450SL with odd but specific starting issue: - Mercedes-Benz Forum
 
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post #1 of 4 (permalink) Old 03-27-2019, 09:19 AM Thread Starter
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Date registered: Sep 2017
Vehicle: 1973 450SL
Location: Seabrook, TX
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450SL with odd but specific starting issue:

1973 450SL D-JET - Fuel pressure great at 30PSI at injectors (correct PSI for this model). Last time I checked a few months ago, it held fuel system pressure just fine after car is shut off.

I little background regarding the idle and the AAV (auxiliary air valve). When we first bought the car last summer, it cold idled at 1600-1700 rpms. Even when warm it still idled high at 1000 rpms. I tapped down the AAV very carefully with a hammer. After tapping down the AAV, it cold idled at 1100 or so (too low?) then 600-700 when warm (we put the car in the shop shortly after so did not have time to test under different driving conditions).

Now that we have it out of the shop and driving again (details of driveline repair in previous post) we are having idle and starting issues under certain circumstances.

In the morning the ambient temperature is about 55F. The car starts right up and idles at 1000 rpm cold (too low?). My wife then drives no more than 15 minutes to a park and ride to catch a carpool to work. When she comes back to the car after work (about 8-9 hrs) with an ambient temp of about 75F, the car takes many tries to get it started. And when it does fire up it fumbles up to a smooth idle but is only idling at 500 rpms, cold. After that, its fine. Idles smooth, runs great and you can start it right back up when she gets in the driveway. Even a few hours later it still starts right up.

The only time the hard start happens is after she tries to start the car after work (8-9hrs after being parked). I also think I have the idle adjusted out further than spec just to keep the idle up when warm (i could be wrong).

This is confusing because its starts fine cold, its starts fine when warm if you start it within a couple of hours after shutting it down. Only hard time starting is after work. Could it be that the AAV is sticking in the "warmed up" closed position? Maybe since its 55F in the morning the AAV opens back up and the cold start valve kicks in, but the 75F in the afternoon is keeping the AAV in the closed position? Sticky AAV? Any other things I should check out?

Could I have tapped down the AAV too far? I tapped it down to where the lip of the valve is flush with the housing. It was sticking out of the housing about 1/8 inch.

To be sure, tapping down the AAV lowers the RPM and pulling it up raises the idle, correct?

Any help is appreciated.
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post #2 of 4 (permalink) Old 03-28-2019, 06:54 PM
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Date registered: Oct 2006
Vehicle: '72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Quote:
Originally Posted by Mercy107 View Post
about 1/8 inch.

To be sure, tapping down the AAV lowers the RPM and pulling it up raises the idle, correct?

Any help is appreciated.
I have two AAVs. Both of these were always fully seated. By that I mean that no amount of tapping would have made any difference. The upper cylinder is hard up against the bottom insert. Tapping top would just push bottom out. You can confirm this by looking at Rowdies 1/2 section of a Djet AAV.

https://www.benzworld.org/forums/r-c...ml#post4252414

What is important, is where the inner piston is relative to the cylinder openings at cold and hot temperatures. On some(but not all) AAVs, adjustment is possible internally using nuts on threaded screw(Can be seen in Rowdie's pics) The piston should fully close off the air opening at 150F and should retract back to fully open as it cools. Several ways to test, but I have cycled mine on bench using hot air gun and cold water. Plus IR thermometer to measure temps.

The piston and cylinder do get eroded , presumably by dirt in the air. When they are eroded, they can and do stick. I took one apart and polished the surfaces so that they would slide easily. This link might help if you decide to dismantle the AAV: https://www.benzworld.org/forums/r-c...399-aav-3.html

If your idle at cold start is low (1000rpm) and also low when hot (500rpm) it seems like the idle screw should be opened a little. On my car, those numbers are more like 1200rpm/700rpm.


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1972 350SL 4.5L, E320, 300D, Outback 2.5 - near Kingston, Ontario

Last edited by MBGraham; 03-29-2019 at 06:21 AM.
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post #3 of 4 (permalink) Old 03-28-2019, 10:47 PM
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I suggest you remove the AVV, disassemble and then test.

"Tapping" it will not do much to help the internal sliding piston.

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post #4 of 4 (permalink) Old 03-28-2019, 10:50 PM
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See my post:

https://r.tapatalk.com/shareLink?url...7&share_type=t



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