fuel starvation w123 - PeachParts Mercedes-Benz Forum

whyameye · #1 01-31-2016, 01:35 PM

1985 300D w123 running SVO and diesel since 2008.

In the last several months I have had a fuel starvation issue. The problem is apparent on both diesel and veggie but much more extreme on the veggie side.

Symptoms: runs OK but coughs occasionally on idle. After about 5-15 seconds under load the engine starts to lag. The problem is most apparent on the highway. I feel it hesitating especially going up a hill. The hesitation isn't consistent. Rather it sputters along with bursts of full power. The problem seems to get better and worse. Sometimes it completely disappears. Then a day later it won't run on veggie barely at all. It think it's likely air is getting in the line based on the symptoms.

I have new filters on both veggie and diesel. I changed the hand primer pump. I put the fuel line under vacuum before the lift pump. It held 20 inches no problem. I have replaced the diesel injector return lines. Years ago I removed the ALDA.

I also have put a fuel vacuum gauge inside the car to measure how much vacuum the lift pump is creating. It goes up to only an inch or 2 on the highway on diesel and 5-8 inches under severe continuous load on veggie. That seems reasonable so I don't think a constriction in the line?

I've read about the fuel relief valve spring on the back side of the injection pump. What are the symptoms of a compressed spring?

I'm also wondering if there is any way the lift pump could be leaking air other than at the hand primer and if there is a way to test if it is able to create enough vacuum consistently. (I suppose I could I purposely restrict the fuel intake and watch the vacuum gauge.)

Diesel911 · #2 02-01-2016, 12:26 AM

Concerning the Fuel Pressure Relief/Overflow Valve. If yours has a ball bearing pressed into the center you cannot easily take it apart. If it can be taken apart do that and measuer the spring. It is supposed to have a free length of 27mm.
The Service Manual has that you can stretch the spring till you have a free length of 27mm.

As far as the Fuel Supply/Lfit Pump goes there is rebuild kits for them that are $35 on down to about $17 for the generic version. However, due to the veggie use it could simply be that the valves inside are gunked up. In particular the valve return springs are not very strong.

There is a Vacuum reading where you can like remove the Plastic Filter and T into it there and hook you Vacuum Gauge. The spec is at idle you should get 0.1 Bar of Vacuum. I don't know what that converts into as far mm water or mm of mercury.

There is also some pressure tests but there is no ready way to install a pressure gauge; but there is some threads on that. Also even with a liquid filled gauge the needle bounces.

Funola and another Member were offering a adjustable pressue Fuel Pressure Relief/Overflow Valve. But, I do not know if they are still offered.

I would do any testing with Diesel Fuel as that is what the specs are for.

biopete · #3 02-01-2016, 04:57 AM

Did you check your fuel tank strainers ? Does your veggie tank have a strainer in the tank?

Re lift pump -- i'd put an electric one on there. Way better when priming and changing filters. Congrats on your long veggie run. That's awesome!

Personally i think i would put a electric lift pump on the diesel side see if it helps then get another for veggie side if it does or just use one after the valve. then check the tank strainers if you haven't .

mbolton1990 · #4 02-01-2016, 08:55 AM

Try doing a diesel purge?
On some of these old cars,it's the best $15 you'll ever spend.

ollo · #5 02-01-2016, 09:40 AM

On one of my Mercedes or BMW's back in the day, I had a leak in the rubber hose that runs from the fuel tank to the metal fuel line. Cannot even remember if it was on a gas or diesel car, but if there is such a set-up on your car, a cracked hose could be introducing air. Don