mpg for a 1975 450sl

[Gurunutkins]

Hi Folks I need to pick your brains again. I now have my 74 450sl running but it seems to run rich, 10 - 12 mpg highway driving. I assume it should be higher than this. I have checked the cold start injector and also the temperature senders, all seem to work fine, I will test the MAP sensor this weekend but it holds a vacuum. I ran through all of the posts in the archives and have isolated just the engine vacuum system. Using a mechanics stethescope I can hear all of the solenoids in the injectors clicking, all seem fine. does anyone have any thoughts?
thanks in advance
Barri

[Benzmeister]

around the city 10mpg is normal. These cars are gas hogs, I had my tuned up new plugs, fuel mixture adjusted, had the engine cleaned internally, there heavy, for the type of car, High compression engines and they were made in the 70's when gas was cheaper. It is the nature of the beast, great cars, over 275K model run and considered one of the most collectable of the this era Mercedes. So we both have very collectable cars, but they burn gas, 4.5L DOHC V8 engines, even American ones were very fuel efficient back then either. But you are a bit low. I wouldn't change andy injectors, check your air cleaner thou, and perhaps get some Bosch plus 4 plugs. Also, be careful on what kind of gas you use, in my owners manual if says 87 (ROM 91), Here in the states we use ROM, stay with the 91 and throw a bottle of Lucas fuel system cleaner in it now and then, I have a K&N filter and noticed a improvement with the new plugs almost immediately. If your old plugs have black soot on them then you might be running a little rich, but I think your car has a computer under the drivers seat that is supposed to control air fuel and all that. Mine is old fashion, I can adjust idle, Co2. One other thing I just thought of make your your accelerator linkage is properly adjusted, as mine was out of wack and that can also cause a decline in mileage.

Great cars to drive aren't they. My everyday drive is a 99 C280 and this is a great car also, but there is nothing like a older car you actually drive.

[JBR]

I don't want to steal this thread but I beleive this info would be good for all concerned.

I have a 73 450sl and I was wondering how do you adjust the air\fuel mixture? The plugs only had about a 1k miles on them and they were very black and crusty. The air filter, plugs and wires were all new. A few things I have noticed.
A vaccum line coming from a X splice near the center fire wall, leads to the pass fender and just lays there (open).
The car does not smoke at all but the plugs make me think it is too rich.

[Benzmeister]

on the top of the engine there is are two adjustment to adjust the idle speed and the air/fuel mixture and CO2 or idle speed emission value, but not a specific adjustment to adjust the air fuel mixuture. Take off the air cleaner and you will see a adjustment screw with has a spring load on it. by turning it left or right, will adjust the idle, the CO2 must be adjusted using a CO2 reader. I assume you car is fuel injected? How does the car idle? Slow, fast etc. the vacumn like if it is on the driver side and disappears into the area of the door, is probably for the vacumn going to the door locks. What color is the like, Clean it up and I can look up the color code. More important is to check the vacumn lines uner the air cleaner, make sure they are not cracked or not connected and you accelerator linkage is in proper adjustment. The X splice your speak of could lead the line to other places and you said it just lies there, are you able to check if it is connected to anything else. When I first got my car 6week ago, My frist DIY was to check as many vac lines and connections as I could, I found two running fromthe intake maniford, one was cracked and one was disconnecte. but my biggest problem was the accelerator linkage which I had to take to a mechanic as it was ready to fall apart. This all be said there could be a number of situation that would lead to bad gas mileage. If you live in a warm climate, or have a heated garage, I would cover in Saran wrap everthing you can find that is electric, that is with wires and clean the engine first, sounds like it is dirty. the colors of the plastci vac lines can be very helpful. If you plugs are black then your running rich. I am going to do some reading about your year car, as the shop manual I have covers all 450 to 1980, when I guess there were some major changes in the electronics of the engine. I'll be back. send me a private email my address is posted on the user cp, as there might be a lot of "things" to look for, One basic question are the correct plugs installed and gapped correctly?

[Walrus]

Originally Posted by Benzmeister

4.5L DOHC V8 engines...

Hi, JBR...
Just so there is no confusion, the 117 engine used in the 450Sl is a SOHC as there is only one cam atop each cylinder head, as opposed to two cams per head on a DOHC. That said, 450s certainly are not noted for good gas mileage. 10mpg is too low, and can be correctd with a tune-up, but don't expect too much improvement.

Also, you will find all kinds of opinions about what plugs to use, but there is commonly agreement to stay away from platinum or plus 4 plugs, and stick with the
W9DC... When adjusting the mixture, convential wisdom says to attempt to do so without a CO meter, or similar device is an exercise in futility (follow benzmeister's advice on this... use a meter or other similar device such as a dwell with a duty cycle or such). Proper procedure also calls for the breather to remain on the car. Access is provided thru the breather lid to affect mixture adjustment.

I do not agree that black, crusty plugs are an indication of an engine running too rich, but one that is burning oil... This is so subjective, as what one person calls crusty black, may be sooty to another. A picture is worth posting, if you have one or more to share...

Benzmeister is correct on another thing... Fuel injection systems did change during one period of manufacture, but I can't remember the specifics... Seems like it was from 1974-1976.

Theree is a great post on fuel systems at sl107.com, that describes how to properly tune the mixture... search for it under the topics of mixture adjustment, or similar, and you should be able to find it.

Good Luck!

[230/8]

Guru:

Your engine uses the Bosch D-Jetronic injection. It uses a computer box on the firewall inside the pasenger compartment. The electronic portion of the system is basically controlled by engine vacuum. Therefore, you need to examine all the many vacuum points on the engine to ensure they are all tight and leak-free. The smallest vacuum leak will adversely affect the engine and cause it to run rich. The good news is that these engines are so flexible they can run very well with the vacuum system really out of whack, but they will only give you 8-10mpg. Get the system back into adjustment with leak-free vacuum and your routine mielage will be 12-16 mpg, not great but better than you are getting now. D-Jet is simple but not very precise. The engines were never designed for great fuel economy.

If you have done the routine checks and your sensors are working try the ignition advance. The later D-Jet cars used a more complicated advance/retard for the spark. Make sure it is working properly in all respects. Once you have confirmed this, try setting the total advance to 30-34 degrees at 3000 rpm with all the advance hoses disconnected and plugged. This usually results in an initial advance of 15-18 degees, without vacuum at idle. This is roughly 10 degrees more than the standard setting and will closely match the european settings for these engines. I have found this to be the best bang for buck change you can make.

Once you have confirmed good solid vacuum and adjusted the timing the car should run pretty well. If not, you can round up a CO meter and begin to seriously adjust the MAP sensor. Before you attempt this you need to get informed on the workings of the whole system. A search of this site will reveal many good references and links for books and articles on the D-Jetronic system.

Factory CD ROMs are available here for about $20, a good buy and they are chock full of good info.

http://www.classictechlit.mbusa.com/MB/

Good luck with your project.

230/8

p.s: JBR, all this applies to your car, too. That vacuum line is probably for the vacuum door lock reservoir, under the passenger fender well. Fix it and all the other leaky vacuum lines.

[Benzmeister]

A 79 450SLC is a DOHC. Also these engines were not economy driven as far as gas mileage. But as I said I get 15 to mpg on the highway an 10-12 in the city. Even in perfect tune and no vacuum leaks, good plugs point,( if there are any in the poster's model), the 3 speed auto, like mine an the 14 inch wheels don't add up to a winning mgg combo, IMHO. But I am impressed with my engine, for a 114K miles it pulls loke crazy and I have cruised at 100mpg, ( not too long) on I 90 and there seems to be a lot of power still waiting to be unleashed. I am going to look for the post you mentioned myself. I also agree black is very sunjective, could be oil, could be a very rich mixture. Also I ,must agree the plus 4 plug are probably too tech for these cars. and the owner should stick with the recommended plug.

Also, you comments about the timing were very interesting, I am going to check the timing on my car. Also, on the computer controls you mentioned. I really don't know where mine are located, people have said under the seat, other near the coil, NADA on both locations, perhaps on the firewall?

Thanks for the enlightment on these cars.


Just so there is no confusion, the 117 engine used in the 450Sl is a SOHC as there is only one cam atop each cylinder head, as opposed to two cams per head on a DOHC. That said, 450s certainly are not noted for good gas mileage. 10mpg is too low, and can be correctd with a tune-up, but don't expect too much improvement.

Also, you will find all kinds of opinions about what plugs to use, but there is commonly agreement to stay away from platinum or plus 4 plugs, and stick with the
W9DC... When adjusting the mixture, convential wisdom says to attempt to do so without a CO meter, or similar device is an exercise in futility (follow benzmeister's advice on this... use a meter or other similar device such as a dwell with a duty cycle or such). Proper procedure also calls for the breather to remain on the car. Access is provided thru the breather lid to affect mixture adjustment.

I do not agree that black, crusty plugs are an indication of an engine running too rich, but one that is burning oil... This is so subjective, as what one person calls crusty black, may be sooty to another. A picture is worth posting, if you have one or more to share...

Benzmeister is correct on another thing... Fuel injection systems did change during one period of manufacture, but I can't remember the specifics... Seems like it was from 1974-1976.

Theree is a great post on fuel systems at sl107.com, that describes how to properly tune the mixture... search for it under the topics of mixture adjustment, or similar, and you should be able to find it.

Good Luck![/QUOTE]

[Walrus]

Benzmeister,

Not only the early SL's, but all the V8 107's used engines with single cams atop the head (SOHC). This includes the 350SL (In the US, this was a 450, badged as a 350... the engine was a 4.5L 117), 450sL, 380sl, 420sl, 500sL, AND 560sl. the 300SL (107) was a SOHC inline six, and the 280Sl (107) was a DOHC inline six.
I have heard several people refer to the V8's as DOHC, because they count each cam (one on each head x two heads = DOHC), but this line of thinking is incorrect. True DOHC, like the cosworth engines, etc, are a beautiful piece of engineering.

Also, just a note on plugs. I read a post concerning soot and black smoke after an owner changed plugs. He was given too cold a plug at his parts house, which resulted in incomplete burning of the fuel mixture. After changing out his plugs to the correct heat range, W9DC for his and most all 117 engines IIRC, the black smoke disappeared, and plug fouling went away.

[Roncallo]

Search for my post under tech help for results on the Bosch Platinum +4 plugs. It gives 1/4 mile drag race ET, Fuel consumption and emmisions. The numbers tell the truth. Im not saying the Platinum +4 is better but I do have the results. The next plugs I will try will be the Bosch copper. That will be in a year from now. I fope to repost the results then.

John Roncallo

[Benzmeister]

double overhead cam:
(DOHC) An engine with two camshafts located above the cylinders. One drives the intake valves and the other operates the exhaust valve. In a single overhead cam engine (SOHC), one cam has enough lobes to drive both the intake and exhaust valves. The DOHC engine is considered to be a very sophisticated and more efficient engine; but is sometimes more difficult to adjust the valves. Also called "twin overhead camshaft." See engine type.

[Gurunutkins]

Thanks for all of the input guys I really appreciate it. I have now tested all of the sensors and they all give results within specs, however when I run the motor with a vacuum guage in line I get really low vacuum teadings. As I have already plugged the car vacuum system and only have the brakes and control lines connected I must either have a big leak in the plenum or a lot of little leaks.

I will eork on these this week and let you know. I have also done a lot of research and found a place called Python Injection in Atlanta that rebuilds the d jetronic pressure sensors, has anyone had any experience with them?

cheers
Barri