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  1. #1
    Carburetion 'sucks' !
    Join Date
    Jun 2013
    Physically within the M25 force field - mentally anywhere else!

    Fuel Pressure/engine stavation issues

    Puzzler for the week. Haven’t even looked at the car yet. I'm posting this up on here as some of you fellow petrol-heads (like myself) have been around a 'few' years and have a wealth of experience, so this is bit like a crossword with me giving the clues and you looking for the right answer, with me answering when I’ve finally found the cause.
    Car is my 489 cu in Vette. Carb is Proform 950 cfm double pumper. Fitted with Holley high volume mechanical fuel pump. Pump was only replaced last year - not that there was anything wrong with stock fuel pump (it was 10 years old - I just thought it was time to replace it) the stock fuel pump never seemed to have a issue 'feeding' the carb.
    I had a major fuel starvation problem a couple of weeks ago when accelerating hard through the gears it lost power in the upper speed band.
    It did the same thing on my first run up Kop Hill this weekend – so, full bore up the hill in 1st and 2nd and it lost power for a seconds or so, only regaining it when I backed-off the gas and allowing fuel bowls to re-fill.
    Got back to the paddock, left the engine running and opened the hood. The in-line fuel pressure gauge was showing zero, yet engine was still running. Felt rubber fuel line and couldn’t feel it pulsing (indicating fuel flow). Stopped engine and removed the rubber fuel pipe to fuel rail feeding both fuel bowls (which has the pressure gauge) and fuel squirted all over the place – so there was residual pressure between the fuel rail and the on-line canister fuel filter (which is between the fuel pump and fuel rail).
    Re-connected, started the engine and fuel pressure was back to normal 4.5 lbs approx. Blasted up the hill on my second run without an issue – no fueling issues.
    Cruised the 65 miles or so home on the M25 without a hitch. Last bit before I got hone and I hit the gas for a few hundred yards or so – and the starvation issue was back again.

    Is it crap in the fuel line between cartridge fuel filter and fuel rail.
    Cartridge filter has a 1/4” diameter spigot for the fuel return back to the tank.
    Is there crap in the fuel line between tank and fuel filter.
    Fuel rail ends are connected by a 6” length of braided hose. Its not unknown for this stuff to deteriorate with modern fuel – perhaps this restricted internally caused a pressure differential between primary and secondary float bowls.
    Perhaps its the Holley fuel pump that is braking down internally (after only a year)
    Suggestions are welcomed – I’ll publish the results of my investigation in a couple of days.

  2. #2
    Site Team
    Join Date
    Jun 2013
    United Kingdom Heathrowish.
    Be wary of under-bonnet fuel gauges. Some oil filled ones show low pressure readings once temperature in the engine compartment gets up. It's a known issue and plenty online about it. I once had fuel issues on a 12 sec. car. It showed up as spluttering at the top of second gear, all had been fine at launch and initially in second. Back off for a second and it 'caught up with itself'. The problem was that you are fighting the G of acceleration and the first pump just wasn't up to that and providing the fuel needs of the engine. On the street driven reasonably it was never an issue. Moved up to a half inch fuel line throughout and a higher volume pump and all was perfect.
    I would have questioned whether a mechanical pump was having enough pull as well as push under full acceleration but your stock pump was fine so that tends to rule that out. I suspect more of a fuel volume issue so possibly a hose restriction as you say? I certainly found that 1/2" upgrade worked for me but i did change the pump at the same time. On my current Procharged set up I run #10 feed and #12 return with a very high volume pump and no fuel issues at all. I appreciate that is a slightly different game.
    I do agree on hose deterioration with modern fuel and that could be an issue causing a restriction.

  3. #3
    Join Date
    Jun 2013
    possibly lost....?
    I was going to say hose too..just done some on a mates Merc.

  4. #4
    Might as well be part of the furniture.
    Join Date
    Jun 2013
    Maybe tank venting is not sufficient for the hard acceleration but fine for normal use, however I would be changing any braided hose that isn't ptfe lined, this does mean different fittings but the modern fuel should not have a damaging affect on it like it does with the old rubber stuff.

    One other thing, just because you have bought a new pump does not mean that it is ok, a friend of mine is in the throws of finishing his 36 hotrod and all was set for his first run to a show but he had to cancel as the following, which were all brand new failed or were not up to the job:

    1) Main electric cooling fan, bought a smaller one to help supplement the larger one only to find the smaller one pulled more air than the big one, got the fan swapped and the second one pulled like a train, didn't need the smaller one after all.

    2) TWO coils both from the states that fried themselves within less than 100 miles, now got twin ballast resisters in to help.

    3) Brake master cylinder leaking from the piston, new replacement one not an issue.

    4) Mechanical fuel pump leaking internally.

    All these are fairly standard fair and all bought from the states and all apart from the cooling fans were for a standard 5l mustang.

    Just thought, isn't there also a rubber hose that connects from the fuel tank to the hard lines on a vette, is this in good condition?
    Last edited by stilltrying; 23-09-17 at 03:47 PM. Reason: Additional thought.


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