slower than it should with a factory or repo valve because I have a slightly different check valve in mine. But, you get the idea.

What valve are you running? I have a re-pop valve & wouldn't mind it closing slower.

LK

 

[mr 4 speed]

i just want to know who at GM thought it was a good idea to put a cowl flapper door on mass produced cars that stays open when the engine is off? these cars were designed to be driven in all climates and left outside in all conditions- i can just see brushing 6" of freshly fallen heavy wet snow off the hood of the car while half awake at 6 am and taking the flapper door off with the broom. and then think of all the snow that gets packed inside there, and all the leaves and junk that could wind up in there while parked when the car lives a totally outdoor existence.
i'm talking about the cars back when they were just another driver- and not the pampered pieces of automotive art they have become.

...or those pesky Oldsmobile that had the under the bumper ram air inlets

 

[Slick71ss]

I know this is an old thread, but I have the same problem. My vacuum never drops low enough to open the cowl door. I have it hooked to manifold vacuum at the base of my Holley 850 double pumper. I also have a vacuum gauge t'd in the same line and the lowest I ever see it is at 12" at idle. I took the car out today and floored it through the gears(th400) and nada. I didn't get a chance to see what the vacuum reads under WOT though. Anyways, is the vacuum stronger at the base of the carb compared to the rear of the manifold? Engine is a zz502.

 

[Big James 4XL]

Your vacumn is low enough, there's some other problem! All these engines "suck" under acceleration. If it didn't pull some vacumn it wouldn't accelerate.

A crimped line/bad servo/check valve in wrong or malfunctioning.

 

[Slick71ss]

It works fine when I hook a vacuum pump to it and then release the vacuum. The door closes on start-up and opens at shut down. My vacuum just doesn't seem to be getting low enough during WOT.