Pump Puzzle

[Gismo]

Hello, this is my first post on the forum , sorry it is so long.

I have a misfire which I think is fuel pressure related and I can't sort it. My car is a 72 CP which developed a misfire about 6 months ago. I realised the pump note was fluctuating and the current draw through the ammeter similarly fluctuated (pump fed via relay directly off the battery positive terminal). I thought it could be fuel starvation to the pump and I checked the pre pump filter before removing and cleaning out the petrol tank.

When all back together, no change. I contacted Malcolm at Prestige who supplied the pump kit (with bigger tank outlet, pre / post filters) and diaphragm PRV. Malcolm was helpful, said it sounds like a bad earth and suggested I try running the pump directly off a spare battery lashed up in the boot. Misfire gone! I found no wiring fault so I tried 2 long cables directly from the car battery terminals to the pump, misfire still there. Swopped the battery in the engine bay with my spare, no change, tried a temporary engine earth with a jump lead to bypass the engine earth braid, misfire still there.

A mechanic I know asked if had tried pulling the small wire off the back of the alternator, worth a go I thought and misfire gone! I'll get a new alternator. Misfire is still there with new alternator. I ran the pump directly off the spare battery again to check I am not going mad and it pulls great, run the pump off the car's system with the new alternator connected, pump speed varies (like a wasp caught in a bottle) and it misfires under load.

I have convinced myself it is fuel starvation because of a fluctuating pump speed / current draw. I have no fuel pressure gauge but the fact the pump provides enough pressure (no misfire, runs fine) without the alternator connected, has absolutely baffled me. Just to add that the car ran fine with the Prestige pump kit / PRV for about 700 miles before this all started.

The car has Lumenition ignition, Prestige pump kit, uprated pump wiring with relay and a baffled owner. Has anybody any ideas?

Thanks,

Colin.

[astontr6]

Colin,

 

This is very difficult to do a diagnosis from a distance but have you checked the internal contacts in the inertia cut out switch and the 2 on the outside base? These can play up with dirty contacts or the plunger is not push down fully. They can be taken apart but you have to be careful. You start by pushing in the pin on the bottom edge with a small punch and then lever out the base with a small screw driver. Also do not rely on the earth on the rear inner wing behind the fibre board in the boot, another known problem area. The pump s/b earthed by the pump and then that earth linked to the original boot earth after cleaning up the contact areas of the original boot earth..

 

Bruce.

[Gismo]

Hello Bruce,

 

I know the inertia cut out causes many problems but I think my checks so far have eliminated that and a bad earth. I have bypassed everything with cables directly from the battery terminals to the pump terminals (positive and earth) and the pump runs erratically / it misfires with the alternator connected. With the alternator unplugged and normal wiring in place (via inertia switch etc.) runs fine / no misfire.

 

If the alternator is connected it misfires, no alternator and it runs fine. It does the same with the replacement alternator I fitted.

 

Colin.

[DaveN]

What grade wire you using? Try a heavier gauge 56/03

Check the connections on the back of the ammeter!

 

Dave

Edited April 21, 2017 by daven

[Guest ntc]

Look at the wiring diagrams you have the wrong alternator fitted.Easy to sort.

[Gismo]

I thought I should update with the solution. Having exhausted everything else, I bought a replacement pump despite the old one having only done 700 miles. The new pump is much quieter and seems to have cured the misfire. I have no idea why the old pump worked ok (but noisily) without the alternator connected or directly off a spare battery but there you go.

 

Colin.

[astontr6]

Colin,

 

You dont say what pump you are using Bosch or Lucas or if it is Bosch what model number. If it is the original substitute Bosch ----------952, which I doubt as these are obsolete, this pump shifts an very large amount of fuel for instance and does not like the supply hose to be less than 10 mm bore size. If starved of fuel its noisy and if you use one of these stiff smooth bore PTFE HP hoses rated at 1000psi????from pump to PRV which are sold by some of the well known suppliers you get the harmonic hammering noise, from the Lucas PRV. This why they now offer the diaphragm PRV? So as you have had 2 pumps recently what make and type are they? Noting your last sentence how exactly was (is) your pump wired up as some of these replacement pumps need as much as 15amps current supply and were the two pumps the same make and model?

 

Bruce.