Air in Fuel, take two.

CRDMiller · **Posted:** Sat Jan 09, 2010 1:44 am

A long time ago, after replacing the fuel head with the updated version, i added a carter transfer pump, located behind the battery.

The pump was in place before the stock upgraded fuel water separator and fuel filter. So fuel would flow under suction to the pump, and under pressure into the head, and out to the cp3's transfer pump.

My primer has always been firm, since i added the pump (and at the same time upgraded the head) Until today.

Today i placed the carter pump on the discharge side of the primary fuel head, and installed a secondary fuel filter of 2 microns on the discharge side of the carter lift pump.
So the primary is under suction (15 micron) the secondary is under pressure (2 micron). This is a pretty common set up for a diesel engine. (Except for the electric lift pump)

Today i find myself carrying a 12 mill wrench in the cup holder again.
I guess it's possible that i simply have not gotten all of the air out of the lines but the secondary always discharges clear fuel, with no air or froth.
What do you guys think?

I'm not stuttering or otherwise "suffering" with aif symptoms, but so far every time i have driven and stopped i could get air out of the primary bleeder.

BlackLibertyCRD · **Posted:** Sat Jan 09, 2010 3:03 am

Since you moved the pump, you are now putting the primary fuel head under vacuum and will stay that way after shutdown. The air that is naturally in the diesel fuel will separate and rise to the top of the fuel head (the highest point). The next morning there will be plenty of air to bleed. We all put the lift pump before the primary fuel head to avoid separation of the air in the fuel and suction leak.

CRDMiller · **Posted:** Sat Jan 09, 2010 3:14 am

Interesting theory, however even if this were possible, the problem is time, there has been no time for this to occur. I can bleed the air, drive 3 miles, and bleed more air

.

Regardless of this, i DO NOT WANT to PUSH air INTO the cp3 That is my goal. Yeah i know the way i had it kept the primer full, but it had to be pushing this very same air into the cp3.

Sir Sam · **Posted:** Sat Jan 09, 2010 5:28 am

CRDMiller wrote:

Today i find myself carrying a 12 mill wrench in the cup holder again.

It's a 11mm, trust me I know, I have one in my pocket here(-10F fuel, just making sure the diesel is still liquid enough to pump).

If you dont have a 11mm then a 7/16" will do fine.

AZ CRD · **Posted:** Sat Jan 09, 2010 8:50 am

The in tank Cummins lift pump works great.

Joe Romas · **Posted:** Sat Jan 09, 2010 9:06 am

AZ CRD wrote:

The in tank Cummins lift pump works great.

I certainly does :BANANA:

geordi · **Posted:** Sat Jan 09, 2010 2:11 pm

Why someone would choose to duplicate the air-suction design of the original while adding the very part that would solve the problem is beyond me. :2cents:

My Mr. Gasket pump is working just fine located right next to the source of the fuel - Just on the outside of the fuel tank... And I've had no problems with air in my fuel since.

BlackLibertyCRD · **Posted:** Sun Jan 10, 2010 1:57 am

BlackLibertyCRD · **Posted:** Sun Jan 10, 2010 2:05 am

CRDMiller wrote:

Interesting theory, however even if this were possible, the problem is time, there has been no time for this to occur. I can bleed the air, drive 3 miles, and bleed more air

.

Regardless of this, i DO NOT WANT to PUSH air INTO the cp3 That is my goal. Yeah i know the way i had it kept the primer full, but it had to be pushing this very same air into the cp3.

Remember the check valve is in the primary fuel head, therefore it will always be under vacuum (soft plunger) with your setup. When you open the bleeder after three miles it will suck air in first, then you pump it back out when you can make pressure (hard plunger).

CRDMiller · **Posted:** Sun Jan 10, 2010 5:23 am

Well, i placed my pump before the primary filter.

I was simply trying to imitate the standard fuel system found on, many, many, many (all except some Cummins, basically) heavy trucks.
I'm so used to doing it this way :dizzy:

geordi · **Posted:** Sun Jan 10, 2010 4:50 pm

Hey, no worries. But ya gotta remember: Why do all the "American" cars made in the last 40 years not come with a transmission drain plug? Because GM figured out that it could SAVE THEM a whopping 5 cents per car... And everyone else decided to follow their lead and be CHEAP.

Why do so many diesels have a suction-based fuel system? Because it would cost the BUILDER money to design it properly and use extra parts... When they can simply save that money and deliver a problem for the end-sucker to deal with later.

Bingo - Save money NOW, who cares about tomorrow!

Just because everyone is doing it... Doesn't mean it is a smart decision. Always ask who has to spend money to deal with the issue. If it's YOU... Then it is a design flaw that they just didn't want to fix.

Wobbly · **Posted:** Mon Jan 11, 2010 11:23 am

geordi wrote:

...Why do so many diesels have a suction-based fuel system? Because it would cost the BUILDER money to design it properly and use extra parts... When they can simply save that money and deliver a problem for the end-sucker to deal with later...

The Mercedes 300SDL has no auxiliary fuel pump. I never had an air lock problem with mine.

UFO · **Posted:** Mon Jan 11, 2010 11:53 am

Wobbly wrote:

geordi wrote:

...Why do so many diesels have a suction-based fuel system? Because it would cost the BUILDER money to design it properly and use extra parts... When they can simply save that money and deliver a problem for the end-sucker to deal with later...

The Mercedes 300SDL has no auxiliary fuel pump. I never had an air lock problem with mine.

That's because they designed in an air bleed in the fuel return line. The CRD has problems precisely for that lack of design.

geordi · **Posted:** Mon Jan 11, 2010 1:03 pm

Its also probably b/c you can just about run one of those Mercedes on SAND and it will still get 30+ mpg.

Gotta give the Germans credit, they built a hell of an engine.

Wobbly · **Posted:** Mon Jan 11, 2010 3:41 pm

UFO wrote:

Wobbly wrote:

geordi wrote:

...Why do so many diesels have a suction-based fuel system? Because it would cost the BUILDER money to design it properly and use extra parts... When they can simply save that money and deliver a problem for the end-sucker to deal with later...

The Mercedes 300SDL has no auxiliary fuel pump. I never had an air lock problem with mine.

That's because they designed in an air bleed in the fuel return line. The CRD has problems precisely for that lack of design.

As I remember GMCTD's postings, the CRD bleeds air from the IP to the return line.

UFO · **Posted:** Mon Jan 11, 2010 4:07 pm

Wobbly wrote:

UFO wrote:

Wobbly wrote:

The Mercedes 300SDL has no auxiliary fuel pump. I never had an air lock problem with mine.

That's because they designed in an air bleed in the fuel return line. The CRD has problems precisely for that lack of design.

As I remember GMCTD's postings, the CRD bleeds air from the IP to the return line.

Not from the high point of the system. That's the filter head. That's where the bleed belongs.

Wobbly · **Posted:** Mon Jan 11, 2010 5:15 pm

UFO wrote:

Not from the high point of the system. That's the filter head. That's where the bleed belongs.

It's difficult or impossible to vent air from the suction side of an open loop system. The filter head is on the suction side.

UFO · **Posted:** Mon Jan 11, 2010 6:51 pm

Wobbly wrote:

UFO wrote:

Not from the high point of the system. That's the filter head. That's where the bleed belongs.

It's difficult or impossible to vent air from the suction side of an open loop system. The filter head is on the suction side.

Now you understand. A lift pump is necessary for a robust design.

Wobbly · **Posted:** Mon Jan 11, 2010 8:15 pm

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.

Joe Romas · **Posted:** Mon Jan 11, 2010 8:48 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.

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.

I thought the primer pump had a check valve in it :?:

Air in Fuel, take two.

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