Stumbling "Missing"

[Robyn]

Well some time ago I wrote a post on the Dahooooley having a stumbling issue.
I replaced the PMD, lift pump and the OPS.
The truck did it once more and I took at back to the ranch to replace the filter harness (94-95 trucks only)
The issue was NOT TO resurface again until Thursday afternoon.
The kid has been using the truck with no issues all winter.
I unhooked the horse trailer and headed into town and as I pulled onto the main highway I got 2 quick stumbles and then again a couple miles later.

I also thought that the power was down a tad since the last time I ran it.

Hmmmm, the fuel tank was close to empty so I ran the truck over to the card lock and pumped in 30+ gallons of fresh #2

Within just a few miles the power was back and Old Dahooooley was purring like a champ again without any more stumbles.

It is begining to look like maybe the original problem may have had nothing to do with the items I replaced at all.

I am supect now that the fuel pickup may very well be sucking air when the fuel level nears the bottom.

I am thinking about yanking the tank down as soon as the weather permits and replace the tank if its all skunked up along with the sender/pickup assembly.

Good bet there is an issue there.

Now just before I replaced the filter harness on the IP I had filled the fuel tank and then run home to do the harness.
The truck had not been run this low on fuel since.

Hmmmm, ya never know just what is the culprit sometimes.

Im thinking that the fuel system is getting a small amount of air in it that is mesing up the optical sensor or ??? and the pumps ability to do its job.

Just thought I would pass this on

Robyn

[CapnAmerica]

Robyn--

Thanks for the "heads-up" (as the corporate :cool: cliché-makers like to say) on stumbling. I occasionally get them on my newly-rebuilt 6.5 with new IP and PMD, and haven't been able to tie them to anything. :confused:

Normally, when the PMD is giving up the ghost, it gets progressively worse and is related to its local temperature...but these latest are more subtle: just a "SPUT"-:eek:-far short of a "SPUTTER". I'll try to make this connection next go-around. I always figured if something's related to low fuel inventory, it would start and be somewhat consistent, but maybe an air bubble is all it takes.

Isn't it amazing how we took a simple concept like diesel technology, which had as its major point that it was really steady and reliable and didn't require electricity to run, and morphed it into a computer-driven and dependent beast, complete with warts and wrinkles?!?!!?

By the way, do you think folks would be interested in pix of my PMD, mounted on a cooler in front of the driver's-side battery, with its own cooling fan? Been working well, so far...:D

[Robyn]

You are refering to the PMD in front of the battery Or also refered to as FSD (Pump Mounted Driver or Fuel Solenoid Driver)

Please do post the piccy's, always good to have visuals on what others have done.

Yes I agree the whole idea of the diesel was simplicity. Leave it to a bunch of enviro freeks to worry them selves sick about things and drive the engineers to warp good technology into a tangled mass of quivering electronic modules. :eek:

Thanks

Robyn

[CapnAmerica]

Yep--PMD is what I meant to say. :o

It's too early; I haven't had enough coffee to get conscious; been sick with Vermont flu and bronchitis. (Best to roll out the excuses early...else it sounds like I'm simply igmoront.)

I'll get them shot sometime when I can walk more than a few feet...the location's a bit awkward to shoot photos.

[d.h.]

Robyn, With regards to air in tank. Mine does it anytime guage dips below 1/3 of a tank.not so much on level ground,but on slight hill or if I accelerate quick into traffic.Back when truck was "new" I took to dealer and they replaced sending unit in tank under warrenty as you could never trust guage.Replacing it didn,t really change anything.
As soon as mine gets just below 1/2 tank I usually fill to avoid this.I,m thinking its guage as when full guage goes past full and will stay there for 150 or so miles before moving.

[Robyn]

These trucks have a 34 gallon tank. The tank is long and fairly slender and this allows the fuel to all slosh to the back under a pull or while on a grade.

The Burbs usually have a 40 gallon tank that is more square.

It could be a baffle design problem too.

The tank is tha same from 92 to 2000 on the full length bed trucks.
Same tank gas or diesel just a different sender/pickup and on the gasser the pump is in the tank.

Mine also sees the gauge hard over past the full mark for some time too.

Easy running does not seem to produce the issue when the tank gets low.

The other day it took about 30 gallons or so to fill it so it was low.

The sloshing could well be causing the remaining fuel to foam up some which woukld then be sucked into the inlet and on to the IP.

It does not take a whole lot of air in one of these little creatures to make them unhappy.

The DB2 IP might get a tad soggy if the fuel gets air in it but the DS4 can get downright cranky.

Just seems that the best fix is to keep the fuel level up about 1/4 tank on my truck.
I still want to drop the tank and have a look see at the pickup sock and the bottom of the tank

The factory fuel filter was really fouled with mud/water and rusty crud when I replaced it with the Racor last summer. I can imagine that the tank may also have a fair amount of crap in it.

Truck has 180K now showing on the clock so that can translate into a lot of fuel having been run through it along with HOW MUCH CRUD from poor quality stations/storage tanks and filters

Later

Robyn

[arveetek]

I've been experiencing the same situation with my Tahoe lately. If I let the fuel level get below 1/4 tank, I'll have the engine bucking like you describe. In fact, twice now it's acted like it ran out of fuel. The first time I attributed it to cold temps and gelled fuel, but now I think it's actually sucking air at that fuel level. I try not to let it get below 1/4 tank anymore.

Casey

[chickenhunterbob]

Just a thought here...

The service manual indicates there is a check valve on the pick-up in the tank (I've never had it out to see, nor have I experienced any similar problems), but the check valve is said to be positioned such that if the sock is plugged (example in the manual is cold/waxed #2 diesel) that the last 4 gallons in the tank will not be used.

Perhaps if the sock is plugged for other reasons than cold the same would apply, i.e.: you are actually running out of fuel with 4 gallons remaining?

[Robyn]

Yup that is the little beast

After seeing all the fouling in the filter housing I suspect a nasty bunch of crap in the tank on mine.

I am planning on dropping the tank when the weather permits and cleaning things up or replacing the tank as well as the pickup/sender unit.

That little valve can over time go bad and allow air to enter the system in small enough amounts that it wont kill the thing but will make it run a bit erratic.

Mine will start losing it's "Edge" below 1/4 tank and then it will start bucking if I get into it hard.

Simple and easy to fix.

A new tank is not cheap from Ma General though. I hope mine is not too badly fouled.

The tanks are interchangable from a lot of the full sized (Bed) pickups, Gas or diesel (except the fuel pickup is different)

Once you know what this is about its easy to just keep the tank over 1/3 and get along.

I spent a fair amount of time searching and fooling around as well as replacing some stuff (probaby needed it anyway) to sort this out.

The day I thought I fixed the issue with a filter harness on the IP I had actually filled the tank with fuel before heading back to the ranch to work on it.
After that it ran fine. I proclaimed it fixed and did not get the tank below 1/4 until just a few days ago when it bucked some.

It was then that I filled the tank up and the issue stopped immediately.
BINGO I knew then that it was that in tank valve (or possibly a cracked suction tube)

The pickup safety valve is the most likely culprit.

When mine did it the other day it took 32 gallons to fill the tank. :eek:

So it was almost dry anyway.

Easy fix and we need to remember this little gremlin so that we can help others with the on again off again stumble issue. :)

Later

Robyn

[hayhauler69]

Good grief,
I hope I didn't shell out 700 bucks for nothing. Oh well, I needed injectors anyways, right?
My truck has low miles, but I think it sat a lot (95, with 116000).
So something else to put on the list, drop the tank.
Gotta get all this fixed because a year from now I :)RETIRE!!!!!!!!!!!!!!:):):):)Buzz

[Hubert]

Robyn thanks for the heads up. Yet another suspect in the drivability issues.

It would help if we can learn best how to target this particular issue. Its so hard to quantify and discern subtle differences and descriptions. Was there any RPM conincidences that you can note?

The toughest of all is how best to describe it? Is it a chugging miss or maybe lighter fishbite miss? Are they the same? Did fuelrate play any part? Do you monitor fuel pressure? I measure right at the IP inlet and have always wondered about the continuous 0.5 -1 and sometimes 2psi needle bounces and dips. I have read lots of posts on fishbite and precise fuel metering issues that affect smooth drivability. I have read where some fight it continuously and only solve it finally with new IP.

How does the system help purge air in fuel delivery system especially at the IP ? I think the sock in tank is suppose to stop air from entering the lines but does it always? The IP has internal some mechanism for fuel feed pressure regulation and housing pressure and returns excess fuel. How much air can it tolerate and recirculate?

I have read where feeding the beast mod and feeding the DS IP a little more fuel than GM specs helps with a little more even fuel metering per cylinder.

So food for thought is would rock steady pre IP fuel delivery pressure and or volume be the part of the problem? Where as a new IP can better deal with fluctuations and an older one can't?

[Robyn]

All the above

The fuel pressure is not going to stay rock steady. The demand for fuel during the course of driving will change the flow through the IP.
As the flow increases the pressure will drop some and as it decreases the lift pump will catch up and the pressure will rise.
The main thing is you dont want a negative number on the fuel pressure. A plugged (even Partially) filter can casue a negative pressure at the IP even with a good lift pump.

The transfer pump (Inside the IP) is not designed to pull fuel from the tank. If the lift pump fails the transfer pump will get worked very hard and this can cause a failure as well as some driveability issues.

The variation in the fuel pressure is in and of itself not going to cause a miss.

The condition I have experienced was from a couple missed beats to some serious stumbling after a hard romp to get into traffic. The issue being that a small amount of air bubbles will travel through the IP and then can end up in a couple plungers and will then cause those cylinders to missfire due to a lack of fuel pressure.
The air is self purging normally and will flow out within a few cycles and things will return to normal as long as the fuel flow is not continuing to see air being introduced.

It feels just like a gasser with a bad plug. The issue I have seen was indeed air related and as soon as the tank was filled it went away within a few moments.

Allowing the rig to "Float" with light throttle would see the issue resolve itself.

It all makes sense as the demand for fuel drops off and the suction on the system drops as does the intake of air due to the faulty valve or plugged sock.

Now if its other issues such as a bad optical sensor or ??? then there should be a DTC set if it happens often.

The function of the tank sock is to be sort of a "Clod" Sifter. This keeps any large particles from entering the fuel system and ruining the lift pump.
The fine mesh of the sock "May" help disburse any foaming of the fuel as the level drops and the fuel sloshes around during operation of the vehicle.

The safety valve is positioned above the sock to allow fuel to bypass the sock if there is a condition of severe plugging (Waxing) or ????????

The valve is placed high enough that hopefully any large amounts of contaminants will still be below the inlet. 1/4 to 1/3 level on the tank.

Corrosion of the valves spring over time due to condensate possibly can cause a valve failure ??????/
The issue with these is most likely poor fuel storage with the resultant contaminants being pumped into the tank at the point of fueling.

The sock is a Nylon mesh and can over time just plain plug up from the accumulation of crud.



Just some thoughts.

Robyn

[rhsub]

Had a similiar problem with the sub and it turned out to be a pin hole in the fuel line beside the tank allowing air into the system, they use salt on the roads here to stop all those idiots that don't know how to drive in the snow from killing each other:mad:. Only time it leaked fuel was when there was a big swing in the ambient temperature and the truck was not driven.
I don't know if they use salt where you live but I would check the lines
when you have the tank out
Good luck
Ron