Running Rich

[Zagato]

My 450SE D-jet is running rich. I did a calculation to see what my mileage is, and concluded the thing is getting 6 MPG. Which is shockingly bad.
Unless I did it wrong (8 gallons to fill up after doing 47.8 miles).

The problem is, I don't know why it's running rich. I don't see black smoke, but when the car is warming up (idling between 1000 and 1750 RPM) there is a faint scent of fuel. Also the engine sounds like it's struggling when accelerating, if I don't let it warm up for more than 2 minutes.

Any ideas? Spending $40 per week on fuel ain't good for a student.

[BAR]

For the savings in economy and cost of wasted fuel, take it to a qualified mechanic to tune up.

He [or she] will have all the proper equipment to test and fix your problems.

[13B]

I'd pull out a couple of your spark plugs (one at a time, don't mix up your firing order) and inspect the tips to see if they are black and sooty, indicating the car is running too rich.  If they are brownish then the car is not running too rich.

I can easily get my 6.9 down to 6MPG during spirited driving, and my 450SE race car on full song down to 3MPG. 

Are you doing lots of full-throttle take-offs?

[Zagato]

I'd pull out a couple of your spark plugs (one at a time, don't mix up your firing order) and inspect the tips to see if they are black and sooty, indicating the car is running too rich.  If they are brownish then the car is not running too rich.

I can easily get my 6.9 down to 6MPG during spirited driving, and my 450SE race car on full song down to 3MPG. 

Are you doing lots of full-throttle take-offs?

Last time I checked the spark plugs, 3 months ago, they were brownish. The car had the same poor mileage back then.

I often do full-throttle take-offs because it takes a lot of effort to keep up with traffic. It takes a heavy foot to accelerate the same as a normal Mercedes C-Class, or even a Toyota Camry. That and I love the exhaust note.

Are you saying that single digit mileage figures are normal for these cars? I always thought 12 MPG is the average for V8 W116s'.

[13B]

13MPG average was what Road & Track magazine got out of these cars when new in 1973.  Thats with a brand new perfect condition engine with 100% compression and perfectly functioning D-Jet EFI system.

If you do lots of highway driving at a constant speed 55-65mph you'll get slightly better, if you do lots of city stop-start driving and full throttle take-offs then you'll get worse than this. 

Still, like BAR says, get a mechanic to have a look, even if its just to fit new points and check the ignition timing.

I.

[Zagato]

Alright, I'll have it looked at by an independent Mercedes shop I know of. I'll also try to drive more leisurely and recalculate the mileage for the following week. Hopefully I will get it up to at least 9 or 10 MPG to be at odds with a Range Rover. 

Thanks!

[TJ 450]

A 450SE should be able to keep up with the traffic effortlessly. It sounds like you might have some performance issues. If it's running too rich, you will have poor performance and certainly poor fuel economy.
If it's running way too rich you will be able to smell fuel in the engine oil. Also, what has the weather been like lately? If very cold it will be running rich for a while anyway. What does the temp gauge register, if it is a long way below the half-way point all the time, then your thermostat probably requires replacement. This will lead to the EFI thinking that the car is cold all the time, dumping in more fuel.

Tim

[Niclas]

Hi,
have you looked up the ignition timing?
Mine had similar symptoms when the timing was set 20 (!) degrees late. It was very weak and used loads of fuel.
If it's wrong set it ang change the spark plugs, my wrong timing driving of only 100 km destroyed 3-4 plugs!

If the plugs still get brown and the exhaust smells of fuel you can check the manifold pressure sensor. The little bolt should be covered with  molded plastic, otherwise it has been adjusted and is probably set wrong. Don't adjust it if it's covered and if it's not think of adjusting the MPS (MAP) sensor as the last thing to adjust.
Another, more easy to check, foult is a leaking hose between the manifold preassure sensor and the intake manifold, this would couse the engine to run rich.
There is also an very good thread from Nutz about how to (easily) check your wiring for the electronic injection system on this forum.

Or let a Merc mechanic check it, but that is cheating

/Niclas

[pez]

There are  very few cars on the road today that your 450 should not be able to outrun at a stoplight or on the open road, even without wide open throttle. There is something, or several somethings wrong.  Start checking.  I get 20 MPG on the highway at cruising speed and 16-17 around town on my '76 euro 450se K-Jet motor.

[Zagato]

Thanks for the suggestions guys! I'll be sure to mention them to the mechanic, to be sure he knows what he's doing.
I hope the timing isn't at fault as that sounds expensive. I'm going to get the upper control arms changed later this month.

Also, what has the weather been like lately? If very cold it will be running rich for a while anyway

It's sunny out, but chilly as well.

What does the temp gauge register

Upon start up the gauge slowly comes off the bottom and usually hovers around 175'F. So seems normal, though the aux electric fan doesn't seem to work.
Plus, the oil pressure always shoots to the top and only comes down after the car is completely warmed up (15-20 minutes later during driving).

If the plugs still get brown and the exhaust smells of fuel you can check the manifold pressure sensor.

Is that under the air box?


Also, when I got home today after a 3 mile drive, I popped the hood and made a small visual inspection. I found a small amount of liquid leaking out of a hose (which has a screw tightening clamp around it) and onto a electrical plug. When I poke the hose, more fluid seeped out. I tightened the clamp, dried off the electrical plug and socket, and poked the hose a few more times to be sure I stopped it leaking. With that sorted, I fired up the engine to see if I screwed anything up when I unplugged the wire to dry it. No difference besides a slightly smoother engine idle exhaust note (as though the car was fully warmed up).

Here is where that leaking hose is:

[pez]

Zagato:
Your oil pressure should be at max until the car is fully warmed up [not just coolant], then it will come down when the car is at hot idle, but should shoot right back up to max as soon as you apply some revs.

Timing is worth checking. Your mechanic should not charge more than 20 minutes time to check and set the timing....it's a cakewalk.  While you are at it, have him check the condition of your cap & rotor, and also verify good conductivity of the lead wires. These will have nothing to do with running rich, just good to have checked.

Can you pull the air cleaner assembly and get us a better photo of where that leak is coming from?

[Zagato]

Sorry for the extremely late update! Weeks ago I finally took it to an independent Mercedes shop to look for the cause of the horrendous fuel consumption. They checked the ignition timing, spark plugs, etc. They came to the conclusion that the problem is the idle valve for warm-up. Apparently it's malfunctioning, causing the car to rev-up for too long than it is necessary, sometimes taking it all the way up to 1,750 RPM. So I'm using too much fuel when I put it in park. They quote $700 for a replacement valve and the stiff hoses. As I'm far more interested in investing in a new exhaust, I'll just observe the idle for now. Spring is just beginning, so maybe I'll spot a difference.

In other news, I checked my oil for the first time in 3 months and was shocked to find the level to be far below minimum. Colour was an unhealthy black as well. Got the oil and filter changed the next day. Happily, all my performance woes have gone. Acceleration is much smoother and 39mph cruising is no longer fussy. I feel like such a fool for neglecting my pride and joy like this. I'm just glad it's so well engineered, it didn't take any mechanical revenge on me!

On the brighter side though, I've changed to a relaxed driving style and brought the city mileage up to 9 MPG! Finally, my ol' warhorse is as 'economical' as a Range Rover.

[brettj]

You can change the idle valve yourself and I believe you can purchase a new valve for $375.00 from Mercedes Benz. The idle valve is held on by 2 screws. Just unscrew them, pop out the old valve, install a new valve gasket and valve. Lots of coolant will come out when you remove the the idle valve ,so be prepared for that. It can be a bit messy.

I drive a 75 450SE and having been having some of the same issues as you. Poor fuel economy has been one but at last fill up my mpg was averaging 14.5 mpg. I've changed just about every sensor in the EFI system and also discovered that changing the electrical relays in the drivers interior kick plate and fuse box have really made a difference in power and fuel consumption.

[pez]

I have a '76 euro [k-jet] and I can get close to 20mpg on the highway here in Colorado [high altitude = more fuel burned & less power].  If you keep tweaking and measuring and refining you can make these cars damn good.  BTW, all those mini SUV's and over sized over weight sedans barely manage 20 mpg in real world driving today....

[Zagato]

discovered that changing the electrical relays in the drivers interior kick plate and fuse box have really made a difference in power and fuel consumption.

Really? I'll try that out if it makes a difference in power in fuel economy!