Very fast Idle, 5.0 in 1980 450 SLC (Euro) - PeachParts Mercedes-Benz Forum

Leewolf · #1 07-08-2004, 03:12 PM

This car, a 1980 450 SLC 5.0 (Euro) had set for up to 3 years in the Tampa area before I acquired it. The fuel delivery system and intake gaskets, vacume system were pretty much toast. Needless to say this is a project car, and on my income a long project!!lol.
It is running pretty well but the cold idle is at about 1600-1800 RPM and warm idle is at about 1200-1600 RPM. Note: by the fact that the idle speed is coming down by 400 to 500 RPM during warmup leads one to believe that the warm up system is working BUT it's "zero point" is at 1200 to 1600 instead of around 600 to 800 Rs.
This is the mechanical system and does not have any electronic "idle" boxes behind the glove compartment. The electronics are limited to the electrical signal coming from the tempurature sensor associated with the cold start system.
We have checked and double checked all adjustments for air and fuel, have replaced the metering valve system on the fuel distributer, all vacume lines, port inlets, gaskets, fuel pump, filter and regulator, etc. Checked for air and vacume leaks (none found).
Has anyone had a similar problem or any ideas as to where else we need to look. SURE WOULD APPRECIATE THE HELP. I'm out here in the OZARKS (South Centeral Missouri). We are a little short of help in the Mercedes department. Thanks Lee
1979 450 SL (sold)
1979 450 SLC (Totalled, very unfortunate)
1988 300SEL
1980 450SLC 5.0
1988 Lincoln Town Car

Deltacom · #2 07-09-2004, 05:12 PM

Your Auxiliary air valve needs replacing. The wax filled thermostat that closes the piston in the valve perish with time and remains partially open allowing high air volume. Had exactly the same problem in my 5.0. There are two types of aux valves: see pics.

Leewolf · #3 07-10-2004, 12:22 AM

Sincere thanks for your reply. Two questions if I might: You mentioned pictures of the Aux Air valve. I didn't get them or I didnt look in the right place. Two, Where is this valve located? Again, many thanks, Lee

Deltacom · #4 07-10-2004, 02:54 PM

Lee, this url will give you a picture of the types of valves.

AUXILIARY AIR VALVE

Leewolf · #5 07-10-2004, 03:16 PM

I recieved your message and again thank you very much. Went to the site and they have valves for 380, and 450 in the proper year range. They also have one for they call the European Auxillary Air Valve. They give no pricing or other information. They indicate they are no longer participants in the "Affiliate" program. Would you know if the following is in fact a MB part # This is from the BIZ site, B3020=60180. If so I might be able to get some info from the MB Classic offfice that MB has in New England Incidently I did find a valve for the 1980 4.5 priced a $182. I hesitate to experiment at that price. Again 2 questions, what year is your 5.0 liter engine and where is this vavle located. It very well may be that this was removed by a previous mechanic that had no idea what he waas doing. Ive been stung pretty badly on this car. Hopefully with a little help from people like yourself I'll finally get this thing running. Thanks Lee One more Q. does the European valvle look like the one your replaced?

Deltacom · #6 07-10-2004, 03:48 PM

My 5.0 is 1980 and uses European aux valve. Also check your PM's, there are details of the location of the valve.

Deltacom · #7 07-10-2004, 03:57 PM

Location

Deltacom · #8 07-10-2004, 04:12 PM

Another pic of the valve next to the Warmup compensator. You can test the valve efficiency by warming the engine first to temp. Then take off one of the the hoses from the valve and block one of the inlet of the valve with a corck or similar, reconnect the hose and start the engine. The revs should drop to 600-700 rpm. You might need to adjust the idle screw clockwise to lower revs anti to rise them.

Leewolf · #9 07-10-2004, 04:55 PM

My sincerest thanks, I will be under the hood the first thing Monday. Got to wait till then, the car is having some interior work done. The latest shock, a hole rusted completely through the floor under the pass enger seat. FUN, FUN, FUN. I knew I had some rust, but This was well covered up. Anyway thanks again. Lee

Damian · #10 07-11-2004, 05:01 AM

I also am have some idle speed problems with my 82 500 sl and have read your comments on the aux air valve. I will have to check mine, thanks for the info.
I was woundering if you would happen to know wat the vacum line in your picture of the warm up compensator does? The on with the arrow next to it.
I had my warm up compensator replaced with a second had unit, which does not have this line and the mechanic has just blocked the vacum line up. Could this be some of my problem?

When my car gets to operating temp (more when it's quit warm) the idle drops very low. I have adjusted the idle screw to bring the idle up to compensate, but when it is cold the idle is alot higher than it should be. The idle is also high when it's warm and you put in in park and take the load of the engine.

If you could shed any light in this vacum line that would be great.

Regards

Damian

Deltacom · #11 07-11-2004, 06:36 AM

Damian,

There are 4 types of WUP. The vacuum line on top of the WUC (also known as fuel pressure regulator) is used for vacuum-controlled control-pressure to provide full-load enrichment and altitude compensation. It does not affect idle revs, but check that the contour hose on the aux valve in your car does not have vacuum connector inlet. There was a cut-out production date to eliminate this vacuum-control-pressure requirement (1979) and the WUC top vacuum line simply vents to atmosphere in some models.

Your symptoms seems more to do with the vacuum-switchover valve (active when air conditioning is on) to rise revs. Other posibilities are the fuel accumulator or the check-valve on the fuel pump. A series of tests would be required to ascertain de drop on revs when warm and sudden rise when load is reduced (gear shift in park).

kip Foss · #12 07-11-2004, 09:38 PM

Leewolf,

If you are going to buy more than a few aprts from the local MB dealer try to register yourself there as a restoration company or some such. I did this at my local MB dealer in Corpus Christi, Tx. and they matched the aftermarket people's prices very closely. Granted I had a very cooporative partsman, but all I did was show him the prices I could get my parts for from the cataloges and he said that with a dealer's discount he could match or beat them-and he did in most cases. It never hurts to ask.

kip Foss · #13 07-11-2004, 09:43 PM

Leewolf,

If you are going to buy more than a few aprts from the local MB dealer try to register yourself there as a restoration company or some such. I did this at my local MB dealer in Corpus Christi, Tx. and they matched the aftermarket people's prices very closely. Granted I had a very cooporative partsman, but all I did was show him the prices I could get my parts for from the cataloges and he said that with a dealer's discount he could match or beat them-and he did in most cases. It never hurts to ask.

Damian · #14 07-13-2004, 06:25 AM

Thanks for your info Deltacom.

Actually my contour hose does have a connection point for a vacum hose, but both are also blocked with small screws.

As for the vacum swith over valve where would I find that?

Some one has disconnected the positive feed to my A/C switch before I got the car, cant seem to find it either. So it never worked. So it may have something to do with it .

Many thanks for your help on this

Damian

Deltacom · #15 07-13-2004, 07:26 AM

Damian,

The switch-over valve (change-over valve) on most 107 is located in the firewall just behind the air filter (in others is on the fender just below the washer reservoir). Since your A/C is not working you might as well disconnect the electric plug to the switchover valve to eliminate this possibility.

Your vehicle with the vacuum inlet in the contour hose indicates the vacuum-controlled pressure regulator is required. (without it the acceleration response is more sluguish and noticeable lack of performance at high revs.) to do away with this requirement this function is taken over by the pressure-step switch to activate the control unit in the lambda control wich it might not have been upgraded in your car.

See pic of the switch-over valve