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  #1  
Old 05-21-2005, 01:21 PM
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73 450SL High RPM Idle

Fix one problem and find another one. I need some help. I've just tuned
up my 450 - new plugs, wires, distrib cap, rotor, and changed out the vacuum lines. I also cleaned out the combustion chamber. The engine idle (warmed up) was at about 1400 rpm before I started. I found a number of issues, but everything I tried I couldn't get the idle down. I have not yet adjusted the timing. The "book" says I can adjust the idle by the idle speed adjustment screw, but this has no effect (provided I'm adjusting the correct one). The engine is the 117 V8 FI with auto tranny. I"m kinda at a loss for where to look next. Anybody have any ideas?

HH
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  #2  
Old 05-21-2005, 08:25 PM
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You either have a vacuum leak somewhere, or a tear or leak in the idle air valve's plumbing, or a problem with the warm-up air valve. One or more than one is allowing additional air into the system, outside of the idle air valve.

Check the rubber hoses very carefully, and try tapping the warm-up valve since it may be stuck.
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #3  
Old 05-22-2005, 03:29 PM
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Which is the screw you're turning? It has a red arrow pointing to it in the diagram I attatched.

Any leaks at the hoses marked in yellow will also cause MAJOR idle speed changes (as would bad intake manifold plenum seals or really bad injector seals).

Turn this screw clockwise about 1 full turn. If you don't notice a decrease, turn it another full turn. If it bottoms out and you STILL don't notice a change, then there's something drastically wrong with your hoses or plenum seals and injector seals, and also possibly your idle air control valve.
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73 450SL High RPM Idle-airscrew.jpg  
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  #4  
Old 05-22-2005, 10:18 PM
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Reflecting on this some more on the airplane...

It could also be a linkage adjustment issue. Is the linkage against the throttle stop at idle?

One other possibility is that the throttle plate gap could be too wide. This is a very sensitive adjustment. There is a set-screw and a lock nut. Too small and you get a vacuum lock condition that makes it hard to push the gas pedal down, too large and you lose the control of the idle valve.

If you send me an email, I will try to send you my K-Jet troubleshooting guide - all 12 MB of it.
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #5  
Old 05-23-2005, 12:01 AM
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In the D-Jet tuning/troubleshooting manual, there are 3 reasons listed for "Idling speed too high, idling speed cannot be adjusted." All 3 have been mentioned here

Leaks in idling speed air system -> Check idling speed air system
Rubber sealing ring under injection valves leaks -> Replace rubber sealing ring
Throttle valve badly adjusted -> Adjust throttle valve correctly

If it's the third, I can email you the technical pages you need to resolve it as well, although my copies probably aren't as good as Chuck's!
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  #6  
Old 05-23-2005, 12:10 AM
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Look inside the throttle plate area. It could be dirty and have a build-up of crud. Use carb cleaner and a rag to clean the throttle bore plate area. Also, clean any vacuum holes in the throttle bore.

You might conside removing the throttle off the manifold and cleaning it on the bench. Don't forget, use a new gasket.
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  #7  
Old 05-23-2005, 01:20 PM
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Hardhead: Before you dive in and start turning screws, removing and spraying parts etc, take a deep breath. Chuck and Tomguy gave you good advice now take a minute to go over it and the photo to orient yourself. First check that all your air supply hoses are tight and in good condition. The large ones going to the aux. air valve are the main air hoses. Check them and every, underlined, every vacuum hose under the hood. All should be pliable and tight on their fittings. On some eary D Jets the large main air hoses were clamped tight, but for some strange reason MB decided to save a few pfennigs and eliminate the clamps. Replace any suspiscious hoses and clamp them to ensure tight fit at all times.

You will have eliminated the obvious easy to fix sources of vacuum leaks. Check your vacuum with a gauge. Find a manifold source, at the back of the engine is where most easy connections can be made. You'll see the area back by the firewall where the lines come up from the maifold base and take off to the accessories. Somewhere in there you can find a place to tap into the vacuum for your gauge. If you have a very low vacuum reading, low normal could be 14-15"hg for an older engine, middle normal could be 16-18"hg. A low reading in the 12s or single digits and you have another big leak probably like the manifold seals Tomguy suggested. And if you do it will be affecting many other things on your engine. If you get something in the mid-teens then the manifold gasket is probably OK.

Next, try adjusting the large brass idle screw like Tomguy suggested. If it bottoms out w/o lowering the speed then air is coming in someplace. It is probable that your AAV is stuck open and no longer closing off as the engine temp rises. Try this site for a tutorial on AAVs: www.jag-lovers.org/xj-s/book/AAV.html
If it needs replaced, figure about $140 for the valve. It is fairly easy, but you may be able to clean it enough to get it working again.

If you have messed with the throttle body throttle plate screw instead of the idle air screw, then you'll need those instructions from Chuck or Tomguy to get it correct again.

230/8
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  #8  
Old 05-23-2005, 04:06 PM
1972 280SEL 4.5
 
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One vacuum hose that you should absolutely check is the one that feeds the retard canister on the distributor. If that's disconnected or leaking, the timing will be advanced at idle, speeding things up regardless of the idle screw setting.
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  #9  
Old 05-23-2005, 06:44 PM
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All - thanks immensely for the suggestions. I did not
replace the larger hoses with the clamps on them. I
do believe they probably need replacement too as did
the other hoses. don't have ready access to a vacuum
tester, but always wanted to get one, and now I have
an excuse.

I havenít looked at the warmup valve yet, but will take
A closer look at it when I replace the larger air hoses.

Tom Ė thanks for the pic. I am turning the screw youíve
dentified and it has very little effect. As a matter of fact,
itís bottomed out and the idle still is at 1400 rpm.

Chuck Ė I appreciate the offer of the k-jet manual. Iíll send you
my email address and see if I can handle the big file. I
think I can. Also, if the throttle plate is the circular plate inside
the throat (similar to the choke on a normal carb) I did clean it
with some Sea Foam. It appears to me that, at idle the plate
that Iím thinking of is virtually closed. When throttle is applied
it opens and when the throttle is released it closes again. I
thought that was a little strange because Iím familiar with
gas engines that after warmup the throttle/choke place goes to
open. Is this called the ďthrottle plateĒ?

230/8 thanks for the pointer to the AAV. Given what Iíve
heard for suggestions so far, Iíll check that too after I've
gone through the more obvious parts.

You guys are Aces! Thanks again!!!

HH
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  #10  
Old 05-23-2005, 06:45 PM
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Quote:
Originally Posted by ctaylor738
It could also be a linkage adjustment issue. Is the linkage against the throttle stop at idle?

One other possibility is that the throttle plate gap could be too wide. This is a very sensitive adjustment. There is a set-screw and a lock nut. Too small and you get a vacuum lock condition that makes it hard to push the gas pedal down, too large and you lose the control of the idle valve.

If you send me an email, I will try to send you my K-Jet troubleshooting guide - all 12 MB of it.

Chuck - here's my email address(es):
srdahlstrom@msn.com
steven.dahlstrom@medtronic.com

Hopefully, one of those will allow the manual to get through.

Thanks again.
Steve
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  #11  
Old 05-23-2005, 09:19 PM
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Be aware, you have D-Jet, not K-Jet. I think Chuck made a simple typo (or at least I hope)!

What you need to do, most likely, is replace all those hoses, clean your aux air valve out (or possibly replace it), and replace your injector and manifold seals. If, after replacing the hoses and cleaning the valve out, it still idles that high, you probably need to replace the aux air valve. I will be throwing one up on eBay shortly, but if you want first crack at it, let me know and I can sell it to you before it hits eBay.
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  #12  
Old 05-28-2005, 03:28 PM
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The high idle is fixed! (temporarily anyway). All the new hoses helped a lot, but the real culprit was the AAV. I "tapped" it a lot and it helped. I suspect I'll have to change it out when I can find one at a reasonable price. Since
the rpm is down now, I can go ahead and time it closer to what it's supposed to be.
I want to thank everyone again who gave me advice. Next, on to the transmission leak....

HH
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  #13  
Old 09-02-2005, 10:51 AM
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FYI, mine was also fouled after years of usage and the idle was not consistent. Whacking it did not fix it for me. I pulled the AAV and there's a nipple that has a spring-loaded air check valve that was not seating. I cleaned it out as best I could with carb cleaner and a pipe cleaner and persistence. After it all dried, I "suck tested" it and it now seats in one direction, opens in the other. Problem solved - for now. Perhaps replacing the AAV isn't necessary?
S

Quote:
Originally Posted by Hardhead
The high idle is fixed! (temporarily anyway). All the new hoses helped a lot, but the real culprit was the AAV. I "tapped" it a lot and it helped. I suspect I'll have to change it out when I can find one at a reasonable price. Since
the rpm is down now, I can go ahead and time it closer to what it's supposed to be.
I want to thank everyone again who gave me advice. Next, on to the transmission leak....

HH
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