Dieseltech please

[Fritz Maurer]

I have a '79 NTC 290 with air in the return fuel. How can I tell if it's coming from the pump or the injectors?

I put a clear plastic test hose between filter and pump. No air is entering the pump. Truck sat for a year. Ran fine when parked. Now has mushy vague acceleration and hunting idle.

 

[cjunrau]

I have a '79 NTC 290 with air in the return fuel. How can I tell if it's coming from the pump or the injectors?

I put a clear plastic test hose between filter and pump. No air is entering the pump. Truck sat for a year. Ran fine when parked. Now has mushy vague acceleration and hunting idle.

I may not be smart and what I do is not always right. I have sprayed wd40 or either all over the connections to the point of getting it wet and dripping and have found where something has been sucking in air.

 

[Fritz Maurer]

I may not be smart and what I do is not always right. I have sprayed wd40 or either all over the connections to the point of getting it wet and dripping and have found where something has been sucking in air.

I will try that , but what has me dumbfounded is I had a truck shop make new DOT fuel lines a year prior, and its the best its run in 20 years... especially the drain-back.... now this?

 

[used red MN]

I may not be smart and what I do is not always right. I have sprayed wd40 or either all over the connections to the point of getting it wet and dripping and have found where something has been sucking in air.

I don’t understand the principle or methodology behind this approach. Are you watching for the fluid or wetting on the outside to disappear or lessen as it is drawn into the leak?

 

[wore out]

I have a '79 NTC 290 with air in the return fuel. How can I tell if it's coming from the pump or the injectors?

I put a clear plastic test hose between filter and pump. No air is entering the pump. Truck sat for a year. Ran fine when parked. Now has mushy vague acceleration and hunting idle.

Have you "run the rack' on it?

 

[Dieseltech]

Does the PT pump have the aneroid control with a boost line to it? if so the pump might need the boost diaphragm replaced which requires pump removal. Last couple of PT pumps I had in for repair had the diaphragm blown.

 

[Fritz Maurer]

Have you "run the rack' on it?

If you mean adjust the injectors, no, not lately, and I will certainly try that, but yesterday is the first time it's run poorly in years.

 

[Fritz Maurer]

Does the PT pump have the aneroid control with a boost line to it? if so the pump might need the boost diaphragm replaced which requires pump removal. Last couple of PT pumps I had in for repair had the diaphragm blown.

Yes, it does have that.... would explain sudden symptoms

 

[MARK ROBKE]

Yes, it does have that.... would explain sudden symptoms

You can take a radiator pressure tester, remove the line to the diaphragm, and pump it up should hold 15-20 psi. If it is blown it won't hold anything. Mark

 

[Fritz Maurer]

Thank you all , I will try all of these things but I am wondering one thing: if the aneroid diaphragm is bad, would that not cut the power, since the pump is receiving no signal from the turbo to allow full fuel?

I hauled a load on her maiden voyage but noticed no decrease in power. It is, however, difficult to shift because it takes forever to unwind.

 

[cjunrau]

I don’t understand the principle or methodology behind this approach. Are you watching for the fluid or wetting on the outside to disappear or lessen as it is drawn into the leak?

If you spray starter fluid and it is sucking air it will run differently. If it misses because of air being sucked in it will run smooth when it gets the starter fluid sucked in. I have used it on gas engines to see if it had vacuum leaks.

 

[m16ty]

If you spray starter fluid and it is sucking air it will run differently. If it misses because of air being sucked in it will run smooth when it gets the starter fluid sucked in. I have used it on gas engines to see if it had vacuum leaks.

Yes, I’ve done either on gas engines to find vacuum leaks, but I don’t think it would work as well on a diesel.

 

[MARK ROBKE]

Thank you all , I will try all of these things but I am wondering one thing: if the aneroid diaphragm is bad, would that not cut the power, since the pump is receiving no signal from the turbo to allow full fuel?

I hauled a load on her maiden voyage but noticed no decrease in power. It is, however, difficult to shift because it takes forever to unwind.

If the diaphragm is bad, then yes -no power. It has been a LONG time since I worked on these engines, and I actually don't remember a blown diaphragm allowing air into the fuel. It seems that the suction/supply side lines/fittings/ to the pump were the problem, which I don't remember being a common problem, just occasional.
I do remember one truck that only sucked air when it was in hard bind turning. It was toughie to find, but it was a short line near the fuel separator. Sucking air can be diagnosed by making temporary lines sometimes- eliminating a section at a time. Good luck, Mark.

 

[used red MN]

If you spray starter fluid and it is sucking air it will run differently. If it misses because of air being sucked in it will run smooth when it gets the starter fluid sucked in. I have used it on gas engines to see if it had vacuum leaks.

Okay, you don’t really have any new revelations. Although I think he has been steered to something else as a greater possibility of being his problem, this is quite a bit different than looking for a vacuum leak. This would be looking for a very small and probably slow leak of air going in to a fuel line. Far different from a manifold vacuum leak on a spark ignition engine. I was really hoping you had something going that I had not heard of or considered.

 

[Fritz Maurer]

Found the problem. Cracked nozzle retainer nut ... blowing compression into the fuel.

I plumbed in a glass in-line filter encased in steel, so I could observe the fuel coming out of the pump. There was no air going into the head but sure was coming out.

 

[MARK ROBKE]

Found the problem. Cracked nozzle retainer nut ... blowing compression into the fuel.

I plumbed in a glass in-line filter encased in steel, so I could observe the fuel coming out of the pump. There was no air going into the head but sure was coming out.

Cummins 290's had injectors, not nozzles and were held into the injector cup with a 2 bolt hold down, if I recall correctly. The fuel was delivered thru passages in the head, and they had no nozzle retainer nut. Perhaps I misread what engine you had? Mark.

 

[wore out]

Cummins 290's had injectors, not nozzles and were held into the injector cup with a 2 bolt hold down, if I recall correctly. The fuel was delivered thru passages in the head, and they had no nozzle retainer nut. Perhaps I misread what engine you had? Mark.

Dunno where you've been, NOT in the old Cummins engines world, apparently?

Those Cummins unit injectors DID have a "tip" held in place by a sort of nut.

If the "rack" was not set up properly the injector plungers slammed harder than they needed to into the top of the nozzles which could cause nozzle or nut breakage. Or, sometimes they'd break for no apparent reason, just out of spite!

 

[MARK ROBKE]

Dunno where you've been, NOT in the old Cummins engines world, apparently?

Those Cummins unit injectors DID have a "tip" held in place by a sort of nut.

If the "rack" was not set up properly the injector plungers slammed harder than they needed to into the top of the nozzles which could cause nozzle or nut breakage. Or, sometimes they'd break for no apparent reason, just out of spite!

I was in the old Cummins world- between 30 and 50 years ago. We did call them unit injectors, and never referred to them as nozzles. The tip was referred to as the 'tip'. So, I guess it's just a terminology thing. You are correct-if the injector was not set correctly, it could over travel the plunger and cause damage to the tip and the cup. Mark.

 

[farmer656]

I have a '79 NTC 290 with air in the return fuel. How can I tell if it's coming from the pump or the injectors?

I put a clear plastic test hose between filter and pump. No air is entering the pump. Truck sat for a year. Ran fine when parked. Now has mushy vague acceleration and hunting idle.

sounds like it is starving for fuel, that was a sure sign of running out of fuel before engine died, you do not state how old truck is, aged trucks have a bad habit of any rubber hoses on suction side will collapse on the inside and you cannot see it my suggestion would be top run a temporary hose from tank to fuel filter and see if it does not run better , I have found this on many trucks over the years and the older R model mack where really bad about this

 

[farmer656]

I have a '79 NTC 290 with air in the return fuel. How can I tell if it's coming from the pump or the injectors?

I put a clear plastic test hose between filter and pump. No air is entering the pump. Truck sat for a year. Ran fine when parked. Now has mushy vague acceleration and hunting idle.

sounds like it is starving for fuel, that was a sure sign of running out of fuel before engine died, you do not state how old truck is, aged trucks have a bad habit of any rubber hoses on suction side will collapse on the inside and you cannot see it my suggestion would be top run a temporary hose from tank to fuel filter and see if it does not run better , I have found this on many trucks over the years and the older R model mack where really bad about this