I think there is something wrong with my 923A1 fuel pump....

[Hooper]

I was combing through the TM's when I found the touble shooting catergory that says surging idle. Which is what I have. All it says is that if the it can not be adjusted with a cable adjustment to contact DS maintenance. I was in the middle of adjusting the kick down cable when I disconected the throttle cable. This truck has always had a hunting idle that I figured was dirty injectors. With the throttle cable disconected the idle was all over the place. When the throttle is increased by hand and then quickly moved down, the idle stays up for 1-2 seconds by itself. Any ideas?

 

[Hooper]

Meet a neighbor who saw me working on the truck, turns out he is a diesel truck mechanic for a local truck shop. He is sure the pump is bad. I had a feeling that may be the case. Found one on ebay for 500 junior bacon cheeseburgers.

 

[M35A2-AZ]

My M923A1 had a problem when I got it of surging idle and it turned out to be an air leak in the fuel tank pickup tube.
I have seen other guys with the same problem and it has been an air leak in a fuel line or the fuel filter.

Here is the link,
http://www.steelsoldiers.com/showthread.php?79327-M923A1-losing-prime&highlight=

 

[jedawson1]

Same surging issue here, mine will hold good idle for a few seconds after driving just like you mention. Also, my truck is loosing prime when it sits for an hour or longer. The plunger barely draws fuel too. Got to find the air leak. Pump on my truck looks brand new so I don't think that's the problem. Yet.

 

[Hooper]

My looks new too, replaced in 2010. I guess I need to look for an air leak also.

 

[jedawson1]

From a cold start does your plunger prime right up or can you pump it in an out repeatedly? My other truck does not have the surge issue and it can sit for weeks and the plunger takes 1/2 stroke and the fuel line is full. I say all this to show the difference between my truck with the surge problem and my truck without. Could be a clue.

Good luck and post what it takes to fix.

 

[Hooper]

My truck does run worse when it is lower on fuel. hmmmm

 

[M35A2-AZ]

Sounds like a air leak to me. Try to pressurize the fuel tank to about 4-5psi.
If you have the vent that runs up the intake on the drivers side of the cab that is a good place to
tap in to do it.

 

[73m819]

My truck does run worse when it is lower on fuel. hmmmm

Pull the pickup tube from the tank, LOOK for a rust line (this is where the fuel level was for a LONNNNNNNNNG time) scrap the rust line LOOK for a pin hole, this hole is sucking a tiny bit of air which in turn is causing the "run worse when it is lower on fuel"

Anybody with steel in tank pickup tubes SHOULD pull the tubes and inspect them, especially if about the same level of fuel stays in the tank after use, fueling, ect.
On a tank that stays full look for the rust at the full level line, REMEMBER some of these trucks sat for as long as two years.

I learned this little tidbit on knowledge from a OLD CAT. mechanic who told me it is cheaper to do a in house inspection/repair then call CAT. out to diagnose/pull/replace the pick up tube or replace the ip because it ran out of fuel one to many times (some of the older ips were fuel lubed). If fact the Cummins ip IS fuel lubed.

Hope this helps

 

[Hooper]

My looks new too, replaced in 2010. I guess I need to look for an air leak also.

Tuesday is fuel tank removal day, I suspect I will find something wrong. Will report back. Thanks to all for the advice.

 

[Hooper]

Dang, not the fuel tank pickup tube. It was tight. Took the fuel filter off, there was a clear gell around of the bottom. Probably not the main issue but will replace it anyway. Oh yeah, before I started the truck I primed it. It took 20 pumps before fuel came out.

 

[jedawson1]

Dang is right, was hoping it was that simple for you. When your plunger picks up fuel, does it get too stiff to pump after the 20 or so pumps? My truck with the fuel problem will just keep pumping, always has air mixed with the fuel.

 

[Hooper]

Dang is right, was hoping it was that simple for you. When your plunger picks up fuel, does it get too stiff to pump after the 20 or so pumps? My truck with the fuel problem will just keep pumping, always has air mixed with the fuel.

Yes it does get hard. In fact I did a little test. Without doing nothing upon first start up the rpms would race for 2 or 3 seconds. A Ford diesel tech told me that may be a sign that it has an air leak. Tonight before I took the tank out, I primed it the 20 pumps until it got hard then started the truck. It did not race but ran smooth for 30 seconds and then back to the eratic idle. Will be back at it this weekend.

 

[Hooper]

This morning I began to replace the fuel filter. I had it apart so I replaced it. Primed the system and like expected, it idled all over the place. Crap. Next plan is to go over all the fittings. Feels like a long shot at this point. My first fitting is a brass one at primer pump. Loose, finger loose!. That probably doesn't matter since its at the primer......go over all the rest and find no other loose fittings. Prime the system, start truck.........FIXED! No more hunting idle and the throttle is now snappy. RPM's come right back down when you let off the pedal. Oh boy, I am one happy camper.

 

[jedawson1]

Glad to hear you found it. That leaves promise for me finding my idle problem. Thanks for posting the solution, it will come in handy for those who use the search feature.

 

[jedawson1]

I meant to update on here the other day, I tightened the fittings associated with my plunger assembly and it fixed the problem. I also tweaked my fuel solenoid 1/4 to 1/2 turn as the motor wasn't wanting to shut down.

 

[Hooper]

I meant to update on here the other day, I tightened the fittings associated with my plunger assembly and it fixed the problem. I also tweaked my fuel solenoid 1/4 to 1/2 turn as the motor wasn't wanting to shut down.

Ha! that's a good feeling. Glad to hear you found yours too.

 

[ichudov]

On one of my trucks, I have a similar issue. After startup, the idle is "hunting", goes from 450, to 600, 700, 500, etc. Is that the same as what you guys are experiencing, or did I misunderstand? thanks

 

[ichudov]

I meant to update on here the other day, I tightened the fittings associated with my plunger assembly and it fixed the problem. I also tweaked my fuel solenoid 1/4 to 1/2 turn as the motor wasn't wanting to shut down.

What are those fittings, where are they?

 

[Hooper]

On one of my trucks, I have a similar issue. After startup, the idle is "hunting", goes from 450, to 600, 700, 500, etc. Is that the same as what you guys are experiencing, or did I misunderstand? thanks

Yes, mine would hunt at an idle but ran fine down the road. Mine would race right after start up and then hunt. It's the two fittings for the fuel primer. It should a short plastic line that goes from the fuel primer to the fuel pump. Mine was loose on the primer side. If you have a good running truck to look at, double check that the valve on the primer is closed in the right direction. Meaning it should be closed when it is threaded all the way down not up.