14.9mpg in my SDL, no, the math is right...and the odometer works too... - PeachParts Mercedes-Benz Forum

WANT '71 280SEL · #1 01-05-2005, 11:28 PM

I filled it up and reset the trip odometer. When I fill it up, I let it click off the push the lever twice. I noticed I wasn't even getting 300 miles out of a tank! Well, todaty I filled up and put in 20.3 gallons after driving just 302.6 miles. 50% of those are highway.

When going about 45 or faster and the accelerator is floored, thick smoke comes out the back. I checked the boost valve today and blew in the manifold-side vacuum hose to see if air went out the top, well it didn't. Also, the switch looks new and has a tag saying it was replaced jan. of '02. The EGR hasn't been disconnected yet. Also, once when my mom filled it up, my little brother lost the gas cap. Would that have hurt it? Right now on this tank (I added the new gas cap today when filling up) I've driven 85 miles and it's down to the 3/4 mark. 75% of those miles just today are road miles of about 50-60mph average with a lot of CC on. I thought the gas cap might help some, but it doesn't seem to change anything except the smell in the back seat.

Also, I don't appear to have any fuel leaks, or at least not major enough for me to see and the injector return lines all appear to be dry.
Thanks
David

P.E.Haiges · #2 01-06-2005, 12:31 AM

Want,

I think the solution to your problem lies in the black smoke problem. Something is causing the injection pump to add way too much fuel, thus causing the poor mileage. It might me a bad injection pump.

So you might have to R&R the IP and have it calibrated.

BTW, its a fuel tank cap not a gas cap.

P E H

#3 01-06-2005, 01:17 AM

Replaced your air filter lately?

fahrgewehr2 · #4 01-06-2005, 01:21 AM

I can't really help you with your fuel consumption problem, but I can help you pick a name for that car.

While waiting for responses, do a search on www.mercedes.cx and the alt.auto.mercedes lists.

Good luck!

WANT '71 280SEL · #5 01-06-2005, 09:18 AM

The air filter is Hengst and looks new. PE, is that really you? You're saying I should go straight to an IP rebuild? I hardly would think that's my problem. I thought the problem might lie with boost since the car seems to be pretty sluggish.

Thanks
David

KCampbell · #6 01-06-2005, 09:24 AM

If you are generating black smoke then you are pumping fuel into the engine that is not being burned, the last thing you want is for the boost enrichment to add still more fuel.

What about the possibility of weak injectors allowing too much fuel in? I imagine that if the pop pressure was too low you'd get too much fuel delivered, and symptoms much as you describe.

Kevin

Brian Carlton · #7 01-06-2005, 09:26 AM

Quote:

Originally Posted by WANT '71 280SEL

The air filter is Hengst and looks new. PE, is that really you? You're saying I should go straight to an IP rebuild? I hardly would think that's my problem. I thought the problem might lie with boost since the car seems to be pretty sluggish.

The IP will typically not provide additional fuel if there is no boost pressure. PEH is concerned with the overfueling even without the boost.

That being said, I would strongly encourage you to hook up a boost gauge via a T in the line to the ALDA. You can then see, immediately, whether you are producing boost pressure to the ALDA or not. If you have insufficient boost (less than 8 psi) then you need to track it down and see what is clogged.

BTW, normal boost on the 603 is 11-12 psi.

boneheaddoctor · #8 01-06-2005, 09:27 AM

Checking you injectors wight be a good idea......if they have a poor spay pattern you will get poor burn ...poor power than makes you push a little harder on the go pedal giving it even more fuel etc.....

Dervman · #9 01-06-2005, 09:39 AM

A 45% reduction in fuel consumption from a 'normal' 27mpg is huge

If you really don't have any fuel leaks then there is a real problem with your fuel injection system. I think any issue with the Boost Controller would affect power but not cause such a major fuel consumption. Is the power OK or are you flooring it all the time to get anywhere? Does it smoke badly on light acceleration or only when you floor it? Some smoke is normal under maximum acceleration.

Possibly your IP may need re-timing or maybe your Timing Chain has "stretched" causing incorrect valve and IP timing. Start with the simple stuff. Do a search for 'Smoke' and 'Poor Mileage' on this forum and go from there. I would also carefully check all your vacuum lines and the fuel supply lines from the tank to the IP Pump.

Good luck

Hatterasguy · #10 01-06-2005, 11:10 AM

I would pull the injectors, get them tested and rebuilt. They cost $10 each to test in my area.

I didn't think a 603 could burn that much fuel!

You should see another 10mpg.

WANT '71 280SEL · #11 01-06-2005, 08:03 PM

Timing chain was replaced by the PO's mechanic who specializes in BMW's and MB's. My mom drove on a 3 hour trip today so we'll see where it goes from there.

I constantly have the throttle down resting on the kickdown or about 3/4 for normal acceleration.

What about the tank strainer? What do I need to do to check it? I was under the car changing the oil yesterday and didn't notice anything sticking right out at me. Is it much work to remove then clean?
Thanks
David

Brian Carlton · #12 01-06-2005, 08:09 PM

Quote:

Originally Posted by WANT '71 280SEL

What about the tank strainer? What do I need to do to check it? I was under the car changing the oil yesterday and didn't notice anything sticking right out at me. Is it much work to remove then clean?

I was going to pull mine and check it when I changed the hoses recently, but you need a 1 13/16 socket, or crow's foot, to turn it without damage. So, it will have to wait until next time.