problem air in fuel system

[dmetalmiki]

I have an RPM problem with the 5 ton M54a2 running all over the place..sometimes ok sometimes stops sometimes runs lumpy and then usually stalls out.
we changed the in tank pump to an all singing and dancing aircraft type replacement. changed all the filters, and the hydraulic head. Then the hi pressure inlet pump on the side of the injection pump. Still no go.
When we bleed at the filters it runs fine..for a while, then "here comes lumpy" again!.
I then changed the rubber inlet pipes...no different.
A little bleed, and fine for a while.
Frankly I am beat, so any advices appreciated.

 

[red]

It is sucking air so if the fuel pressures are good then it's a cheap (but pain in the ass to locate) problem. Check the seals at all fuel line fittings, including the filter canisters (maybe one didn't seal right when last replaced). Pinhole leaks in the fuel pickup tube would be the next place I'd check, followed by all the hard lines. The hard lines are at least 50 years old now.

 

[74M35A2]

I had this exact same situation, found it to be one of the brand new replacement hoses was sucking air at the crimp. New doesn’t mean good. New just means clean.

 

[Floridianson]

Would seem that if air is a problem then it would have to be between the in tank pickup and the booster pump. How was the primary filter gasket on the bolt that holds that canister to the base?

 

[dmetalmiki]

Thanks all for the suggestions. One point of note. The fuel at the first filter bleed (on the left as looked at the engine) chucks fuel out when opened...like.....frothy coffee!.
the plastic pipe from the hydraulic head that loops round and back under the steering column (to the supply) did loop,..it was kinked shut where it couldn't be seen!. the main supply flexible joint from the tank swivels easily even when bolted to the up tight. OK we changed both those.
It still takes for ever to start, but not if bled first.
Now it runs without stopping..but it is still threatening to misbehave..with just a hint now of erratic tick over.
Just the 'coffee mix' to cure?

 

[74M35A2]

Should not be any froth. This means air. A good bleed should only be pure liquid only. Keep working backward until you have fluid only when bled.

 

[dmetalmiki]

Thank you for that, I am replacing more pipes as in the ones associated with the fuel density compensator. Never could fathom out which what all those pipes do..they just seem to go round in circles. I have had a long held belief that the air in the system is, perhaps, coming via that fuel density compensator. Looking at forums I do not see advices to confirm 'known issues' regarding air ingress with those units though. Advices?. I have others I can swap in.

 

[red]

No reason to keep the FDC, just bypass it. Run the fuel supply hose from the final fuel filter directly to the inlet on the hydraulic head. It's not a needed part for the fuel system.


Frothy fuel coming out at the primary filter drain means it still has an air leak between the tank-primary filter. Good posibility that the in tank fuel pump is either partway blocked, the hose in the tank is failing, or the fuel line fitting at the top of the tank isn't sealing.

 

[dmetalmiki]

Would seem that if air is a problem then it would have to be between the in tank pickup and the booster pump. How was the primary filter gasket on the bolt that holds that canister to the base?

I must be dozy..where is that 'bolt' on the 'primary filter' ..primary? filter is that the one on the side of the 5 ton chassis? I am checking every and all suggestion

 

[dmetalmiki]

No reason to keep the FDC, just bypass it. Run the fuel supply hose from the final fuel filter directly to the inlet on the hydraulic head. It's not a needed part for the fuel system.


Frothy fuel coming out at the primary filter drain means it still has an air leak between the tank-primary filter. Good posibility that the in tank fuel pump is either partway blocked, the hose in the tank is failing, or the fuel line fitting at the top of the tank isn't sealing.

Thank you for the advices..but if the fuel line fitting was leaking, would not fluid under pressure be seen to be leaking under pressure there? The in tank pump and hoses are new "aircraft" types.

The line from the top of the tank is solid copper, and if there was a hole in it, then surely the same would apply, fluid stain or leaking under pressure?.
Again, I am just trying to cover all bases. (and) slowly working through this frustrating problem. (With all the advices).

 

[red]

Thank you for the advices..but if the fuel line fitting was leaking, would not fluid under pressure be seen to be leaking under pressure there? The in tank pump and hoses are new "aircraft" types.

The line from the top of the tank is solid copper, and if there was a hole in it, then surely the same would apply, fluid stain or leaking under pressure?.
Again, I am just trying to cover all bases. (and) slowly working through this frustrating problem. (With all the advices).

Air is smaller than fuel. When lines begin to leak it's air intrusion first, no fuel escaping. Happens much more often with soft hoses but after 50+ years copper lines will do it as well.

The primary fuel filter is along the chassis and the bolt in question is the long one that secures the canister to the mount. But if the fuel is frothy like that by the time it reaches the primary filter, then the issue is further back towards the tank. Being either the copper lines, fitting, or the in tank components as mentioned.

 

[dmetalmiki]

Thank you. I will report back after I further work into this problem later today (It already being 12-20 am) I will be checking all the points further suggested.

 

[dmetalmiki]

Changed and checked all the lines. Bypassed the hydraulic head..bled it..And it ran stronger than before. For the same duration, Started hunting and running lumpy..Then stopped..
I'm beat!
Bled it. Same again!.
I have ordered a Cummins 280 H.P. Turbo C series (non electronic) Engine.
Over the (many) years I have solved all our problems and carried out many fixes..Not this time, and our motto We are the Kiddies We have the techno, We can't be beat has not stood up.
So, I remain, Baffled And not a little disappointed. (And in (great back pain)).

 

[red]

Did you open the fuel tank to check the pump?

What is the fuel pressure coming out of the in tank pump? When the engine is first started up, and when it begins to stumble/die.

 

[Scrounger]

There obviously must be an air leak somewhere. The best way, however time consuming, to find an air leak is with clear line. It involves clear fuel line and appropriate fittings. Start at the injection pump and work back. Install the clear line between the pump and the fuel line going into the pump. Run the engine and look for air bubbles in the clear line. If you see them go to the next connection. Keep doing that till you see clear fuel. Once you see clear fuel with no air bubbles you will have found the leak.

 

[dmetalmiki]

Ok. Thank you all for further advices again.
Today Despite the pain in my back, I, Bypassed the fuel density compensator, Bypassed the Manifold pre heater. (The in tank is a new Higher pressure aircraft type).
The truck started and idled the best ever and really strong. Revs better than it ever did,
40 seconds later here comes 'lumpy' again followed by stop.
Tomorrow I follow the 'clear line' checking "orders".
I will report.

 

[dmetalmiki]

Parade! Line up. 'Ten Shun. Thank you all who advised me (in this thread saga) on the dreaded 'lumpy' running problem.
Now I think about it, this problem has plagued that truck for over two years.
Striding 'down the line' (of members), I award each and every one of you, This months Persistent, CORRECT advice ((Medal)) on our problem.
Air gents...flippin' air. (bubbles). >the messages FINALLY got home<.
I spent hours tying to access all the fuel lines, I cut most from inaccessible places with metal shears!.
I have replaced ALL the lines with high quality rubber fuel line.
I manufactured all connections to be single push on twin jubilee clamped fittings.
This results in ONE unbroken line Without any couplings from tank to filter - filter cans. And injector pump.
The truck fired up and runs solid. I mean 'solid'..just like the M45 I fixed a while before:
the tickover is super regular and STRONG.
The test drive was equally impressive. And the vehicle ran cleanly and strongly right up to 2600 RPM (that was enough, It would have gone to the maximum 2800 RPM with ease. But I don't require the engine to attain that figure. As regular driving sees 2000RPM. Most all our journeys.
Again, thank you to the members that got us out of this 'obvious?' hole. Guess We (I) could not see the wood for the trees!.
I now (can) look forward to attending this years events.

 

[dmetalmiki]

The fix included all return lines as well.
I am going to re-fit first, The manifold heat start unit.
I will then re-fit the fuel density compensator.
I also have thank young Brad, (the cadet 'helper') for actually re fitting the piping as my 'pains' lately have rendered me almost immobile. No help..No truck!. No events.

 

[red]

Glad it's been fixed.

Personally I wouldn't bother hooking the FDC back up, all it does is try to keep the power level the same regardless of what fuel is being used. They also have a bad habit of dumping fuel into the engine oil when they have a leak. By leaving it bypassed the pump is more reliable and the engine is making a bit more power, nice pair of upgrades.

 

[dmetalmiki]

Thank you, Outvoted again..OK the fuel density compensator stays out of circuit. I will however fit the manifold heater unit back in as 5 tonne's are loath to start in the cold without it, and sometimes not in the warm either. (Not engine warm..the climate from a 'coldish start).