Why is my sump filling up with fuel ???

[flywheel1935]

This may sound like a daft question, but a series of explanations has been offered by a 'specialist'

1/  A porous fuel pump diaphragm,( that has now been dismissed as not being viable)

2/ The carb is siphoning fuel into the sump via the inlet manifold ( Again, if that's the case then numerous A7,s would do the same ) ???

3/ My own theory is that as the engine has failed to run/start correctly recently, then excess/unburnt fuel is washing down the bores and filling the sump.

Any other idea to consider ????
Over to the experts.

[Hedd_Jones]

1 every time

[Oxford Jack]

Why is the first suggestion not viable, it is the most obvious? Has the diaphragm been changed for one to use with unleaded/ethanol fuel? Has it been renewed within living memory? Has it been checked for holes and tears? Fuel that passes the diaphragm, will drain through the body of the pump directly into the sump.
I don't think the 2nd explanation is at all likely, unless you are parking facing down hill on a 45 deg slope.
The last idea is possible, but the likely amounts that might pass in this way are small in my view, however you would need to say more about how much the level is rising.
You do not say how much fuel is present or in what period of running or simply standing still, I think a few more clues are necessary to be able to provide a proper diagnosis.

[flywheel1935]

1 every time

Sorry, but I don't understand your reply, please expand.This is a serious question.

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Hi Oxford-Jack, The f/pump diaphragm was changed recently by the person who suggested that may have been the initial problem , having suffered a fuel filled sump previously, after a work on my engine, so this is the second time it's happened to said engine by the 'specialist' in question.

[Derek Sheldon]

I agree with Hedd, one being the obvious example. Two and three are extremely unlikely .

[flywheel1935]

Hi Oxford Jack again, you mentioned a diaphragm suitable for these unleaded/ethanol fuels, do you have a particular brand/ supplier in mind. I'm sure that these 'modern fuels' could well be one of the issues causing problems with my engine.

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Hi Hedd-Jones
Sorry, just realised what you meant, been a very long and frustrating day, had a new engine rebuild, and from the day it was returned has failed to perform as expected,
so I getting stressed & skint. Fuel in the sump has occurred twice now, despite a new diaphragm being fitted after the first fuel in sump episode.

[John Cornforth]

Hi flywheel1935

I'm in agreement, I can only think of three ways neat petrol can reach the sump:

1 Past a hole/tear in the pump diaphragm
2 Down the inlet valve stems from the inlet manifold
3 Past the pistons/rings

2 and 3 will be aggravated if the mixture is rich (choke for cold start) and if the engine is cold (incomplete evaporation of fuel to vapour). 3 will be aggravated if the rings/bores are new and not bedded in. Once the engine is running and warm then these leakage sources should be very small unless you have a carburation fault causing excessively rich mixture.

I once had an engine with an incorrect compensation jet fitted to the zenith 26VA carb, which gave a very rich mixture at part throttle. Enough over a week or two to cause the oil to thin and smell of petrol due to route 3, but not enough to cause the level to rise on the dipstick.

How much petrol is reaching the sump, and how much cranking without starting has happened ?

[Parazine]

Friend had this problem with an early 1950's Morris Minor series MM (8hp Morris 8 side valve engine) that we recovered from the scrapyard in 1979. Turned out the engine had a hole in one of the pistons. The crankcase filled with petrol fumes and exploded somewhere in Bath one afternoon, the dipstick and oil filler cap flew off the engine and the cap made a dent in the bonnet, before rolling down a drain.

[Bob Culver]

I take it the car is not an RN or earlier with a scuttle tank mounted above carb level but using a later pump. Any needle valve leakage might then fill crankcase.  Is it definitely petrol causing the rise and not a water leak collecting under the oil.

[Robert Leigh]

Having read all the answers I still supect the fuel pump, and suggest changing it for a known good unit. I once found a pump with corrosion internally, so it was short-circuiting itself. In that case the fuel was not draining into the sump, but the diecast parts of the pump may have gone porous. In my experience if an item is faulty but still passes all the usual tests the only way to reach a proper conclusion is by substitution.