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High Idle Advice - Delete Decel Valve?

 
Old 02-28-2011, 10:15 PM
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SanDiego928
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Question High Idle Advice - Delete Decel Valve?

1981 4.5 - 5 speed convert from auto

My mechanic is thinking the decel valve may be causing my very high idle.

After reading the below thread I was wondering if removing it could be the answer and I could do this myself instead of spending oddles of cash on a replacement.

https://rennlist.com/forums/928-foru...ve-delete.html

I'm in California and wondering about passing the smog test and current thinking on this subject in general and also in regards to a reply that questioned the safety of doing the removal.

Thanks so much for advice

This part is way to expensive I think

Mark
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Old 02-28-2011, 10:34 PM
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also check the vacuum lines from the fuel dampener and regulators, leaking fuel pools in the decel valve and kills it. happened to mine. so i removed it. never looked back.
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Old 03-01-2011, 01:18 AM
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SanDiego928
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Originally Posted by Ducman82 View Post
also check the vacuum lines from the fuel dampener and regulators, leaking fuel pools in the decel valve and kills it. happened to mine. so i removed it. never looked back.
Interesting, thanks. Smoke tested the system so I think I am good there. v lines checked, cable checked fine. she runs real good except for the idle at 1600.
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Old 03-01-2011, 02:03 AM
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nowlings
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Assuming your state doesn't require emission testing, this is an easy bypass and you will never notice it's been removed from a driveability standpoint. This was 70's era smog technology intended to minimize raw fuel dump right after heavy engine loads/acceleration.
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Old 03-01-2011, 02:09 AM
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Woops-sorry.... You are from California; You are going to need to pass the visual if the technician performing the test is smart enough to know what he or she is looking for. Lot's of the current test only smog techs were book smart enough to get the licesnsing, but don't really understand or know their way around our 928 engine compartments. I just don't think this would be causing your high idle however.
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Old 03-01-2011, 02:13 AM
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mark kibort
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just spray some chemtool or brake cleaner in the area. if the idle goes up even higher, its the decel valve. if not, just turn down the idle screw!
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Old 03-01-2011, 04:28 AM
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A good decel valve will not effect your smog test readings as it only opens up when, as the name implies, decelerating.

A bad decel valve can screw up a smog test as it can create a pretty good size vacuum leak.

I'm don't know if not having the decel valve will fail the visual part of the inspection here in California, but I'm guessing it would. You should check with 928 Intl for a used one, I picked one up Christmas of 09 during their yearly sale, it's still working great on my '79.

The decel valve has 3 ports on it, one small one that gets the signal to open and close the valve, and 2 larger ports on the opposite side that allow airflow to pass when the valve is open. Make sure the diaphragm holds vacuum, and also that when no vacuum is applied that the two larger ports are isolated from each other. You can have a good diaphragm but if there's any gluck inside the valve it may not seal properly in the closed position and the result is a vacuum leak. Spraying carb cleaner won't reveal it as this type of leak is internal.

If you want to totally bypass it, you can pick up some heater core plugs (look like large vacuum caps) from Pep Boys in their Help section to plug off the vacuum ports on the throttle body.

I see you're in Alpine, there are 2 other 928 owners besides myself down here in the lower elevations of El Cajon. Give a shout if you ever need anything.
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Old 03-01-2011, 09:28 AM
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I'm sorry, but you don't want to "bypass" the decel valve.

The vacuum line pulls on a diaphragm that opens a path between the two large connections on the throttle body.

The failure mode is that the valve sticks open. Removing the vacuum line may allow it to close and stay closed. More likely, if it is a problem, it'll just stick where it is.

The solution is to remove the decel valve and cap the two ports on the throttle body and the vacuum line.

These go bad but can be replaced. I got a new one a few years back. Maybe get a used one from 928I? It ain't the buffalo's tongue.
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Old 03-01-2011, 10:55 AM
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Neither a smoke test nor a vacuum test will catch a leaking fuel regulator or dampener. The only test is to pull the small vacuum line from the unit and check for the presence of fuel. Any trace of fuel in the vacuum line is cause for replacement.
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Old 03-01-2011, 02:48 PM
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Might be better left installed on a street driven CIS car. The 'injectors' are just spring loaded check valves. High vacuum will pull fuel through them. (L-Jet has proper electric solenoid valves. )

The factory deleted the limiter on '83-'84 auto L-Jet engines. (5-speeds may want to raise the idle by 50 rpm or so to forestall stalling.)

Factory caps might help with a visual inspection.

999 703 171 40 cap 14x15
999 703 172 40 cap 18x15

'84 auto:
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Old 03-01-2011, 05:04 PM
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Originally Posted by GlenL View Post
I'm sorry, but you don't want to "bypass" the decel valve.

The vacuum line pulls on a diaphragm that opens a path between the two large connections on the throttle body.
Sorry, should've been clearer. By by-pass I meant to remove the decel valve and plug the two large connectors just as you describe. But used ones are so reasonably priced why risk not having one when it comes time for the smog test. Who knows what kind of red flags go up in the system once you fail a visual portion of a smog check.
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Old 03-01-2011, 10:11 PM
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Very interesting, varied, advice and approaches. This will take some digesting to get this in my small brain. Even a popular parts dist. today said 'take it off and plug it'. Smog isnt due for quite a while. This is great help and takes me to yet another level in learning about this cool auto. Not sure what I am actually going to do though.

I am very thankful for all the info. and to find 3 others close by with the same car, wow. I dont recall ever seeing a 928 in the 40 years of living here.

I learned that;
1. probably okay to delete delec valve by removing plenum for access and plug 2 hoses, maybe 3. Even have part numbers for plugs i saw, I think. cool. Or Pep boys, good one.
2. this could be the source of fume smell when windows are open, still trying to figure that one out.
3. could solve the high idle of 1600 after warm.
4. some things i need to read again and think about.
5. maybe a good idea to plug and leave in place or replace with used one.

Thanks all.

Any other suggestions welcome of course. - Mark
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Old 03-01-2011, 10:21 PM
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I re-installed the decel valve on my 81
The engine speed fell so fast you had to "blip" the throttle on up-shifting or shift fast enough to catch the rpm before it falls below the engine speed it would be at in the next gear. Made it impossible for my wife to drive the car smooth. By the time she would let the clutch out in 3rd the engine was at idle speed. So now you are forcing the engine to speed up with all the stress going into the clutch / drive-train.
I fail to see how this is an improvement unless you like speed shifting at all times.

On my track car I left it off.

I even took the time to re-install the decel valve on my 81 after going to the EuroS LH / EZK once I "discovered" the EuroS actually used them. Had me puzzled at first why the engine speed was dropping so fast.
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Old 03-01-2011, 10:40 PM
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Mark,
I have a stock, never molested, owned from new 81 Auto down here in Rancho Bernardo - I have all the test gear and 30 years of wrenching on the old guys to figure out ( 90% certain) what your problem is.
If you feel like taking a drive and coming on over, PM me if you are interested.
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Old 03-02-2011, 12:09 PM
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SanDiego928
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Originally Posted by Hacker-Pschorr View Post
I re-installed the decel valve on my 81
The engine speed fell so fast you had to "blip" the throttle on up-shifting or shift fast enough to catch the rpm before it falls below the engine speed it would be at in the next gear. Made it impossible for my wife to drive the car smooth. By the time she would let the clutch out in 3rd the engine was at idle speed. So now you are forcing the engine to speed up with all the stress going into the clutch / drive-train.
I fail to see how this is an improvement unless you like speed shifting at all times.

On my track car I left it off.

I even took the time to re-install the decel valve on my 81 after going to the EuroS LH / EZK once I "discovered" the EuroS actually used them. Had me puzzled at first why the engine speed was dropping so fast.
Thanks for your opinion. That would be a +1 for replacement I think.
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