CAV injection pump question

[Harry in Ky]

This question is aimed towards those who rebuild pumps, or those who know and understand the inner workings of them. I guess I understand the basics, but certainly not everything. The DPA pumps used on many of the Ford tractors from the seventies through the early nineties had a side cover with a fitting/return(?) line leading to a port on the filter base. Also a return line on the top cover that returns fuel to the tank via the injector return line. Non of which used any sort of restrictor orifice to affect case pressure. Why was that necessarily (if indeed it WAS) better than the more common return fitting mounted on top of the main case mounting flange as used by most other engine manufacturers like Perkins, John Deere, Fiat, Kubota, ETC? Was that just another one of "Ford's better ideas"?

 

[Sean in PA]

I don't know the answers to your questions, but I do know that the excess fuel return line needs to be unrestricted because if there is any restriction in that line, then the pump won't build enough pressure at the injector lines to fire the injectors properly.

 

[Harry in Ky]

Seems to me that's the case with pretty much any injection pump, although the ones that come to mind first are the CAV and Roosa Master distributor types.

 

[Dieseltech]

Most CAV DPA pumps use a feed pump, but NOT FORD models, they are gravity. The two bolt return side cover DOES have a pressurizing valve in it, and the top cover is a constant air bleed fitting or banjo orifice on some models. MF/Perkins place the constant bleed orifice on top of the final filter base usually so any trapped air will not ever get into the injection pump inlet. When that orifice DOES plug then any trapped air WILL get into the injection pump and cause it to shut down, until the air is bled out again. Problem will stop soon as the orifice is cleaned. Also, the feed pump MUST be in good shape or that will cause trouble too as air can come in BACKWARD through that orifice too when the feed pump does not keep the system pressurized.

 

[Harry in Ky]

So the pressurizing valve is a form of relief valve for case pressure? Having worked on a number of Fords with this package I've never removed one of those covers for inspection. I thought it was just an open hole. That doesn't make much sense if the other port on the top cover is a direct path to tank anyway, seems case pressure wouldn't rise to the point of needing a second relief path.

The reason I'm wondering about the system in general is because I've been working on a 3600 with "pump problems" for a few weeks now. Having concluded the issue most likely was beyond external issues the pump went in for repair. When I finally got it back and installed I couldn't get it to bleed out and run. Having done this more times than I can count I assumed either I had missed (or maybe crossed) something in the plumbing, or the "rebuilt" pump just wasn't performing. I guess we've all been there a time or two. The rebuilder agreed to take it back, retest, ETC, so it went back. I got it back on yesterday and still had the same problem. Concluded it was probably too low on fuel level to prime correctly, so tried again today with more fuel in the tank. It bled out much better (to the pump bleed screw) but still wouldn't pump anything to the injectors. I'm sure the pump was working as it should on the test stand, and after two rounds of that I'm ready to accept his word. If it pumped fuel for him, why won't it pump for me? The ported cover and both banjo bolts are new, (supplied with the pump) the injectors are rebuilt (same source) and I'm sure the return lines are back where they came from.

Not having too many options I decided to drag out my trusty electric fuel pump. The single element filter base has two in ports and two out ports. One in port was plugged so I removed the plug, found a fitting that matched, and connected a hose onto that. Dropped the suction hose into the filler opening and pumped fuel into the filter base instead of relying on gravity flow from the tank. That spiked the pressure at the bleed screw and eventually it started delivering at the injectors. It finally started and I ran it that way for a while. Stopping and starting it a couple times it seemed to be fine. I removed the pump, put the plug back in, opened the tank valve, bled the filter base, then started it back up with no problem. Seems to run just as expected. I don't remember ever having so much trouble getting one of these old Fords to run. At this point I started wondering again about the return line setup, even though I can't see what changed that would cause me so much problem. The side port on the pump connects to an out port on the filter base. So I decided to see what I could find out about how all this is designed to operate.

 

[Dieseltech]

That side return pressurizing valve will build HOUSING PRESSURE do to the small orifice fitting in the top cover. Since those Fords use a gravity system the tank should be at least half full or more so they will bleed out easier. MOST CAV DPA pumps will have 15/20 PSI pump internal TRANSFER pressure at 100 RPM on the test stand, but NOT THE FORD pump, they barely make 5 PSI, and that lower pressure won't move the rotor plungers as easy during the bleeding process. If engine CRANKING speed is slower that makes the problem even worse. When I get a Ford in for service I like to see the fuel level high as possible, and leave the rear pump inlet fitting loose awhile so gravity can work to get air free fuel there. If the cranking speed is border line I'll put the battery charger on awhile too. Just went through a Ford 2600 pump last week, and have a 3600 in for repair now.

 

[Harry in Ky]

You lost me on the side return pressurizing valve. That's inside the ported side cover? I don't see how that can help build housing pressure when it's getting fuel from the filter base which itself is only gravity fed? Where is the small orifice in the top cover? It's certainly not in the banjo bolts as it is with some systems. They both have standard size holes in them.

Cranking speed was adequate and I did have a charger on it while doing all this.

 

[Dieseltech]

THE pump HYDRAULIC HEAD has an orifice screw inside to dump more fuel inside the housing is why the low speed transfer pressure is so low on Ford applications. That side cover SHOULD have a spring loaded valve INSIDE. Early pumps used an aluminum side cover valve that did have leak trouble and has been replaced with a steel pressurizing valve, The 3600 pump I have in for service now HAS the top cover return line bolt WITH the small orifice hole in the bolt side. The side return line BACK to the filter base is used to help the filter last longer instead of returning fuel back to the tank and then need to go through the filter again to pump inlet. Housing pressure is more to help advance curve after engine is running, there will not ever be any housing pressure during cranking/bleeding process. MF/Perkins feed/supply pump pressure is different in that BOTH ends of injection pump is pressurized to about 5 PSI to keep air out of the injection pump.

 

[Harry in Ky]

The man that did the pump work mentioned something about the side cover fitting that he said may have contributed to some of the original problems with how the engine was running so poorly. That was confusing to me as I had no idea how that had any bearing on the system. He put a new cover on it (steel as I recall) and new return line banjo bolts as well. Neither one has any restriction features I could see. It seemed to run fine once I finally got it going. The owner said he was going to use it today to drag some tree limbs to a burn pile and I haven't heard from him yet with any issues. No news is good news.