Techy fuel pump query

[Guest]

I have a 1983 924 and have had problems with starting it from cold (engine turns over but won't start), replaced the fuel pump relay and it was OK for a couple of weeks and then wouldn't start again, []although it would run if we by-passed the relay .

I replaced the fuel pump (and the relay) last week but it still isn't starting properly or running very well particularly when there is under half a talk of fuel. In the last couple of days it has developed a slight buzzing from the near side rear, understand that there is a second lift pump in the tank.

Question is ... does it sound like the lift pump is the problem (is there a test?) or can anyone come up with an alternative ?? secondly, how do I go about fitting a new lift pump, I can't see that it's mentioned in the haynes manual ??

 

[924nutter]

It does sound like the boost pump in the tank isn't working. There is a boost pump at the bottom of the tank on the offside to ensure that the main pump has flooded suction, as they dont self prime that well.

I am going to suggest that you UNSCREW the pump from the tank and check the strainer gauze is not blocked with 20 odd years worth of accumulated cr4p, and also check that the pipes haven't perished. I recommend you put a supply across the pump terminals to see if it spins up, but don't run it for long, as the fuel acts a the lubricant.

The boost pump is about 190mm from the main pump, and infact the feed pipe to the main pump is attach to the outlet of the internal pump at the other end, and there should be two 15 amp spade connectors (connected in parallel with the main fuel pump) there as well. It is entirely possible that due to the quantity of salt dumped on our roads now that councils are sh1t scared of motorists suing their collective bottoms off, the electrical terminals have corroded away. Check these things first.

 

[937carrera]

Worth mentioning that you need to drain the fuel tank before unscrewing the lift pump............otherwise you get wet & smelly and will become very nervous about any stray sparks [:-].

Two ways to drain the tank

1. Raise the car & let it drain by gravity into a container below from the rubber pipe of lift pump exit
2. Unscrew the delivery pipe to the metering head, place in a container & bridge the fuel pump relay & let the pump do the work....much quicker.

A buzzing sound from the rear will be the main fuel pump. "They all do that sir", and can go on like that for years until they fail completely.

You cannot hear the lift pump. When testing in situ the only way to know if it working is to feel for the buzzing on the pump itself. - having disconnected the main pump so that you are sure it isn't the vibration from that you are feeling.

If you are having starting problems from cold & poor running when warm I would suspect the fuel pressure (aka warm up) regulator.

Get the system checked, system pressure should be about 5.5 bar, control pressure cold may be as low as 1.0 bar, which should rise to about 3.2 bar when warm. Make sure the electrical connection to the warm up reg is working, even when warm this is carrying current (it heats a bi metallic strip which moves to affect control pressure).

I think the car starting by jumping the fuel pump relay may be a red herring........though the problem may be more complex than I think.

I have made previous posts about testing fuel pumps in this forum. "Events" with the 924 GT race car mean I have far more experience on this subject than I really ever wanted to.........

ps..... lift pump requires a man size 36mm spanner IIRC

 

[Guest]

Thanks for your help.

disconnected the feed to the main fuel pump, there is current to the in tank pump and the pump runs very noisily (it sounds like someone shaking a bag of rusty nails) and is drawing about 2 amps.

I s the consensus of opinion that I should replace the in-tank pump ?

 

[937carrera]

The lift pump is normally very quiet, so it looks like you should certainly drain the tank & remove it for inspection at least.

I don't know what current it should be drawing, but if the filter gauze is blocked that could cause it to work harder than it should. Once the tank is drained it's only a 5 minute job to remove the lift pump.

 

[924nutter]

Duh!