Air in fuel system??

[ericboutin]

I was running off the tank that was just about out of fuel tonight - trying to squeeze out every last drop...well I forgot I had it on the empty tank and I was coming down the street just about to the house when she started revving up and sputtering. I switched to the full tank as quick as I could and rode it home. When I parked in the driveway she wouldn't idle lower than 1000 rpms. I let her sit for about an hour and then I had to go back out. She started right up but it was idleing really rough. She shot up to 1500 rpm and then crashed (stalled out). I had to crank her a few times to get her started back up but it was idleing alot better. I took her up the road and got on her alittle and she didn't seem to mind. When I got back to the house she was idleing fine - right at 600 to 700 like normal. Everything seems ok - was it just air in the system that caused all the crazy fluctuations in the rpm's? Does air get into the system when it almost runs out of fuel? Can the truck purge itself of any air on it's own by driving it?
As usual thanks for any and all insight!

 

[JPR]

Yes and yes. The difference between our trucks and a tractor or similar diesel is the mechanical lift pump that primes the injector pump when the engine is run out of fuel.

The reason for the sudden increase in rpm is the first bit of air is compressed in the injector lines causing more fuel to be delivered to the engine. Think of it as shaking a new bottle of coke with lots of CO2 and one that is flat. The air acts like the CO2 in the coke. Of course with more air, the injector pump is no longer pumping fuel so it stops.

And yes, I did the same two weeks ago, switched tanks with 4.5 gallons left as it started sucking air.

 

[pafixitman]

Eric,
When you get a set of and change the filter, you will experience the same thing.

 

[L.Wilkinson]

I've got to respond to this thread, perhaps I shouldn't but
As far as running out of fuel, and the sensation of increased engine rpm and pulling state. you have to look at what is going on inside of the DB2 pump. First, air entering the pump makes its way to the little transfer pump @ the end of the rotary group. Pressure from the transfer pump is used to control the operation of the cam position within the housing, along with other circuits and the cold start feature. I believe air at the transfer pump leads to a cam change, and timing change, hence rpm and power change. Perhaps the governor has a hiccop and turns the metering valve a smidgon too. Correct me if I'm wrong with that.
Otherwise, once you have air making its way into a fuel line, it compresses, and you have less fuel. Solid fuel is far less compressable than air. So if you've gone that far in running low, your slowing down, stalling.
Any pump techies out there - comments?
Lorne

 

[Russ]

Yep what JPR said.

 

[ericboutin]

OUCH Joe!! That's gonna leave a mark!! Yah I really need to do that - least now I will know what to look for.

Thanks to all - I figured that's all it was just wanted to hear it too. Makes me nervous when the Dewey starts acting funny!!

 

[Agnem]

Eric, keep in mind too, that once you get some air in the system, the bubbles tend to stay in suspension in various places, and they don't gather together until you shut the truck off, at which time they often cause a failed start. This can sometimes happen for up to 3 starts in succession. This is common when you change your fuel filter. Sometimes you'll get a rough start for the next 2 or 3 starts until the air is all gone.

As a side note:

This really warrants a new thread, but I didn't want to post one, because the powers that be may be watching, and if your going to give away secrets, it's better that they be buried in a thread which could easily be overlooked by someone on patrol from another site. Concerning linking to articles here on TDS. I tried earlier to link to my fuel injection article from a post on TDS. If you try to link to anything at oilburners.net, you'll get a "Censored" URL link instead. So.... I found a way around it. Go to http://tinyurl.com and paste the link there. It in turn, will give you a link you can use in a TDS post, that the modernazi's won't catch. At least, until they discover my secret. For a demonstration, read my post HERE. Note that it works the other way also, because of the smiley problem.

 

[JPR]

Lorne,

You have brought up a good point with the interaction with the internal components of the pump.

My thought regarding the air compressing is, normally the pump is going to meter a given amount a fuel at a given pressure. For sake of discussion, assume that 5 cc at 2000 psi. The pump pushs that 5cc into the injector line which in turns moves fuel down the line and 5cc enters the engine via injector. The injector line still has fuel, just 5cc was released to get the pressure back down and close the injector. If the pump were to put 4cc of fuel and 1 cc of compressed air into the line, then more fuel would need to be released at the other end due to the expansion of the air as the pressure starts to drop. This is only a guess.
The idea is the same as a surge tank on a well pump. You fill the tank up with air prior to the pump filling the tank with water. As water is used, the pressure drops, but much more slowly than if there were no air.

 

[aaklingler]

i saw that mel, at first i looked at what it was linked to, hum, i haven't seen this before, oh no, oilburners, tricky guy you are.

 

[geonc]

Eric, If this is the first time you have run that low, you just might want to give the fuel system a good ennema

Get a new fuel filter, and 2 cans of Sea Foam.
NAPA has a 1 piece fuel filter and for the life of me, I can't recall the # CRS.............. The only thing you need to switch over is the water sensor.

Fill the new filter FULL of Sea foam and split the remains between tanks.

That should cleanse out any sludge you may have attracted.

Just my couple pennies
GEO

 

[Hoss6.9]

The Baldwin number is a BF1222-SP that has the sensor port and is a one piece filter. Have sold quite a few of them. Haven't had any complaints as of yet!

Hoss....

 

[ericboutin]

Ahh very sneaky Mel!! Very cool!

Hey Geo,
Yah this is the first time I've done that, usually I do better to keep my eye on the tank!
I do need to do a filter change anyway - haven't done it yet since I've owned her and don't want to find out what happens when you let it go to long.
I'm gonna ask yah a stupid question now. I've seen where folks have talked about Sea Foam before but what is it? Where do you get it? Is it kinda like the fuel additive you get at Wally world?

Thanks

 

[L.Wilkinson]

JPR, can't say yes or no to the idea, but the reality of the effect the cam timing has on engine rpm is real. If you get onto your truck to soon when its real cold out for example, the water temp sensor will be venting fuel pump housing pressure till you get up to about 112F. While the housing pressure is vented through the solenoid in the top cover, the internal cam ring is rotated, allowing for about 2-3 degrees advance. If you have ever been accelerating briskly with this going on with your cold engine, the sensation is of almost having a runaway. Well, when you burb some air into the pump inlet by reason of low tanks, etc. the transfer pump provides the same advance signal to the cam ring. If you're pullng at all with a warm engine, it will jump up a notch or two. Not a good thing to do (for more power) don't connect a switch to the solenoid for those moments or pistons will suffer.
Huff actually had an article on the other site a while back, for those of who routinely run @ 10psi manifold and above. It involved knocking the collective timing back from 5 to 3 BTDC, and then having a Hobs pressure switch on the manifold to sense when you pass 8psi to control whether the cold dump solenoid would be open or not. So below 8psi you'd be running at 5 degrees, after you'd be back to 3 degrees. Figures may have been tweeked one way or the other, but this should have provided real good power to economy results, and good life.

Hey, if I'm borring anyone, excuse me.

 

[geonc]

Hey, if I'm borring anyone, excuse me.

Lorne,
Your posts are NEVER boring
Great to see ya here

 

[geonc]

I've seen where folks have talked about Sea Foam before but what is it? Where do you get it? Is it kinda like the fuel additive you get at Wally world?

You can get Sea Foam at NAPA, and most auto parts places.

I used to think Sea Foam was snake oil till I saw it used at Chason Diesel in Raleigh....about 6 years ago.

I asked why a diesel shop was selling that "miracle cure" in a can
and the tech showed me .......While I was buying some Stanadyne for the IDI,
{checks in the mail Fred}

.........a guy from Fayettville had driven up to Raleigh and his IDI was running crappy........after the tech gave it the once over, he changed the fuel filter and filled the new one up with sea foam.... I was sold

I took some flack at TDS when I posted about it, but once people try it....................

Hope this helps
GEO