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Old 07-19-2004, 01:57 PM
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Location: West of Ft. Worth. TX
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Question Starving for fuel? Priming pump?

On my '84 300SD, 350K+ miles, I seem to be having a fuel problem.

The IP sounds as if it is not getting enough fuel? After running a while, esp. in hot weather, the IP sounds like it is running dry when I rev the engine and has a "chirping" sound at speeds above 60. Originally, there was a defective hose to the alda which I have since cured. But this seems to keep reoccuring when it is hot.

I have pulled the fuel screen and found it sparkling clean. The fuel filters are changed nearly every other month ( I do a lot of daily driving - service work ).

I have tried to see if the fuel tank vent line may be obstructed, however, I have not located the "end" of the vent line. When I pulled the trunk divider, I can see where the vent hard line goes through the floor. Looking underneath, there is a flex line with some sort of in-line "filter", then a hard line running forward. Does anyone know where this leads? I am beginning to doubt this is the problem because I test drove it with the filler cap off.

I have a feeling that the priming pump is going. Changing the hand priming pump had no effect. I would hate to lay out nearly $200 for a new priming pump and find out that it may be a free - $10 fix.

Are there any other possibilities? Maybe in the fuel return system?Would weak fuel return lines from the injectors cause this?

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Old 07-21-2004, 11:03 AM
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Anyone have any ideas on this? Thanks
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Old 07-21-2004, 11:28 AM
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Location: Double Oak, Texas
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When you open the fuel cap in hot weather, is there a big vacuum built up in the tank? If so, that means the air vent is clogged. I have just recently had this experience, but have not traced the vent lines yet.

Just last weekend, my car lost power on the highway and died as if out of fuel, but it only had about 430 miles on the tank of fuel. I did notice the vacuum build up in the tank when I opened the cap, but I put my spare fuel (1 gal. of veggie) in the tank, hand primed and went on. When I fueled up, I could put only 18.5 gal. of diesel in the tank.
1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead
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Old 07-21-2004, 12:05 PM
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There seems to be a little bit of vacuum --- sometimes. I am trying to see if there is any relationship to fuel level. However, I removed the filler cap and drove it for a while with no change in symptoms.

The hotter the temps get the worse it seems. I have even suspected weak injectors, they are much quieter when hot, but thought that I would have a more uneven idle. The idle is very smooth, esp. after a recent valve adjustment.
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Old 07-21-2004, 02:54 PM
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diesel purge

Hello, have you tried lubro-moly (liqui-moly) diesel purge? This is a good German-Made product. After I did this on my 300D, the car was very smooth on idle with less vibration coming from the engine. I have provided a link to an article which explains how to do the diesel purge. The only thing I would do different from the article is to do this procedure with your current pre-filter. Once you are done with the procedure, your current pre-filter will be filled with lots of crap. At this point before reconnecting everything back together, put in a new pre-filter. Here is a link on how to do this:

Here is where I bought the cans from (Product LM2005):

Attached Thumbnails
Starving for fuel? Priming pump?-lubromoly.jpg  

Last edited by adiel; 07-21-2004 at 03:04 PM.
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Old 07-21-2004, 04:37 PM
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SD Blue.

What kind of primerpump do you have?

Is it still the old white twist type or the new
black non twist type?

If you still have the old design, does it leak
when priming?
If yes, then you have air entering the fuel
system and that is most likely why this
problem is occuring.

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Old 07-21-2004, 07:51 PM
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I have changed out the hand priming pump to the new "black" type. However, this had no effect.

I have not tried the diesel purge. The car idles very smoothly but maybe there is something plugged somewhere.

Today, I took extra effort to clean the boost input to the alda. and adjusted it 1/2 turn richer to see what effect this would have. Its seems to help but only a little.

Maybe if I described the symptom a little differently. Originally, while accelerating, there would be a little "rumble" before the turbo boost kicked in. Later, I would notice a little "rumble" at speeds of 55-60 mph, but would smooth out when I bumped it up to 70. Now, I am getting this same "rumble" at anything over 60 mph. The idle continues to be real smooth.

Does anyone know what kind of pressure range I should see from the priming lift pump?
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Old 07-21-2004, 11:20 PM
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Location: Savannah, GA
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Have you been having starting problems? Try running down the highway with the fuel cap off and see if that fixes the problem. If so, it's definetly a blocked vent line. On my 240D the vent line comes just ahead of the left rear suspension before turning down (towards the ground). It has a Cone shaped piece of runner on the end.

When I purchased it, I ran into starting problems. turns out the vent line was clogged just about all the way through. Compressed shop air couldn't blow through it. Found an old 8ft speedo cable, cut the end of and used the wire to clear it. Pushed it in until it stoped, grabbed it with some vise grips and started turning it will continuing to push it in. Took all of 2 hours to get it all clear out (As I said, clogged from one end to the other).
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Old 07-22-2004, 12:22 AM
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No starting problems or stalling. I have tried running without the filler cap and noted no differences. I haven't been able to find the "end" of the vent line. However, I am beginning to think it is not the problem.

I just read an interesting article in about ALDA ADJUSTMENT. There is mention of the alda leaking, due to a large o-ring inside failing to seal. A great article showing the alda taken apart. It appears to be pretty straight forward.

With this in mind, I will be checking that tomorrow.
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Old 07-22-2004, 12:35 AM
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If indeed the fuel flow is restricted, and I can't see where you have definitely stated that it is, the tank output filter screen may be plugged. This will cause problems eventhough the vent line, main and secondary filters are all ok. Use search to find numerous threads about this problem and how to fix.
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1983 300D 164,150
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Old 07-22-2004, 10:41 AM
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Symptoms: Running Lean esp. in hot weather at cruise speeds

What I have checked:

Restrictions and/or Air:

Alda Boost Input - Replaced hose (switchover valve is bypassed)
Hand Priming Pump - Changed to new "Black" type
Primary Fuel Filter - Changed
Secondary Fuel Filter - Changed
Fuel Tank Screen - Pulled and checked "Sparkling Clean"
Vent Line - Highway Run with Filler cap off


Priming Lift Pump - Low Pressure?
Alda - Leaking internally?
Injection Pump - Internal problem and/or Linkage adjustment

This may be a combination of things, magnifying each other.
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Old 07-28-2004, 12:25 AM
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An update in case someone else is having similar problems. Well I think I have finally determined part of the problem for my "lean" running condition.

I removed the alda and took it apart since it seemed to not be holding any pressure. After cleaning the internal workings and giving the seals a thin coating of silicone grease, I reinstalled it and took it for a test drive. What a world of difference!!

Now I need to see if the folks from Fastlane can help me with new seals for the alda unit.
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Old 07-28-2004, 11:06 AM
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Originally posted by SD Blue
I have changed out the hand priming pump to the new "black" type. However, this had no effect.
Does anyone know what kind of pressure range I should see from the priming lift pump?
I have a gauge on mine and I always have 15 psi on the gauge, unless pulling hard when the pressure drops by 5 - 10 psi. The MB spec is a minimum of 0,8 bar.
When switched to VEG-OIL, my gauge reads down to 5 psi vacuum before the engine starts losing power.
I suggest that you may have an air leak, which is temperature related. It is getting worse (from your descriptions).

If you wish, you can reverse the fuel line and return line in the engine bay and try it. If the problem ceases, the problen is in the strainer, fuel hose or pipes from the tank to the engine bay. If no change, the problem is not in that fuel line.
If it is the problem, it is a cheap fix, buy 2 ft of fuel hose and replace all of the rubber lines between the tank and IP. Then check metal fuel lines for leaks - you may need to add a little (1-2 psi) pressure to induce leaks, but don't pump it up or the tank will deform..
While you are under the IP side of the rear, under the rear seat, look at the fuel lines coming over the suspension. My fuel tank vent is on one of the metal fuel lines at that location. It has a small rubber fitting on the end of the pipe.
Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
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Elizabeth '81 280E, sporting a '79 300D engine.
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Old 07-28-2004, 05:50 PM
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My car can go up a mountain...

but it takes like 38 seconds to go 0 to 10 mph, and on the highway, it can still go 95mph ez unless I double pump it--i always did think it was a fuel problem and not an engine problem, now I know what to do.....
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Old 07-28-2004, 06:55 PM
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HELP! I am in the same boat. 300SD. Today while driving home from work I started noticing a loss of power above 2500 rpm. Car drives around all day unless you try to run above that, either by hard acceleration or by highway speeds, then it stumbles and resists, going lean/losing power. Here's what I have done...

1981 300SD
tank screen is clean and fuel flows easily.
no vacuum on tank, cap off has no effect.
new fuel filters, old ones clean and like new, no effect.
replaced some old fuel lines with new and clamped, no effect.
line from intake manifold to switchover/alda has been completely cleaned out.
priming pump is of the old white-handle variety, and does seem to leak when priming, but not when running. It's dry now. If it leaking air is the problem, why would it start now out of the blue?

Could this be an injector problem or what? Yes, I run SVO sometimes, but it is new, filtered to 1 micron, and heated to 100C before going into the engine. I've haven't had a problem yet with it. I filled up with #2 and it doesn't help. Car always starts right up, no hesitation or any problems. Like I said, this wasn't a gradual thing, it just started today. what's going on ????

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