73 450 sl High Idle - PeachParts Mercedes-Benz Forum

porscheman89 · #1 11-05-2009, 04:05 PM

I have a 73 450 sl. It has a high idle (1800rpm) at start up for the first 5-10 min or so. Does anyone have any ideas of where to start? Thanks

MAGV5 · #2 11-05-2009, 04:10 PM

Run a search in SL forum. More information than you would ever want on high idle.

Regards

porscheman89 · #3 11-05-2009, 04:11 PM

ok thanks

Tomguy · #4 11-05-2009, 09:47 PM

Common D-Jet issue, especially if your thermo-time switch is failing and dumping gas in the whole time, if your thermostat is stuck and the engine takes too long to warm up, if you have a vacuum leak, if your timing is off, if your throttle position sensor is off, if your aux air valve is malfunctioning, or about 40 other things... No need to search just the SL forum, plenty of vintage posts too about it. It's D-Jet, not SL specific!

meltedpanda · #5 11-06-2009, 08:05 PM

I would check AAV first

Graham · #6 11-06-2009, 09:20 PM

Quote:

Originally Posted by meltedpanda

I would check AAV first

Second that! With engine warmed up, squeeze the hose that leaves the AAV and see if idle speed reduces. If it does, AAV is leaking. It is supposed to be closed at 150F.

But many of them leak. Try screwing the large slotted idle screw in a bit until idle is acceptable - say 1000rpm. Then see what happens on a hot start after heat soaking for say 20 min. Idle will probably be quite low - Adjust it to about 600 rpm and you might be Ok cold and hot.

Try cleaning the AAV - need to remove it and soak in solvent (you can't damage it), then detergent and whatever else you can find

High rpm means you are getting too much air. Either through AAV or idle screw OR through a vacuum leak.

meltedpanda · #7 11-07-2009, 08:43 AM

BTW Graham and I are the resident AAV experts ( lol)

Graham · #8 11-07-2009, 09:33 AM

Quote:

Originally Posted by meltedpanda

BTW Graham and I are the resident AAV experts ( lol)

Once we take something apart and put it back together again, we are the resident experts, whether it does any good or not

Distributor, TPS, AAV done. Maybe time to R&R the MPS?

Actually, I am thinking about removing the fuel pump for a clean and check.

CamelotShadow · #9 08-02-2013, 09:57 PM

Quote:

Originally Posted by Graham

Second that! With engine warmed up, squeeze the hose that leaves the AAV and see if idle speed reduces. If it does, AAV is leaking. It is supposed to be closed at 150F.

But many of them leak. Try screwing the large slotted idle screw in a bit until idle is acceptable - say 1000rpm. Then see what happens on a hot start after heat soaking for say 20 min. Idle will probably be quite low - Adjust it to about 600 rpm and you might be Ok cold and hot.

Try cleaning the AAV - need to remove it and soak in solvent (you can't damage it), then detergent and whatever else you can find

High rpm means you are getting too much air. Either through AAV or idle screw OR through a vacuum leak.

New info on that darn AAV. My 83 500 sec euro has it. I'm in orocess of replaceing hoses to & from it.

I didn't know it was closed after 150F.

I'm just trying to find sources of problems leading to failing smog w higher NO.

Which direction is the system going? Into intake or away?
thermo vacuum switch
I'm trying to find out if the thermo vacuum switch 50 C has leads from egr & throttle body.

I'm going to 1st guess the thermo vacuum switch opens at 50 so it would be open & gives vac to egr?
then again if it closes it block air from intake...Hmmmmm?????

Also I was not sure if it mattered which side port of the thermo vacuum switch the egr was attached to?

I'm guessing open is open for both so it would not make a difference but I want to be sure as diagram has egr vac line hooked on right side port as you face engine

I'm going crazy here

meltedpanda · #10 08-02-2013, 10:29 PM

AAV is into intake, and I dont think it matters, open is open IIRC

Benz Dr. · #11 08-05-2013, 06:47 PM

We had that problem on a 1980 450SL - different injection system but same AAV. I pulled the hose off the AAV, stuck a smaller piece of hose in it and then capped that hose off so their was next to zero air flow.

After doing that, the air screw responded to small turns as it should. Cheap fix that doesn't take long to do, or undo, if needed. The engine may run poorly until you get a bit of heat into it but it's still better than slamming your trans into gear at higher RPM.