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Water Dragon · 04-04-2021, 20:40

I'm working on a CAV DPA injection pump on a 3-cylinder Perkins diesel. The engine was running fine, but the IP had a pretty bad external fuel leak. I replaced both the bleed screw assemblies and copper washers and torqued them to spec. That helped a lot but did not entirely stop the leak, which at that point appeared to be coming from where the hydraulic head joins to the main pump body. I removed the IP and disassembled to replace all the seals and o-rings.

I should point out that I'm not an amateur; I'm an airplane mechanic by training and accustomed to doing things just so. However, I've been feeling very tired recently, and I should have known better than to work on this on Sunday instead of taking a break; and it's entirely possible I did something stupid during reassembly. So...

After it was all together I put it back on the engine (a pain) and got it running... sort of. At max throttle it will only get to 1300 rpm with no load. With it running at 1000, I loosened the injector lines one at a time. When I did this on cylinder 2 and then 3, the RPM dropped slightly and the engine ran rougher. When I did it on cylinder 1, the engine suddenly conked out.

Keeping in mind that I'm using more throttle than usual to get to 1000 RPM, this suggests that cylinders 2 and 3 are getting barely any fuel while cylinder 1 is getting full fuel and almost single-handedly keeping the engine going.

Before I take the pump back off to look for anything that might be put together wrong, can anyone point to something specific that might cause uneven fuel delivery? Is it possible that there's just air trapped in it somewhere and running it for a while will eventually work it out?

**skipperpete** · 04-04-2021, 22:45

Its not uncommon for Dpa pumps to return confusing results when injector lines are opened at idle. As you know there is only the one cylinder with 2 plungers that provides high pressure fuel and the delivery volume at idle is miniscule so any disruption to the injection cycle will affect the “refill” for next shot.
If I understand you correctly the pump had a leak on the hydraulic head, not on one of the attachments ( other than the bleed screws) so you completely stripped the pump ?
Or did you just remove the hydraulic governor housing and do the seals on that. ?
If it has a mechanical governor and not the hydraulic one the the fix is easier. Most likely the large spring became detached during the housing removal and the original attach point positions on the governor plate and speed control lever were accidentally lost and the spring then re attached to the wrong tension hole..... this happens a lot and the code on the pump body is the key to finding the correct tension attach hole. The low rpm symptom and speed control angle is a clue.... but only on the mech governor.
I prefer to crack injector lines to review cylinder balance with a little bit of load on.... and sometimes at full load but great care is needed.
Pete.

Water Dragon · 05-04-2021, 08:51

I removed the top cover and mechanical governor/spring assembly; this was necessary to pull the hydraulic head out. I'm aware of the code and the different holes for the spring, and put it back in where it's supposed to go.

While I had the end plate off I noticed the small cylindrical screen was a bit dirty so I disassembled it and put in a new screen. The end plate should come off easy without removing the whole IP, so I'm going to double check that everything is correct in it.

I did not remove the governor fly weight assembly or the big snap ring inside the bore. The only other thing I messed with is the automatic advance device on the bottom of the IP. One of the bolts also serves as a locking bolt for the hydraulic head, and it's not just a bolt; it has O rings and springs inside it. I can't find a diagram exactly like it in any of the manuals I've found.

Water Dragon · 05-04-2021, 11:38

I think I've figured it out. There's a metal sleeve in the end plate which should have a small piston inside it, and the piston is not there. This is curious since I didn't take the sleeve out in the first place. The piston must have fallen out unnoticed when I replaced the screen. Where it went, who can say.