Weird almost empty behaviour - PeachParts Mercedes-Benz Forum

Theurig · #1 05-24-2004, 05:44 PM

I have this kind of weird behaviour with my tank. When it gets to about a quarter of a tank or maybe a little less, I hear a groan at the back towards the fuel pump, and the car goes off. At first I figured my fuel sending unit was going bad or something and giving me the wrong amount of fuel on an empty tank, but when I fill up, it stops at about 17 or so gallons meaning that there is still plenty of fuel in the tank. Also meaning that the sending unit is working fine. What could be causing this weird behaviour??

MrCjames · #2 05-24-2004, 06:50 PM

Hi

The problem you are experiencing may be contributed to the small plastic pins that secure the fill pipe restrictor in the filler neck. Usually when the restrictor is replaced the pins are knocked into the tank, not a problem for the most part. On occasion it is possible for one of the pins to work its way into the return port of the "splash bowl" assembly. The splash bowl is inversely fitted to the bottom of the tank and the tank screen is inserted within it. One of the primary purposes of this bowl is to retain fuel for the pump when going into turns, otherwise the pump starves for fuel and the engine stalls. The return flow also siphons fuel from the tank as it is returned into this bowl.

The everything is okay until you get a ¼ tank is the first clue. The suctioning sound you are hearing from the tank is the second. The car starts and runs for a short while than stalls is the third. If you remove the fuel hose from the pump and watch the flow it will flow fast then slow down to a trickle. Plug it for a short time then remove the plug and it flows again but begins to trickle.

I have encountered several fuel tanks that needed to be replaced due to this little pin getting jammed in the return orifice. Options are to keep the tank above ½ or jump on in and replace the tank. I have not found anybody lucky enough to dislodge this silly little bugger from the port in the tank, but there is always a first.

Theurig · #3 05-25-2004, 11:15 AM

wow,
folks like you on this forum, are what make it so great!!! I think I'll just stick to keeping the tank a little above 1/4. I don't think I want to play with the tank. if the pin is in the tank, I guess there is not really any way to get it out, right?

cc260E · #4 05-25-2004, 11:40 AM

Hi MrCjames

Have you any drawing of this mounting to help me to better undertsand, the erracic stalling of my car may be due to what is described above (changed a lot of things).

Finally I tried to remove the tank on my 260E 1988 to clean it, I was not successfull after having removed almost everything till the 2 nuts attaching the tank to the back of the rear seat

#5 05-25-2004, 12:06 PM

It's very common on certain other Bosch injection using brands of that era for the "pre-pump" (SAAB term) in the tank to fail while the main pressure-supplying pump is still good, and the car will run well on the main pump as long as there is a certain minumum of fuel left in the tank, but die as if out of fuel below that level.

Theurig · #6 05-25-2004, 12:12 PM

I know my car is the earlier system, which has only one fuel pump, but could it be that there something I don't know about a pre-pump that isn't functioningl when it gets to 5-6 gallons?

cc260E · #7 05-25-2004, 01:08 PM

Quote:

Originally posted by cc260E
Hi MrCjames

Have you any drawing of this mounting to help me to better undertsand, the erracic stalling of my car may be due to what is described above (changed a lot of things).

Finally I tried to remove the tank on my 260E 1988 to clean it, I was not successfull after having removed almost everything till the 2 nuts attaching the tank to the back of the rear seat

Forgot to say that on my car there are two pumps outside the tank and is stalling sometimes as if no fuel is going to injectors

MrCjames · #8 05-25-2004, 01:15 PM

To my knowledge there was no submerged pre-pump in any fuel tanks for MB's of that era. As of late I do not think they have adopted the use of them.

Give me a day or two and I will post an image with a description of the bowl assembly that is mounted inside the tank.

csnow · #9 05-25-2004, 03:51 PM

I get a stall when I take a long sweeping left turn (exit ramp) when the fuel level is low.

cc260E · #10 05-27-2004, 03:48 AM

Quote:

Originally posted by MrCjames
To my knowledge there was no submerged pre-pump in any fuel tanks for MB's of that era. As of late I do not think they have adopted the use of them.

Give me a day or two and I will post an image with a description of the bowl assembly that is mounted inside the tank.

MrCjames,
do you think that a faulty check valve on one of the two pumps can give the same failure ( like lack of fuel) ?

LeaUK · #11 05-27-2004, 04:02 AM

Very interesting.

On my R129 I have experienced this 'growling' sound on the odd hot occasion too, when the fuel is below 1/4 full. I replaced the fuel filter and haven't experienced it again (not that it was due for replacement as it was only a year old).

I cannot say this resolved the issue as the growling was very infrequent and the infrequency probably compounded by only very rarely running that low on fuel, but I did notice the two fuel pumps and return valves. I favour the fuel pump as the culprit as this is probably common across many models where as the fuel tank isn't.

Maybe we should make a list of what models suffer from this behaviour and what remedies people sort?

MrCjames · #12 05-27-2004, 11:44 AM

Quote:

Originally posted by cc260E
MrCjames,
do you think that a faulty check valve on one of the two pumps can give the same failure ( like lack of fuel) ?

I do not believe it would, the check valve is designed to retain pressure in the system when the pump(s) is/are electrically off. Should one of the pumps be faulty there is a noticeable hesitation during hard acceleration but if you ease into the throttle the vehicle seems to be acceptable. (Primary Symptom)

In one of my previous posts I mentioned removing the fuel hose and monitoring the rate of flow from the tank, it should remain continuous until the tank is completely drained. Should it flow for a minute or so and begins to trickle, plug the line for roughly 5 minutes. Remove the plug, fuel begins to flow heavily but shortly afterward it begins to trickle again. Repeat this process one more time to confirm the condition.

Condition(s) & Primary Symptom: Fuel level @ ¼ tank-approximately, vehicle runs for a short time then dies. Vehicle stands for several minutes and starts up after lengthy cranking time. Drive for a short distance and it dies again.

Test(s): Monitor fuel pressure, slowly drops until vehicle stalls. Let stand roughly 5 minutes. Crank engine & fuel pressure returns, engine starts. Drive for a short time & fuel pressure drops, vehicle stalls again!

MrCjames · #13 05-27-2004, 07:09 PM

I have a few images regarding the basic principal of the bowl assembly in the fuel tank. I'll figure out how to covert them to a valid attachemnt extension and I will send them Tuesday.

MrCjames · #14 05-28-2004, 12:53 PM

Here are a few rough drawings regarding the fuel bowl in the fuel tank.

MrCjames · #15 05-28-2004, 01:01 PM

Second Image