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77 air regulator - fast idle

4278 Views 17 Replies 6 Participants Last post by  Tony D
Is there any way to adjust the fast idle on cold start ups. I know the air regulator acts as a choke but I don't see any adjustment. Mine starts up idling at close to 1800 rpm and I think that's too high.

Any help appreciated.
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not really

it doesn otn act aas a choke, but rather as a bypass AROUND the throttle plate (the virtual opposite of a choke) to allow more air in to compensate for the extra fuel being injected during the "cold start loop" of the ECU.

1800 is not that high, and it should some down very quickly after the engine starts to normal idle speed. If you aren't idling at around 750 / 800rpm, then the idle speed screw (bypass) is adjusted incorrectly, and is causing your speedup above the "normal" 1500 fast-idle speed.

Then again, your BCDD may have a leak, also, causing excess air bypass.

You gotta check the system for leaks around the throttle plate before assuming a set and non-adjustable device causes a high fast idle speed.
Re: not really

It idles perfectly smooth at 900 rpm when warmed up. I just thought 1800 was a bit high for a cold engine, with no oil flow right at first, to be starting up at.
Math lesson:

"It idles perfectly smooth at 900 rpm when warmed up."

recommended idle speed 750.



1650 is only 150 higher than the "normal" 1500 fast idle speed. Set your base idle speed where it should be, and the fast idle speed will also decrease.

So like I said, there is something else causing your high idle, not the least of which is a higher base idle speed. By simple math, you are only 150 rpms high after you get your warmed up idle speed set correctly.
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Re: Math lesson:

Funny thing is my idle adjustment is turned all the way in and thats as low as I can get it. I used to be able to take it lower but a mechanic told me once that he made an adjustment there and now thats a low as I can get it. What adjustment could he have made to change the range of idle speed adjustment?
The air regulator has a small nut on a threaded post on one side of the unit. That's the adjustment mechanism; the nut locks the post in place, the post has a small range it can pivot through. it's spring loaded, i think pushing against the spring closes off the air, but i can't remember for sure.

it is possible that your AAR is starting off very open, and ending up somewhat open once hot. in that case, you could easily max out the actual idle adjustment and still idle high.

with the car warm, i would block off the air path to the AAR and see what rpm it idles at then. properly set the idle with the idle screw, and then take a look at adjusting the AAR. i think Blue's tech tips has something about the AAR; i would check there too.
clean the throttle plate...

your throttle plate may be gunked up and sticking open.

or the BCDD may be leaking by causing a high speed situation.

either way, if someone says "it's as low as it will go" that means something is worn to the point the factory adjustment range is not enough, calling for repairs.

With the idle bypass all the way down, I would start looking for things stuck open, and valves that are admitting bypass air.
To reiterate,

BCDD diaphragm ruptured rederring the idle speed screw ineffective to control bleed air passage past the TB.


False air ( unmetered by by the AFM ) entering downstream of the TB's butterfly in the forum of a vacuum leak.....ie

dip stick,
cam cover gasket
oil filler cap O-ring
PVC or breather hoses
Carbon Canister purge hose to intake
Head insp. plate gasket

The idle speed screw being bottomed out tells the story.
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actually I remember the mechanic telling me he "did" something to change the range of the idle speed screw. I just can't remember what it was (or even why he thought it was necessary). It wasn't something that wore or gunked or leaked air..... it was something he consciously did that changed the operation of it immediately. He did it while he was cleaning a contact (or whatever) inside the AFM that was causing my fuel pump not to operate.
He may have moved the throttle plate stop to keep it slightly open.
He adjusted the throttle stop screw.
so is this throttle stop screw inside the AFM or outside?
It's on the throttle body, not on the AFM. It's a philips head screw on the engine side of the throttle body facing down and has an 8mm locknut on it.

BTW the air regulator doesn't actually "compensate for the extra fuel being injected during the "cold start loop" of the ECU", it's still feeding metered air into the engine i.e. the air is still going through the AFM. My point is: the air regulator has absolutlely NO affect on the fuel mixture, it just raised the idle speed. And honestly I don't see a 1500-1800 fast idle being an issue unless the engine has bad rod bearings and rattles when it first starts until it gets oil pressure.
Thanks for the info Steve....

In fact the engine is brand new and that's why I thought a lower rpm warm-up might be beneficial.
tomato tomatoe

So with the cold start loop engaged on the ECU, you will not run PIG RICH without the aux air valve's extra air?

I beg to differ.

While it does add more air to speed up the engine, it is also a calibrated amount to keep the thing from going pig rich under cold-start situations.

Remember the AFM is no really off the first resistance point wihile at idle or fast idle, it really has no effect on mixture---though if the bypass screw on the AFM is mis set, it can screw up a lot with the mixture, as it will move it to the slow speed resistance track point...
Thanks Steve,
I found the screw and locknut and was able to get the idle down to the 750-800 range.
While it does add more air to speed up the engine, it is also a calibrated amount to keep the thing from going pig rich under cold-start situations.
Sorry but you're way off base here. Check this with a CO machine (I have) and you'll see it makes absolutely no difference. My car ran for years with NO air regulator whatsoever and didn't run rich without it. What you're saying is the same as saying opening the idle bypass changes the fuel mixture (which it doesn't).

Remember the AFM is no really off the first resistance point wihile at idle or fast idle, it really has no effect on mixture---


??? Again try moving the gate slightly at idle speed and notice the change in mixture. There aren't "resistance points" in the AFM's movement, it's a linear meter and ANY movement changes the mixture, especially more metered air that opens the gate more.

Think about what you're saying, if the AFM movement at idle has no effect on mixture, how can you adjust the idle AF ratio with the AFM? Instead of argueing just to argue, go look at the system and THINK about what you wrote, the air regulator has ZERO effect on mixture and can no way make a car "pig rich" if it's disabled. Again check the AF ratio with a CO machine with and without this regulator and get back to us on your findings. I've already done this years ago and it didn't have any affect on the mixture.
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CO Machine?

It would stand to reason the CO machine will not show any difference if it's tailpiped.

I can speak from SAAB experience that doing it on them WILL show a difference PRE-CAT. The system is the same in design, just a difference license, so it would stand to reason the same thing would happen to a Datsun.

All I will ask is that you are saying that while the cold-start injector is spraying into the intake, the AFM is at the "idle position", and the air regulator is CLOSED, there will be ABSOLUTELY no difference in CO coming out of the engine's EXHAUST MANIFOLD (not the tailpipe), that if the Air Regulator is open.

And also, that the cold start injector, when NOT adding fuel during the SAME pair of tests, will return the EXACT SAME readings?

Physics dictates otherwise. And on a SAAB (who pipes so clean [PRE-CAT] due to extensivelly better fuel mapping than the Datsun) these same tests will show VISIBLE AND DRAMATIC pre-cat emissions and CO readings when these tests are conducted.

Now if you tailpipe test, you have to make sure you have a GOOD instrument, and that it's a NON-Catalyzed car...

I have watched Tech test and "reset" a bypass screw on an AFM while tailpiping the car, and then call the AFM "bad" due to NO DIFFERENCE. Well, the CAT being there might have something to do with it...
But I won't go into that area, I'm just curious how you will logically tell me those scenarios above work out. And really, if they are "unrequired" then why did Datsun spend soooo much money to include them?

It' almost counter intuitive.
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