Mystery Thump from the rear

[peanut]

heres a strange noise for you to diagnose. Well to be accurate its more of a thump ....just the one huge single thump as though another car has just bumped the back of your car in a car park..

Very occasionally when I switch the ignition to on, ( prior to cranking ),there is a huge thump from the rear of the car and the car jumps fowards about 3-4" or so ?

Its probably only happened about 5x times in as many years but it generally only occurs during those periods when I am experiencing cold start issues .

Commonsense tells me it has to be the fuel pump as its the only mechanical component back there with any power but what, why and how?

Last week I replaced the fuel pump , fuel filter, tank strainer and engine temperature sensor.They were all original parts and I thought after 28 years it was a good investment........well .........ok I guess I really replaced them because after 3x weeks I still cannot find the reason for my current hard cold start issues and erratic rich idle.. But don't get me wrong I'm not going to replace everything on the car like I always tell others not to do....nosireebob ..................... yeah right!

So I'm thinking what could cause the fuel pump to thump like that when energised. It has to be the fuel flow being suddenly shut off surely.
Correct me if I'm wrong but I believe I'm right in thinking that the fuel pump flow is continuous whilst the engine is running and the excess pressure and fuel is constantly fed back to the tank via the fuel pressure relief (FPR) valve . If the FPR valve were suddenly to stick closed or fail to lift off might the resultant surge be enough to cause such a violent event ? You know like when you suddenly shut the cold water tap off and there is a thump and bang in the loft as the ball valve suddenly shuts off the supply to the tank.

Or could it be crud in the tank return pipe suddenly blocking the return fuel ?

Yesterday I pulled off the vacuum pipe to the fuel damper to check the effect on idle and noticed that there was a little fuel in the vacuum pipe . Perhaps I should order and fit a new FPR and damper whilst I'm at it .......but I'm not going to just buy a load of new bits and bolt them on to cure this problem .............groan....

 

[IanK]

But how can the fuel pump make the car jump forwards 3-4" ??

Ian

 

[peanut]

exactly ![] my guess is it must be the abrupt shut off of the fuel .....its 3 bar pressure which is quite a bit I guess. otherwise it could be an alien in my boot ...

 

[Eldavo]

The fuel pump can't make the car move forwards like that.

The only thing that can cause a deiveline shunt is a part of the deiveline itself. Therefore, assuming clutch-in or in neutral when you start the car I can only guess at the gearbox not being completely disengaged. Hence a shunt forward as it clicks out of gear and into neutral. So either a worm clutch or perhaps a worn syncro. Either way - if it does it so rarely - save your cash!

 

[PSH]

I'm confused...you said 'prior to cranking'? i don't see how this is possible/ I have an uprated pump that you can feel pulsating for a minute or two on start up and then it disappears but that's with the engine running. I don't see how a pump can make the car jump forward, I don't see how anything can make the car jump forward if the engine hasn't even been started...or am I misunderstanding the problem?

Pete

 

[peanut]

The fuel pump can't make the car move forwards like that.

The only thing that can cause a deiveline shunt is a part of the deiveline itself. Therefore, assuming clutch-in or in neutral when you start the car I can only guess at the gearbox not being completely disengaged. Hence a shunt forward as it clicks out of gear and into neutral. So either a worm clutch or perhaps a worn syncro. Either way - if it does it so rarely - save your cash!

deiveline shunt ???[]

apologies................ I thought everyone must have heard of water hammer and kinetic energy .
have a peruse of this , it will explain the phenomena https://en.wikipedia.org/wiki/Water_hammer

 

[vitesse]

2 of the special products my company made before I retired were:-
Sewer Ventilation Columns &
Sewer Inlet Cabinets.

These were mainly supplied for Interceptor sewer pipes which collect surge storm water & needed both outlet vents & inlet air vents to prevent the huge horizontal " rods " of collected water causing considerable damage by oscillating along the often 9m dia pipes.

Can't quite see how this effect could cause car problems though-not many garages equipped to solve the many Partial Equations involved in resolution.[8|]

 

[Mike14]

Hi guys are there any meets near Portsmouth . Cheers

 

[IanK]

I understand the theory, but cant see how the amount of fuel that could be on the move in the pipe can shift 1300+ Kg of car.

Ian

 

[colin944]

Stupid question and I apologise for it but are you turning the key prior to you outing your foot on the clutch when you start the car. I would assume you park it in gear. Could it be the starter motor energising and pulsing the car forward for a split second. That would give you the jump and the feeling the car was being pushed from behind as the trans takes a quick hit .Obviously if you try and start the car with it in gear it will lurch forward but if the starter is only getting a quick hit of power then it would just make a thump noise. I'm talking about a tenth of a second or some thing like that.

 

[peanut]

Stupid question and I apologise for it but are you turning the key prior to you outing your foot on the clutch when you start the car. I would assume you park it in gear. Could it be the starter motor energising and pulsing the car forward for a split second. That would give you the jump and the feeling the car was being pushed from behind as the trans takes a quick hit .Obviously if you try and start the car with it in gear it will lurch forward but if the starter is only getting a quick hit of power then it would just make a thump noise. I'm talking about a tenth of a second or some thing like that.

its ok Colin no question is stupid believe me ...

Yes I suppose it is entirely possible that when I have switched the ignition to on.... that the key has momentarily travelled a couple of mm too far and a momentary connection has been made to the starter solenoid. I was never trying to start the car , simply to turn the ignition on for testing purposes with a multimeter
As I always leave the car in first gear it could conceivably be that the starter has caused the car to lurch forward for that fraction of a second.

It has only happened a handful of times over the years and as it lasts just hundredths of a second and happens without warning so I cannot be certain it wasn't the starter engaging. It didn't sound or feel like the starter engaging but then these days I seem to doubt everything .....

 

[Mike_Pollock]

If you have fuel in the vac line - replace the FPR.....

 

[peanut]

easy for you to say Mike ...you try and find one []