Oil & Fuel bleeding problems on perkins diesel - air at fuel injector - cylinder not firing

[Bazzaclark]

A good news story! The Perkins AD3.152 on my MF40 was running after a rebuild, but sounded like only 2 out of 3 cylinders were firing. No smoke, but no power - I thought this was probably just air on the system and set about bleeding all air from the diesel lines. After 15 mins of bleeding, I still had lots of tiny bubbles at the CAV DPA injection pump outlet union for cylinder 1, as well as at the injector pipe union itself. After an hour of internet trawling, I decided to swap the no1 and no2 injectors to make sure the problem was air from the injection pump, or at least from the no1 pump lines.

Wonder of wonders, the problem moved to cylinder no2. I broke down the 'duff' injector and apart from cleaning it, did little else - I did 'tighten' the spring tension inside the top nut a little just by 'feel' - but it was all trial and error as I had no pressure-test gear. After re-assembly and with hardly any bleeding whatsoever, it started beautifully on all 3 cylinders. No smoke, full speed range.
With air bubbles down at the pump, as well as at the injector, I would never have thought the problem was at the injector but it must have let HP combustion gas leak-back into the fuel feed line, giving the appearance of air in the fuel. Weird.
I guess this could apply to lots of different engines not just the Perkins - hope it helps someone else

 

[Roadworthy]

Thank you for sharing that tidbit of information. It may come in handy someday, knowing an injector can allow pressure back from the cylinder sometimes.

 

[Bazzaclark]

A little update - Cleaning the injector was a 'quick-fix' whilst I waited for the arrival of 3 x new chinese injector nozzles (around 19 USD each). Fitted them just now and hey-presto runs beutifully. I did not have injector pressure test gear for the opening setting....but I guessed and used a setting of 1-1/2 turns of the spring pressure, taken from the point where the spring and valve assembly are just 'mated' together. Looking forward to sunshine tomorrow to be able to try it in the woods. Yay.

 

[Bavarian]

I'm not sure about the Perkins injection pump, but the Bosch has check valves installed on the pump side of the fuel line. This keeps the line and injector presurized and might keep it from pumping air backwards.
Those check valves are meant to hold pressure in the lines to save time for the next injection.

 

[Bazzaclark]

I'm not sure about the Perkins injection pump, but the Bosch has check valves installed on the pump side of the fuel line. This keeps the line and injector presurized and might keep it from pumping air backwards.
Those check valves are meant to hold pressure in the lines to save time for the next injection.

Hi Bavarian: Although I never disassembled the HP section of the pump, I don't think there is such an additional NRV on the CAV DPA. Hope you can see this cutaway - red is fuel outlet to injectors, and seems to come straight from the plunger piston port ... although the internals probably serve as a check valve.
The new chinese injector nozzles seem to be working well and so I am pretty sure that was the problem all along

 

[Bavarian]

Since your problem moved over to the next cylinder, yes it sure was. I don't know if pressure in the line helps to keep the injector closed.

I've marked one of those check valves in the picture below. I think the Perkins has something similar attached to the ports. IIRC.

 

[Bazzaclark]

Hi Bavarian; Nope. There was certainly nothing like that on mine when it arrived here - and it looked like it had never been touched. The outlet of one of the ports is here. I worked in Switzerland with a German team for 10 years - and there is no doubt the German engineering (Bosch) is often boots-and-braces : nothing at all wrong with that!!

 

[thepumpguysc]

There is no check valve in the CAV DPA inj. pump..
it relies on the lift/supply pump if equipped & the tightness of the lines to keep fuel pressure in the lines..
On some of the lines on the DPA fuel system, have rubber "Olives" that seal the lines..
they get old & hard & allow air to ingress into the system..
& its VERY COMMON to suck air w/ no external fuel leaks..

I had the opportunity to diagnose an air ingress problem on a Cummins 6BT engine w/ a BOSCH VE fuel system..
I found the injectors were stuck in the open position due to sitting & when it fired for the 1st time, they stuck open..
I have a pop tester & confirmed the diagnosis..