My Bedford TK is acting up

[Nadar]

I have a -71 Bedford TK with the 330 diesel (MP044) naturally aspirated with the hydraulically governed CAV injector pump. I've had it some years, and even though the engine is definitly "worn" (some smoke, turns easily), it's always been very easy to start - even in -20 C.

This winter however, it suddenly wouldn't start one day. After a lot of glowing and grinding I finally managed to get it to "start" on one cylinder. That's not uncommon in the cold (was about -10 C then), so I tried to keep it alive on this one cylinder until the others would heat up enough to start firing. But it never happened, and in the end I gave up. I figured it could wait until the weather got warmer to save the battery and starter. Since that I've had a couple of tries where I haven't been able to start it at all, just some random ignites a very few times.Now it's spring and my plan was to get it ready for MOT.

Some years back my feed pump died, and I made an "emergency repair" by replacing the valves with some I found from an old Massey Ferguson feed pump that were, as far as I could tell, identical. However, I didn't really trust that the valves would stay in place since you have to "stake" the housing to secure the valves, which doesn't work as well the second time. In addition to that the threads for the incoming connection had seen their best days. So, I ordered a NOS pump from the UK - which I've never come around to actually install.

Everything points to lack of fuel, both that it would fire on one cylinder only and the fact that it will temporary start when spraying starting spray into the air filter. I therefore figured that the feed pump must have died, so I replaced it with the brand new one I had. I also changed the fuel filter since I can't really remember when I did that last, and it was definitly due.

My problem now is that I can't seem to get the bleeding/venting "right" for some unknown reason. Btw, I'd like to congratulate whoever figured that it was a bright idea to install the feed pump and the injector pump on opposite sides of the engine - it really makes bleeding easy for one person..! Anyway, I've bled this engine, and many others for that sake, many times before and this Bedford has always been very easy to bleed. This time around though, I keep getting small air bubbles no matter how long I bleed. I also experience a strage phenomena where the feed pump handle is really slow to return even though the bleeding plug is open. It feels just like it does when you have a pressurised system when I pump. It does come diesel and air of the bleeding plug hole though, but I think it flow is a bit small.

Normally when I see small air bubbles like this, I assume it's a leak on the vacuum side of the pump, so I've unscrewed the input connection again, cleaned and reinstalled it with thread tape to make extra sure there's no leak there. Still the result is the same. I've looked over the fuel line from the tank to the feed pump, and it's completely dry as far as I can see, except close to the pump where I've made a mess each time I've disconnected it.

So, in short, I've really got no idea what could be causing this at the moment. The most logical explanation would be that the "input fuel line" would be partially blocked somewhere so that the vacuum manages to pull some air in at joints that would otherwise be "sealed". That would also possibly explain the slow return of the feed pump handle, but I'm not sure simply if a vacuum is strong enough to cause this. A blockage after the feed pump would better explain the slow return, but that wouldn't explain the small bubbles at all. I'm stuck.

I haven't yet pulled the fuel intake out of the tank since I'm a bit scared about how many of the bolts that will break when I try to disconnect it, but I've tried to look inside with a optical scope camera, but they aren't as easy to control as one would imagine. I can't really see that much except that it seems to be a plastic cylinder in there probably containing the fuel indicator float. Does anybody here have any experience with those, often there is a small filter in the bottom which could possibly be blocked. I've also considered it a leak could have arisen inside the fuel tank above the fuel level sucking in air. Is this likely at all, or am I focusing on the wrong part of the fuel system here?

Any ideas/experience appreciated.

[G-CPTN]

You seem to have all the right ideas, but is the vent hole in the fuel tank filler cap blocked?
It sounds like you are trying to suck against a vacuum . . .

Maybe, as you suspect, the suction pipe is blocked - you could fix some clear plastic tubing and see how easily you can suck fuel (stop when it reaches the clear tubing!).

I think that the plastic cylinder is the float for the gauge.

I'll check my manual and see what else I can find.

[Nadar]

There's no vacuum in the tank itself, I've even tried with the cap off. I could try to suck myself, but I'm not really sure how to judge how hard it should be, and I suspect getting a good seal against the existing connector would be somewhat difficult.

I guess I'll just have to pull the whole suction/float module and see - even though I really don't want to

[G-CPTN]

My manual is for the military MK model. The MK engine was a multi-fuel version of the 330cu in diesel engine with an injection pump that could be set to suit aviation fuels as well as diesel.

The transfer pump (on the MK) is an electric pump.

You describe a hand lever, suggesting that yours is a mechanical (engine-driven) lift pump.

My manual shows a shut-off tap between the fuel tank and the primary fuel filter (there are two filters on the MK model). The primary filter is between the tank and the delivery pump (not the injection pump).

Maybe your tap (if yours has one) is partially blocked?

Otherwise, your lift pump should be capable of freely-delivering fuel (if you disconnect the outlet pipe). If the fuel doesn't flow freely then that is your problem.

There is no filter (or anything similar) inside the tank (of the MK) so fuel should be able to be drawn easily from the tank by the lift pump.

If you cannot easily suck fuel from the tank, there might be an obstruction on (or in) the pipe.
You might have a crack somewhere in the fuel pipe between the tank and the lift pump allowing air in.

[G-CPTN]

Reading your description of the sluggish return of the fuel pump lever it sounds that there is something wrong (as if the diaphragm has hardened or a spring has broken). I would think it should return easily.

[G-CPTN]

I suspect getting a good seal against the existing connector would be somewhat difficult.

Do not suck directly on the metal pipe - use a good long length of clear plastic 'hose' (at least a couple of metres) with enough to allow a dip down between you and the tank. You should be able to easily suck fuel from the tank until it runs down into the hose where you can see it before it gets anywhere near your mouth.

[Nadar]

I haven't found a tap anywhere on the fuel line, but it's a bit hard to access parts of the fuel line (inside the frame) without raising the tip, which again is difficult when the engine won't start - so I can't bee 100% sure there isn't one. Anyway, my lift pump is mechanical and of this type. I only have one fuel filter, and that's placed between the lift pump and the injector pump (that's the one I just changed). There is however a very "simple filter" inside the lift pump itself, which is just a brass mesh if I remember correctly. That one is probably instead of anything at the intake in the tank, which would make it possible for something to get sucked into the pipe in the tank and partially block it.

The slow return of the lift pump lever isn't a defect of the lift pump itself, since both the old and the new pump does this, and this is not the case the first couple of pumps. It's obvious that either a pressure builds up after the pump or a vacuum builds up before the pump causing the slow return. I guess the only way to find out where this is is to loosen connectors different places on the fuel line and see when or if the resistance goes away.

My worst fear is that something's gone wrong with the injector pump itself, and that it won't let the fuel pass freely thus building up pressure. I don't know about enough about the inner working of the injector pump, but maybe for example a stuck "engine stop" valve could cause this? At this point though, I'm still trying to find faults anywhere else but the injector pump...

I understood what you meant by the plastic hose, I would never get my head down to the pipe anyway. I'm worried about the seal between the plastic hose and the pipe, since the connector threads seems like something in the range of 3/8" and I don't have any clear hose larger than around 1/4" inside diameter.

[G-CPTN]

Softening the plastic hose by immersing the end in hot (boiling?) water might allow you to stretch it over the connector.

Concentrate first on getting the lift pump to work (disconnected from the injection pump). If that doesn't deliver fuel freely then you have a basic problem.

As I've mentioned, the low-pressure side of the fuel delivery system (from the tank to the lift pump) is simple and should operate freely without obstruction.

From the lift pump to the injection pump (via the filter) there will be more restriction, but it should flow relatively easily. Loosen the connector before the filter and you should be able to pump fuel easily (and without air - unless there is insufficient fuel in the tank of course!) - or a blockage or split in the supply pipe.

Was the fuel pump that you changed the lift pump or the injection pump?
You mention thread tape - is there a chance that there is some blocking the pipe?

Loosen the connection between the filter and the injection pump and you should be able to pump fuel easily. If this isn't possible there may be a blockage in the filter.

It's only when the fuel leaves the injection pump that it is at (very) high pressure.
The injection pump shouldn't pass fuel until it reaches very high pressure (apart from the relief valve which might return the fuel to the tank).

Unless you can supply adequate air-free fuel to the injection pump it stands no chance of working.

[Nadar]

I replaced the lift pump. I was carefult with the thread tape, and the symphtoms where the same before I added thread tape.

When I'm talking about the strange buildup of pressure or vacuum, the bleed/vent plugs are open. If they were closed, it would be normal and expected for å pressure to build up in the injection pump.

[coachman]

Originally Posted by Nadar

I replaced the lift pump. I was carefult with the thread tape, and the symphtoms where the same before I added thread tape.

When I'm talking about the strange buildup of pressure or vacuum, the bleed/vent plugs are open. If they were closed, it would be normal and expected for å pressure to build up in the injection pump.

Are all the fuel lines clear of dirt, it might be a good idea to give them a good blast out with an air line. Also I'm sure those old Bedfords had a glass water trap in the line, if they were over tightened they used to crack.