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  #1  
Old 10-23-2006, 05:27 PM
largelybored's Avatar
1986 300SDL in Austin TX
 
Join Date: Apr 2006
Location: Austin TX
Posts: 43
300sdl faster when fuel tank filled

Hi all,

I've got an 86 SDL and she always seems faster after I top off the tank. I have the same seat of the pants feeling on my 2001 dodge cummins. Does it maybe have something to do with increased fuel pressure? Less strain on the pump?

-M

p.s. I had my injector nozzle o-rings fail recently. Im assuming its due to ULSD and I have not been putting in any powerservice. It would not want to start without a few minutes of cranking. I thought it was glow plugs but they worked fine. I guess too much air in the lines.

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  #2  
Old 10-23-2006, 05:30 PM
Banned
 
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Quote:
Originally Posted by largelybored View Post
Hi all,

I've got an 86 SDL and she always seems faster after I top off the tank. I have the same seat of the pants feeling on my 2001 dodge cummins. Does it maybe have something to do with increased fuel pressure? Less strain on the pump?

-
Yep, my '86 does the same thing. In fact, if I run it down close to the point where it needs 20 gallons to fill, it gets so upset that it won't get enough fuel to climb hills for the next 100 miles or so. Then it clears itself.

Strange indeed. Probably the tank screen, but, I've not bothered to pull it.
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  #3  
Old 10-23-2006, 05:43 PM
dieseldiehard's Avatar
Dieseldiehard
 
Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,368
Try removing the gas cap (well, fuel cap - you know its a habit calling them gas caps!)
In case there is a vacuum being formed in the fuel tank, it would work against the lift pump (on the IP). Maybe your lift pump is getting weak?

In a 123 chassis I saw a pin hole leak in the fuel line result in strange symptoms like yours, ie. no power as the tank nears empty. What was happoening was the ppin hole allowed a stream of air to get pulled inot the supply line as the tank was low. With the tank full there was enough head on the tank to force fuel to weep out and that prevented any air from being pulled in.

My 124 300Dt (same engine as your SDL) has quite a bit of pressure in the tank as fuel gets used up. If you remove the cap air swooshes out and I haven't figured out if that is normal or not.
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  #4  
Old 10-23-2006, 05:47 PM
Diesel Giant's Avatar
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Location: Loganville/Atlanta
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Come on its not that bad of a job. Get on your back, drain the tank, get diesel in your hair, wrestle with the stainer, bust your knuckles.
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  #5  
Old 10-23-2006, 05:50 PM
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Join Date: May 2002
Location: Blue Point, NY
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Quote:
Originally Posted by Diesel Giant View Post
Come on its not that bad of a job. Get on your back, drain the tank, get diesel in your hair, wrestle with the stainer, bust your knuckles.
Oh, I've been there and done that when I changed the hoses. Even planned to pull the strainer. But, without that 46mm socket, the configuration of that nut makes it impossible to get any channel locks on it. I've recently acquired a 1 13/16" socket, so, it's on "the list".
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  #6  
Old 10-23-2006, 06:18 PM
Bruce Kennedy's Avatar
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Join Date: Oct 2006
Location: Cincinnati Ohio
Posts: 230
Quote:
Originally Posted by dieseldiehard View Post
Try removing the gas cap (well, fuel cap - you know its a habit calling them gas caps!)
In case there is a vacuum being formed in the fuel tank, it would work against the lift pump (on the IP). Maybe your lift pump is getting weak?

In a 123 chassis I saw a pin hole leak in the fuel line result in strange symptoms like yours, ie. no power as the tank nears empty. What was happoening was the ppin hole allowed a stream of air to get pulled inot the supply line as the tank was low. With the tank full there was enough head on the tank to force fuel to weep out and that prevented any air from being pulled in.

My 124 300Dt (same engine as your SDL) has quite a bit of pressure in the tank as fuel gets used up. If you remove the cap air swooshes out and I haven't figured out if that is normal or not.
Was this pin hole in the metal line? My 85 300td starts getting air in the system if I let it get below half a tank. I changed all the rubber sections of line and even replaced the expansion tank. My fix for the time being has been not to let it get below half. Puzzled
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  #7  
Old 10-23-2006, 06:39 PM
H-townbenzoboy's Avatar
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Location: Houston, TX
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Quote:
Originally Posted by dieseldiehard View Post
If you remove the cap air swooshes out and I haven't figured out if that is normal or not.
It's not supposed to make a swooshing noise when opened. I know it denotes a problem, what that problem is, I don't know.
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  #8  
Old 10-23-2006, 06:47 PM
dieseldiehard's Avatar
Dieseldiehard
 
Join Date: Oct 2002
Location: Bay Area No Calif.
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Quote:
Originally Posted by Bruce Kennedy View Post
Was this pin hole in the metal line? My 85 300td starts getting air in the system if I let it get below half a tank. I changed all the rubber sections of line and even replaced the expansion tank. My fix for the time being has been not to let it get below half. Puzzled
Yes, a pinhole usually develops at a point where some vibration against a clamp or something. I've seen and heard of rigid transmission lines leaking right where its clamped near the front of the bottom oil pan, when the rubber gasket around the metal clamp "goes away". Mercedes have a few problems that show the old adage "History Repeats Itself" is quite applicable!

You may want to pressurize the tank to a few psi using an old rag around the fuel filler then shine a light all along the entire length of the line from the fuel tank to the engine. In my 300TD the expansion tank was leaking and all it took was my nose (in the car) to determine I had a fuel leak. After unsuccessful attemps to seal the crack in the plastic expansion tank, I replaced it with one from a '79 300TD, made when MB was still using metal for critical things Why did they have to go and cost reduce stuff? hint! that was a prelude for things to come!
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  #9  
Old 10-23-2006, 07:07 PM
dieseldiehard's Avatar
Dieseldiehard
 
Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,368
Another thread on this subject: Sluggish running diesel on last half of tank

This is a problem that surfaces every now and then. From my archived files using comments from members of the SHopforum I offer these posts:

Subject: VERY Sluggish on 2nd half of Tank (Due to leaks in the fuel line!)
______________
Hi All,
About 2 months ago, I bought a 1985 300TD that had been sitting in NC for sometime. Everything is fine with this car until I get to about 1/2 tank of fuel.

Then she hesitates and runs so sluggishly it's actually dangerous to be in traffice. Thinking I may have bad fuel or water/algae, etc. I manager to limp to a service station and filled her up with diesel. After idling for a while, all seems well.

Now this morning she did it again. Valves have been adjusted and when everything's kosher, she runs like a little Swiss watch.

Does it sound as if I have some gunk in the tank that needs to be removed and then clean the tank?

Any ideas/suggestions greatly appreciated!
GoWolfPack

07-01-05
The SDL just did this on the last tank. It lost about 50% of the available power when I got down to the last three gallons.
Funny thing about it was that it took about 100 miles of running, after the tank was filled for the power to completely return.
In fact, I can't be certain that it has fully returned because I typically measure its performance against a specific grade in CT. It should be able to hold 70 mph on the grade if it is providing the proper horsepower.
We'll see........
It's a strange deal, I'll say that for it. Can't be fuel filters, AFAIK, because they are not self-cleaning as the vehicle is driven.
Brian Carleton

07-01-05
This same thing started to happen to me about a year ago and after looking in both the engine compartment and the fuel tank I gave up for a while and just kept my tank as full as I could. But it steadily started to get worse until i could barely drive anywhere, regardless of the level in my tank. I ended up finding out the fuel lines from the tank to the engine were rusting out. With a full tank there was enough pressure in the lines from gravity to not allow any air to suck in, but they sucked air at lower levels of fuel.
I did not have to replace the entire length of the fuel lines, I just cut out the rusted part and replaced it with some flexible fuel hose (approx three feet total). This did the trick. The most common place for this rust is in the rear end above the sub frame where salt and debris hangs out, you can check this by looking around the driver's rear tire for fuel drips- these drips started very slowly on mine so they were hard to find. It helps to clean the fuel line off with a rag so it will encourage the fuel to glide down the line and drip on the ground just in front of the rear tire. If this ends up being the problem check your sub frame for rust too-very dangerous situation if they start to rust through.
As I have been writing this I have also been thinking that you should check your overflow tank in the spare tire compartment and make sure that is not leaking any fuel there or any of the hoses from the overflow tank to the main tank. I could see this sucking air at certain levels of fuel while not sucking air at others.
To test if your system is sucking air, the next time you lose power, pop the hood and inspect your fuel filter. If it is not very full that could be an indication it is sucking air. With the engine running vigorously pump your primer pump and quickly drive a mile or two to see if it fixes the problem temporarily, if it is really idling hard pumping the primer pump should immediately smooth out the idle as well.
After I diagnosed my problem and drove to the shop to buy parts I ended up having to pump the primer pump every couple of miles just to get down the road (they are back roads, so it was o.k. to drive 10 miles an hour with the pedal floored!)
When I was checking my fuel filter it had little black flecks in it, which I found out were tiny rust flakes coming off of the inside of the fuel line. this was fixed when I replaced the bad section of fuel line so that might also be an indicator.
I hope this helps! If it is not caused by sucking in air I don't know what the problem could be.
cheers,
sdparizona Scott sdpettitt@yahoo.com

07-07-05 Final Report.
Seemed to be a loose clamp on the fuel line to the back end of the pre-filter. This is what was causing bubbles in the prefilter. The IP was sucking for fuel and taking in air. I tightened that clamp down nicely and no more bubbles.
GoWolfPack
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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!
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  #10  
Old 10-23-2006, 10:21 PM
Hatterasguy's Avatar
Zero
 
Join Date: Nov 2002
Location: Milford, CT
Posts: 19,318
I never noticed a difference.

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