Air in Fuel, take two.

UFO · **Posted:** Mon Jan 11, 2010 9:19 pm

Wobbly wrote:

Back to my original comment, Mercedes diesels don't use a lift pump, and don't have fuel starvation issues due to air. Their fuel filter heads are not vented.

If the return line were to be looped vertically, the CRD IP low pressure pump could possibly retain its prime even with some air entering the suction side at the filter head or the tank fittings. With the return line above the level of the fuel filter head, fuel would be trapped in the IP low pressure pump when the engine is off. Since the IP low pressure pump can vent air, it might be able to vent air in the line between the fuel filter and the IP pump, and the engine might start and run. As designed, fuel in the IP low pressure pump now drains back to the tank when the engine is stopped. With no fuel in the IP low pressure pump, and some air in the line from the fuel filter to the pump, the IP low pressure pump is never primed. The engine sputters a few seconds until the fuel in the pump high side is used and then the engine dies.

Just a theory.

My Mercedes has a lift pump, and a bleeder at the top of the fuel head. The fuel head is vented, via a very small orifice, to the tank. It's a solid design, simple and effective, and not at all like the CRD. Maybe the SDL is different, but AFAIK it's not.

Wobbly · **Posted:** Mon Jan 11, 2010 11:19 pm

As shown in this diagram, the 86 300SDL has no fuel pump at the tank.

http://www.performanceproducts4benz.com ... gram_id=77

UFO · **Posted:** Tue Jan 12, 2010 12:22 pm

I know of no diesel Mercedes with a lift pump at the tank. The lift pump is on the side of the injection pump where the primer bulb is. The fuel head is fed by the lift pump under pressure, at the high point of the return line, and flushes air back to the tank before it gets to the high pressure pump.

Wobbly · **Posted:** Tue Jan 12, 2010 1:12 pm

UFO wrote:

I know of no diesel Mercedes with a lift pump at the tank. The lift pump is on the side of the injection pump where the primer bulb is. The fuel head is fed by the lift pump under pressure, at the high point of the return line, and flushes air back to the tank before it gets to the high pressure pump.

The difference between the CRD and the Mercedes is that the Mercedes IP low pressure pump sucks pre-filtered fuel
from the tank and sends it through the filter head. As you mentioned, air can exit the pressurized filter head and return to the tank.
The CRD IP low pressure pump sucks fuel from the filter head, which is under a slight negative pressure, and the IP vents air to the return line.

UFO · **Posted:** Tue Jan 12, 2010 1:39 pm

You are correct, sir! Root cause is identified.

Herk · **Posted:** Fri Jan 15, 2010 2:56 am

Ok we know what's the problem. I have replaced the fuel filter head and about month latter the issue is back and much worse than before :cry:

. Has any one had any luck doing some different besides adding a lift pump?? :?:

Thanks

UFO · **Posted:** Fri Jan 15, 2010 11:51 am

I would check all the hose connections back to the tank for tightness. If you can't find anything obvious, you may have to install a lift pump. I've still got an original filter head that I bleed every fill up, no lift pump, no large leaks and the only time it shut down from lack of fuel I had a plugged filter.

Wobbly · **Posted:** Fri Jan 15, 2010 7:27 pm

Herk wrote:

Ok we know what's the problem. I have replaced the fuel filter head and about month latter the issue is back and much worse than before :cry:

. Has any one had any luck doing some different besides adding a lift pump?? :?:

Thanks

The replacement filter head is the new model?

stoutdog · **Posted:** Sat Jan 16, 2010 1:50 pm

Herk wrote:

Ok we know what's the problem. I have replaced the fuel filter head and about month latter the issue is back and much worse than before :cry:

. Has any one had any luck doing some different besides adding a lift pump?? :?:

Thanks

Check all your hose fittings for leaks... The fuel pump isn't a fix to the root cause of the issue. Only replacing the leaky hose/fitting is.

Herk · **Posted:** Sun Jan 17, 2010 12:06 pm

Thanks guys for the input and Yes its the new improved fuel filter head.

Joe Romas · **Posted:** Sun Jan 17, 2010 4:51 pm

I believe the "root cause" on mine was at the quick connect plastic fittings at the top of the tank

They were designed for a gasoline system under pressure not a diesel system under a vacuum :banghead:

And to replace them with rubbber hose and clamps you need to drop the tank. Early on in the life of 05/06 CRD's some dealers replaced ALL the fuel lines from the tank to the filter head and cured the problem. But the biggest problem I and other early adventurers had was getting dealers to believe you really had a problem :dizzy:

Remember many of us bought new and expected some sort of support :dead:

So putting in the Cummins lift pump was a no brainer and has completly cured my air problem while retaining all my stock fuel lines :jester:

nescosmo · **Joined:** Sat Jul 14, 2007 11:17 pm **Posts:** 202

I istall an E8012S on the supply line under the Jeep and it has a check valve built in to it.
Maybe if you install a check valve on the fuel supply line it could solve your problem.
try that nothing to loose......

Herk · **Posted:** Tue Jan 19, 2010 3:02 am

Sorry to ask but a what's a E8012S? Do you have pic's

Wobbly · **Posted:** Tue Jan 19, 2010 11:11 am

Herk wrote:

Sorry to ask but a what's a E8012S? Do you have pic's

It's an Airtex fuel pump. Installing a check valve in the supply might stop fuel from flowing from the filter head back to the tank, but it wouldn't stop fuel from draining back from the IP to the tank. A spring-loaded check valve might not open under the IP low pressure pump suction pressure.

Air in Fuel, take two.

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