740 fuel pumps - lift vs main - & gauge sender - queries

[eeedelli]

Hi all,

I have an intermittent fuel supply problem on my 1989 pre-facelift 740GL (B200E) causing it to cough and splutter when on load.

It fooled me into thinking it was ignition so I’ve been right round all that and even had my oscilloscope on it, which proved that all was well.

While standing there scratching my head, I heard a slight graunching noise that disappeared after a couple of seconds. Eventually, I traced this to the inline (main) fuel pump. Whenever I could hear that noise, blipping the throttle would cause the cough. When the pump ran quietly, all was well. (Later on while idling, it stalled completely a couple of times when the noise came on.) It's not the relay - I've already re-soldered that and drilled some tiny holes in the case to try to reduce the heat in there. You can actually feed the vibration of the pump body as it makes the noise.

Obviously, the main pump is on its way out.

However, pulling fuse 11 (the in-tank low-pressure lift pump) made no difference either way, so it looks as though that has failed too. Measuring the current there shows 4.6 Amps. My cousin (who also runs a 740) reckoned his was pulling around 2 Amps, which suggests that mine has stalled.

Just before I go ordering lots of expensive parts for a car that is getting pretty tatty (though much loved and useful / used daily), I’m wondering whether anyone has any useful advice?

If mine has been running without the lift pump for ages (which I suspect, as I’ve always had to blip the starter to prime the system before starting), do I need to bother with replacing the lift pump, or can I just get away with the main pump? (The lift pump looks like it would be a pig to change!)

Could it be disintegration of the in-tank filter (sock) that has allowed muck through, jamming the lift pump and damaging the main pump? (Mind you, they are nearly 30 years old so it could well be old age/mileage – estimated at 350-400k – odometer dead too (another classic fault)!)

I notice there are pattern parts (much cheaper than the hideously expensive Bosch originals) but they often seem to have screw-on tags rather than the push-fit ones that are currently on the car. I’m wondering whether those push-fit parts are merely adaptors that screw on in place of normal tags and nuts and whether I can simply unscrew the ones that are on the old pump and transfer them over? Does anyone know?

I also have an erratic fuel gauge (classic 740 problem) so, if I have to remove the in-tank unit, I could tackle that too, except that looking at the VolvoPartsWebStore.com site, it seems as though there is no separate gauge sender unit, unless the car is fitted with an ‘extra fuel tank’ (for which that sender is no longer available anyway). Has anyone ever tackled repairing/replacing the tank gauge sender unit?

The FAQs pages don't really seem to answer these specific questions, even though there is quite a lot in there.

Thanks.
David.

[NI_Volvo_Nut]

When the in tank pump died on mine it "ran out of fuel" at around half a tank.

Fuel pump relay controls both units, are you sure this isn't the problem?

The earth wire was broken off my tank unit which turned out to be why the in tank pump stopped working. It's not actually too bad to change the unit, just get new jubilee clips, remove the carpet and get in the boot!

[eeedelli]

Originally Posted by NI_Volvo_Nut

Fuel pump relay controls both units, are you sure this isn't the problem?

Unlikely. I only checked it a few months ago when I first thought I had a problem, which only cropped up occasionally over a few days. I checked and re-soldered everything which didn't help but then changed all the injector seals (which were well past it again) and thought I'd cured it. You can now actually feel the vibration at the main pump when it 'grinds' intermittently if you rest your hand on it, which the relay wouldn't cause.

Originally Posted by NI_Volvo_Nut

The earth wire was broken off my tank unit which turned out to be why the in tank pump stopped working.

Again - that's not going to be the problem here. If a wire was off, there would be no current flowing at all through fuse 11 (in tank pump fuse) but it's passing 4.6A (which also confirms that the relay is supplying power nicely).

Just found this video online - this is what mine now sounds like when it's doing it (intermittently). https://www.youtube.com/watch?v=CQBkLVhDKFg&t=0m18s

Ta,
D.

[Laird Scooby]

The noise/vibration on the pressure pump is cavitation due to fuel starvation caused by the lift pump in the tank.

It's not too bad a job to remove the in-tank pump, just fiddly. You obviously have an estate if there isn't the extra tank so the method is fold the back seats down and find the lift up flap in the boot flooring. Underneath this are 3 13mm headed nuts. Remove these and pull the flooring forwards to release the locating tabs.
Underneath this is a square access plate secured by 4 screws (10mm hex heads), remove it and you can now see the fuel sender/pump unit held in place by a retaining ring. A strap wrench is the best thing to remove this, anti-clockwise as you view it. You'll need to disconnect the various hoses and also the plug halfway along the wiring for the pump and sender.

Refitting - you "Haynes it". In other words, refitting is the reverse of removal. Be careful though as the sender/pump unit is spring loaded and is a fiddle to get in the right position. Vaseline or silicone grease helps on the seal as well.

As for the fuel gauge, that needs a new gauge almost certainly. It's rare the senders go. There's another thread on here about a dodgy 740 fuel gauge, i made a replacement using an LED bargraph and have been trying to find the time to get the diagram from my old PC and add it to the thread. When i do i'll link to it for you.

[eeedelli]

Thanks Dave.

That's a very interesting idea. I hadn't thought of cavitation. I was thinking it might be signs of something like an intermediate shaft starting to oscillate in a slot that was originally circular. (That's what it sounds like too.)

However, if cavitation is the cause, surely the main pump would be running faster rather than slower (which is what it sounds like it's doing when it starts growling)? It also raises the interesting question of why it's only doing it intermittently. Even with fuse 11 pulled this afternoon (thus guaranteeing that the in-tank pump wasn't running), it ran perfectly on idle for about 20-30 minutes this evening without growling once.

I managed to get the rear floor cover and access panel off this evening before it went dark. Not a pretty sight! The jubilees at each end of the main filler hose coupling (between the body and 'doughnut' top of the sender) have completely rusted through and sprung apart!

It looks as though someone has already been in before me (which is not surprising given that it's probably done >350k - dead odometer at 254k about 10 years ago!) as, instead of the right-angled quick-release connector, both outlet and return are rubber hoses with jubilees in a nice arc. However, the lack of rigidity normally provided by the right-angled connector means that the pipe on to which the substitute hose is connected is just bouncing around, which is not good.

The other problem is that the screws on the smaller jubilees are completely rusted away but haven't yet given up clamping their hoses. Goodness knows how I'm going to get those off! (Don't exactly want to go hacksawing metal clips right next to a tank full of petrol!)

Oh yes - and someone has spliced a scotchlok connector on to one of the wires on the plug side of the connector for the pump/sender, which is going to be a pig as I'm going to be unable to pull the harness through without cutting the wire there.

Incidentally, it looks as though there has been some sort of flat metal clip that held the wiring harness to the top of the doughnut by being pushed into a pair of parallel barbed plastic pieces. That's rusted away too, potentially allowing the wire to flop about (though it hasn't yet).

Talking to my cousin just now, he came up with an interesting idea which is to just put a high-pressure in-tank pump in there (a la 940) and do away with the old main pump underneath. That would also allow the main filter to be positioned in the engine bay so there would be nothing underneath. However, the wiring to the existing 740 in-tank pump is likely to be lighter than that to the main pump, so I'd have to run a chunky extension flex from the main pump location to the tank, to avoid overheating the existing harness and potentially blowing fuse 11. There's also the question of whether the hose and pipe between the tank and existing main pump location would withstand full pressure, having only been designed for lift pressure.

I wonder whether anyone has tried that before?

Looking around online, there seems to be much confusion as to which pumps are available for the high-pressure in-line version (cf 940) vs low-pressure lift pump (as per 740). Some suppliers list pressure as 0.2bar but then list it as fitting 940 mark II, so one of those two aspects can't be right!

I also noticed that a wide range of apparently almost identical in-tank pumps is available at much lower cost for more modern vehicles, the only difference apparently being that they use a plug-in connector rather than screw terminals. The problem would then be how to join the wires from a new connector reliably and safely inside the tank to the existing ones. (Need to be sure that heatshrink is long-term petrol proof, which I'm not sure about.)

Anyway, I need to get it out for inspection first!

Ta,
D.

[Laird Scooby]

I wonder if the Scotchlok is an extra feed to the in-tank pimp because the original feed was failing? With fuse 11 removed, did you check whether there was a feed to the pump?

The feed hose from the tank is 12mm bore to the main pressure pump and i haven't been able to find any high pressure fuel hose in that bore - was hard enough finding 12mm fuel hose! There is a short (about 2-3ft) length of rubber hose to the first rigid fuel line then a very short (about 9") length of 12mm rubber fuel hose to the underslung pump.
This gives the low pressure, high volume flow to feed the pressure pump which ironically has a 6mm bore link pipe from the pump to the filter. From the filter there is a length of 8mm bore rubber hose (or was it 10mm?) to the hard fuel line that feeds the injection system.
If memory serves the return lines are all 8mm bore whether hard or rubber. The hard lines seem to be some sort of nylon fuel line.

If you're going to retro-fit a 940 type pump, you'll probably have to change the fuel lines for 10mm (or was it 8mm) bore on the feed side all the way from back to front. To get at the join between the rubber lines from the tank and the hard lines, the tank needs to come out. It's a fiddly job but not bad and it's a plastic tank as well so not too heavy.

Before you get too involved in that i'd check that Scotchlok to see if that's your problem. Then sort out the Jubilee clips, preferably with new hoses and clips. If the old clips are that rusted, a bit of rough treatment with a pair of pliers should twist them off easily.

[eeedelli]

I suspect the Scotchlok is more likely to be for the sender to gauge as it's very thin wire that's connected on both sides. Certainly not beefy enough for the pump!

The supply to the pump must be ok as fuse 11 is drawing 4.5A. (Ammeter across Fuse 11 position when removed.) If there was an open circuit, that would be zero. (A high resistance connection would be low current but a smell of burning as a connector (or wherever it was the bad connection was located) would be getting very hot.)

I've just found a data book that I'd downloaded from somewhere and it suggests that the in-tank pump draws 3-4 Amps, so it's not that far off. My cousin reckons his is only about 2A though, which is why I thought it might have stalled.

Still doesn't explain why, with the in-tank fuse out, the main pump is running without growling today. (Don't you just hate intermittent faults!?!)

Yes I did wonder about those high-pressure hoses and pipes. The data book suggests the pressure relief valve is set to around 43psi. I wonder whether the existing fuel hose would stand that? It's not too high, I suppose - only about 3 bar.

It's all a lot of faffing around to change everything to 940 spec ... but then it's a lot of faffing around anyway. Tricky to know what to do for the best. Might have a better idea once I've got it all out, though goodness knows when that will be.

Ta,
D.

[Laird Scooby]

Have you added more fuel to the tank in between whiles? If so that could be why the main pump isn't growling. I know on mine it sometimes growls if i'm running low on fuel and that's with both pumps running.

It's a Bosch system that was used on quite a few other cars, Saabs, Ford, BMWs, Mercs, Porsches and so on. Finding a capable lift pump should be easy enough, whether the fitiings/connections will be easy or not is another matter though!

All three wires going to the tank pump/sender unit are the same thickness - one is earth, one is +ve for the pump and the third (pink i think) is the sender for the gauge.

[eeedelli]

I filled it up only perhaps 30 miles ago and it was just as bad after that. In fact, at first, I thought I'd got a batch of dirty fuel as I really only noticed it spluttering badly shortly after I'd done so. (I've been using my 940 all week, which I don't like to do in bad weather - that's what this 'shed' 740 is for!)

It was almost dark when I was looking at the connector and I didn't pay much attention to the other wires but I'm pretty sure the ones on the top of the tank sender are thicker than the black one that had been spliced at the connector so I'd guess that extra thickness would carry on throughout the harness. (I've left the Haynes in the car so can't check for now.)

I did call in at a couple of scrapyards on my way home and they seemed pretty clueless when I said it was a bog standard K-jetronic that was used on all sorts of vehicles. They both just said 'no chance' as soon as I said that. I think the problem these days is that scrapyards don't seem to keep anything older than about 10 years - it just goes straight off to the crusher without having any parts pulled off it. I suppose the demand is low enough not to warrant their labour time and storage 'just in case'. :-(

Yeah - there seem to be quite a few varieties around on eBay but, as you say, it's the connectors that are all different.