Killing while idling.

[theanimal]

It looks like my truck is dropping about 400 rpm when I put it in gear at idle and kills. I idled it up to around 900 and it drops all the way to 500 when I put it in gear. Never used to do this. Does anyone know what I should check?

 

[98G]

Which truck? Which engine?

 

[theanimal]

M923a2. 6cta cummins

 

[98G]

Possibly air in the lines. The fitting at the top of the lift pump is a prime culprit.

Can you describe symptoms in more detail? Is it driveable?

 

[fuzzytoaster]

Sounds exactly like air is getting in the fuel system. Trace your hoses and let the truck sit a week and check if it looses fuel prime.

 

[theanimal]

It drives but is erratic. Honestly doesn’t have the power it used to. As long as its in a low gear and winding up its fine but doesn’t have any power once it shifts. It’s been sitting so I changed the lift pump and pulled the tank and cleaned it out. Not really any difference. Going to get a fuel filter now

 

[74M35A2]

Check fuel tank dip tube and its solder joint on the underside. Replace the fuel lines, especially the suction side. Make sure the frame mounted fuel filter (if you have one) is sealed up tight and the square washer is fully seated squarely into the housing, rubber side down, drain valve is fully closed, etc....

Could also be the over-flow valve stuck open on the outlet banjo fitting of the injection pump. You can clamp this hose shut for 60 seconds and release as a field cleaning attempt. Do this several times. This valve should "squeak" when the primer button is pushed if working correctly.

 

[theanimal]

So let me try and give some more info on this issue.
The truck sat for a very long time and the lift pump gummed all up. I changed the lift pump and the truck started. The fuel was old and bad and the truck ran terrible. You give it the gas and it would barely go. All of a sudden it started running better but still not the same power it used to have.
We pulled the tank and there was about 1/2” of sludge in the bottom so we cleaned it completely. The fuel pickup tube seems to be fine and it’s clear of any blockages.
Put all of it back together and filled it with new diesel. Truck fired right off and I took it for a drive. All the way through first gear it pulled good but as soon as it shifts out it falls flat (sometime). The truck will go down the road with the pedal to the floor at 40mph and barely gain any speed. Once the rpm’s get over about 1700 it is like someone turned the motor back in and it pulls up to about 60 then no more.
So I got a new fuel filter and replaced it. The truck was very hard to get started but it finally cranked. Now though it would act even worse on the road not having power at all until the rom’s get up close to 2k. I drove the truck back home and it killed in my driveway. It took about 5 min to start and I moved it in the shop. Now it won’t crank at all.
I can tell you that you can pump the primer and hear the squeaking at the pump. Also when you crank it over for 20 or 30 seconds then turn the key off it sounds like fuel running through all the lines for about 5 seconds.

 

[fuzzytoaster]

So I got a new fuel filter and replaced it. The truck was very hard to get started but it finally cranked. Now though it would act even worse on the road not having power at all until the rom’s get up close to 2k. I drove the truck back home and it killed in my driveway. It took about 5 min to start and I moved it in the shop. Now it won’t crank at all.
I can tell you that you can pump the primer and hear the squeaking at the pump. Also when you crank it over for 20 or 30 seconds then turn the key off it sounds like fuel running through all the lines for about 5 seconds.

You're loosing fuel prime/getting air in the system .You say fuel filter but which one? The smaller one on the block or large canister style filter next to the frame? The larger one is your primary and can be tricky to seal. Check the large o-ring on the canister lip to make sure it's sealing properly and on top where the bolt is make sure the square spacer is sitting in the groove with a good o-ring. If any of that isn't perfect you will loose prime as that's the lowest part in the system.

 

[Jbulach]

Does your return line in the tank still have the rubber dip hose on it, and did you put a new or used lift pump on?

 

[theanimal]

I only have one fuel filter and it’s up in the engine. It’s a Napa 3472 filter. Other than that I only have about a foot of rubber hose from the lift pump into a hard line that runs all the way back to the tank and about a foot and a half of rubber hose from the injection pump into a hard line that also runs back to the tank.
I have no idea what the square spaced or washers are that you are talking about. My filter just screws on. The lift pump was a new Cummins pump but I just pulled it apart and the screen was clogged already. I put it back together and went for a drive. It was crazy hard to get it started and the truck acted no different than before. It seems like if I gas it from a stop then it pulls pretty good all the way up to about 50. If anywhere in that increase of speed, I let off the pedal then push it back down its like it’s almost not even connected to anything. The engine has absolutely no power and only increases speed slowly until it hits 1500 rpm. Then it starts pulling again.
The hose on the return inside the tank is still there but is in bad shape. Why would that hose affect anything?

 

[Jbulach]

I only have one fuel filter and it’s up in the engine. It’s a Napa 3472 filter. Other than that I only have about a foot of rubber hose from the lift pump into a hard line that runs all the way back to the tank and about a foot and a half of rubber hose from the injection pump into a hard line that also runs back to the tank.
I have no idea what the square spaced or washers are that you are talking about. My filter just screws on. The lift pump was a new Cummins pump but I just pulled it apart and the screen was clogged already. I put it back together and went for a drive. It was crazy hard to get it started and the truck acted no different than before. It seems like if I gas it from a stop then it pulls pretty good all the way up to about 50. If anywhere in that increase of speed, I let off the pedal then push it back down its like it’s almost not even connected to anything. The engine has absolutely no power and only increases speed slowly until it hits 1500 rpm. Then it starts pulling again.
The hose on the return inside the tank is still there but is in bad shape. Why would that hose affect anything?

You can actually run without it, but if your overflow valve or checks in the lift pump are starting to fail it can help prevent loosing prime. Did you try pinching the return line off to see if your still loosing prime? I’m leaning toward the overflow valve, but you need to find out why you’re still getting junk in your lift pump if you just cleaned the tank

Not all the trucks had the frame mount filter on them, so you can disregard the square washer check. But I think the original rigid fuel lines where copper and prone to leak/suck air with age as well as the rubber.

 

[theanimal]

Does anyone know the part number for the overflow valve? I will go buy one tomorrow from the Cummins dealer here in town to try it out.

 

[theanimal]

Went ahead and put a new lift pump just to eliminate it from the equation. Haven’t been able to start it since. Primer is pushing fuel I can hear squeaking at injector pump. Tried clamping return line but still no start. Going to get a can of diesel now to put next to the lift pump and bypass all the lines to the tank. Will see what happens.

 

[theanimal]

Ok short hose to lift pump fuel pumping to injection pump but no start. Running out of ideas here.
Bad injection pump?

 

[Jbulach]

Tie the fuel shut off back, hold throttle wide open and try. Did you end up replacing the overflow valve also?

 

[theanimal]

Done the fuel shutoff already no change.
I pinched the line after the overflow and no change. I pulled the overflow out and it looks fine but there is rust inside the hole.
How do I check the pump to see if that’s the problem.

 

[74M35A2]

Have the soft lines to the injection pump every been replaced? Nearly all suck air by this age, yet they do not show it from the outside at all. I don't remember seeing it mentioned in the thread. I know it is easy to sit here at our desks and tell you what to do, but these are some pretty experienced guys that have all went through the same stuff.

In the end, if you are able to put a clear piece of tube on the return line and route it up or restrict the end of it, you'll be able to see if there is air in the system.

 

[theanimal]

Soft hose on supply side is new.
Going to get clear hose now

 

[Jbulach]

I pulled the overflow out and it looks fine but there is rust inside the hole...

That doesn’t sound good.

Maybe pump the primer with the overflow out, and see if a bunch of junk comes out the galley. If so you’re probably out of my league, hopefully Will Wagner will chime in before you pull the IP. Did you try cracking the injector lines to see if you where getting fuel out?