Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Fuel System Not Holding Pressure

Thanks Gomog,


To confirm the fuelpump is in the (right hand) fuel tank and neither the original pump nor the Walbro conversion in tank pumps have pressure control.

The fuel is pumped to the injectors and the fuel filter in the return line has the pressure control - call it a valve or regulator as you wish. This is preset at 58 psi and monitored via the ECU so if the pressure drops the spark is cut off.

Are you sure the ECU monitors fuel pressure? Can you tell me where the fuel pressure sensor is located?

Reading back through this thread I have a sneaking suspicion that something simple and perhaps too obvious has been missed. If I'm wrong then apologies to all...

If the engine isn't running then the fuel pump only runs for about 20 seconds after the ignition is switched on before the electricity supply to it is cut off so the pressure will drop off quite quickly, mainly by bleeding back through the pump into the tank. When you turn on the ignition, if you get fuel pressure at the test point but when it is cranked over on the starter motor the engine doesn't fire then it probably isn't the fuel pump that is the problem. Conversely, if the engine fires, then the fuel pressure drops and causes it to misfire or stall, particularly when under load, then it is very likely to be the pump that is the problem or maybe the in-tank hoses and connections.

The arrangement of the fuel system is; the in-tank pump draws fuel through a strainer and sends it forward to one injector, from there it goes to the second injector then to a return hose which takes it to the combined fuel filter/pressure regulator canister (which is mounted to the rear bulkhead above the swing arm) and from there returns to the right hand fuel tank via a short hose to the top of the pump housing on top of the tank. This means that the pressurised part of the fuel system runs from the pump via the injectors (and test port) then back to the fuel filter. When the pump is not running the pump will allow the pressure to bleed off back into the tank up to a point at least. If the simple valve and spring in the filter isn't sealing as well as it should then it could also allow fuel to pass when the pressure drops off anyway but usually some (but not full) pressure will remain in the system for a fair while after the pump has been running. If there is crud in the tank that gets past the pump strainer then it will travel right round the system to the filter where it will be removed just before returning into the tank. While this may sound like a daft arrangement it is an extremely common one. As far as I can tell there isn't an actual one way or check valve in the system but the pressure regulating valve will largely fulfil this function.

It is more than possible to re-plumb the fuel system to allow thew fuel to be filtered before the injectors and to use a better, dedicated, pressure regulator that controls the fuel pressure much more accurately and consistently than the standard arrangement (no pressure drop at full throttle and high revs which does happen even with a brand new standard filter/regulator and new pump) plus you can even fit one or more check valves to maintain the pressure in the system if you wish, but the benefits are mainly confirmation bias related...guess how I know!

Thank you Chris!

A FUEL PRESSURE REGULATOR and a check valve are different. The regulator is a spring-loaded pressure-activated diaphragm system that maintains the pressure in a fuel system to a pre-set
value above manifold pressure, particularly in a fuel injection system. It sends unused fuel back to the fuel tank..so FI systems normally had a second fuel flow returning fuel to the tank. It largely
functions as you say but many of us use adjustable ones like FSE.

Earlier Morgan EFIs can use them to:
1. create a steeper curve of fuel supply on hard demand, increasing acceleration when one puts the foot to the floor.
2. Adjustable ones can increase fuel pressure across the range, easily allowing a greater supply for larger capacity engines, like mine.

A CHECK VALVE merely stops fuel from moving in one direction, in this case backwards though what should be only a feed line. Without one, your feul will drain backwards every time your pump is
off or gravity (angle of the car) allows it. For example, you have one form of check valve being used (in the diagram) as a turnover valve. If you turn over, it prevents the fuel tank from emptying onto drivers when
upside down. They became mandatory decades ago, though many older mogs don't use them though they DO vent their tanks. Yours pumps the fumes into a charcoal canister. Often the regs of two
different governments fight for commons sense.

You got me interested enough for the first time to examine the M3W stuff I archived. I enjoyed it! I made a simple diagram of the fueling area from the M3W parts books I have.
Below.

Questions.
1. I see a lot of rubber section interfaces between metal (?) pipes and other metal pipes or components. Is that the case?

2. Are the two colored sections of the to-and-return-the engine fuel feeds metal or hose?

A check valve cost is negligible and installation simple with an unpressurized system. It will prevent leakage backwards, which is the issue here unless the technician was
mistaken. Why not? Redundancy is prudent in this area. No need to take it off regardless of the result. I and many others had this EFI leak down problem until we installed
them. The risk is 2 quid.

Lorne

The blue coloured line is the supply to the engine and the green lines are the supply to the engine. See my previous post. It may be counterintuitive but is a very common arrangement and not restricted to Morgan. Check valves do not feature and serve little purpose in this type of system as the flow of fuel is always considerably higher than the consumption of fuel by the engine. A simple pressure control valve in the filter canister restricts the flow of the returning fuel to the tank which is what regulates the pressure seen at the injectors.
The lines that you have coloured are metal tubes of which the two long ones are connected to rubber hoses at the engine.

That is where I fell the problems is. Fuel should not run backwards down the feed pipe. If it does, you get the situation at hand. Whatever is supposed to stop that is not doing its duty,
assuming there is no injector leakdown.


You are suggesting there is an one-way (aka check) valve in the fuel filter. https://board.moparts.org/ubbthread...line-fuel-filter-with-a-check-valve.html


Yes. A strainer is the primary filter. It is there to protect the pump. The second filter, a finer one, is there to protect the injectors et al engine from minute debris. Was the strainer checked (PUN!)


Regulators do not normally (AFAIK), act as check valves. Thoough an improperly seated fuel pressure regulator diaphragm WILL allow back flow more readily. Guess how I know?

Good thinking Red. In any event, both your theories and mine are easily tested if back flow is stopped. Depending on the hose type, which I asked about, that can easily be done by clamping the feed line
with a small needle nose vice grip after closing the engine off. Then see if there is the same problem when starting up 15 minutes later as there is now. If it starts right up (before stalling for lack of fuel from
a clamped line). we have absolutely confirmed that the leakdown is happening backwards (pump, filter etc) rather then forwards (injectors and regulator) . Easy, cheap, more reliable fix.

gmg

P.S. BTW Red, I found I halved my EFI engines on road diagnosis time by adding in, permanently, pretty fuel pressure gauges per car. If the fuel pressure is fine at a glance, then it has to be electrics/electronics.
One of the major UK suppliers, in the day, adopted the idea wholesale for V8s. Cost is 15$.

Nice drawing, BUT as Red Three has pointed out, it is wrong! The blue line is the supply to the engine and green the return. Unfortunately, one of the joys of the internet is that once incorrect information is out there it is out there for eternity!

[/quote]

Apologies you are correct. It's the throttle position sensor the ECU reads and will not allow an engine start if the throttle is open.

However, back to the original post, when everything is working the fuel system will hold pressure with the fuel pump off for quite a few minutes (or longer). So, if the system is not holding pressure and not leaking then fuel must be returning to the tank which is symptomatic of a fuel pump failure.

Was the car running OK before components were replaced? If so I would check that the filter/regulator and pump connections are all as they should be, it's easy to cross them and it really doesn't work well then........ If it is all working correctly you shouldn't need to be putting NRV's in the system or modifying it, it's not perfect but it does work OK. If your friend doesn't have the Walbro fuel pump fitted to his M3W, he should do. Full instructions are available in the owners resources at the top of this forum.

I am trying to think how quick the fuel pressure drops off after turning off. I never timed it but it was a visible fall of the gauge, so not that long, maybe a minute or so? I wouldn't say that 40 seconds was a big issue as long as you are getting a steady 58psi when the pump is running. All the injectors need is a steady head of pressure.

Chris rechecked the diagram and retracted Bunny./b] [/i] I took the "nice drawing" from the two publicly posted M3W parts diagrams we/I have. I merely combined and simplified them
rather than force the readers wade through many lines and old numbers of constantly changing parts #s. I used my colors rather than theirs. I am posting their original below and the
explanation of their numbers to make it easier for you. Page. 26 No apologies necessary. I am glad you are interested, I prefer posting on a forum as good people are the best protection for
all of us. It is rarely done.

Originally Posted by Bunny

The blue line is the supply to the engine and green the return.

No. [b]If you had checked the MMC diagram OR (better still) merely traced the fuel flow as Red Leader did, you might change your mind. As it is, you are suggesting that fuel comes from
the engine and only then is filtered. The world does not works that way.


Yes. But only with public searchable forums where membership and context do not pop-up on a search engine. It is the biggest problem my Gomog Helpline presents me these days days. It
keeps getting worse. Others disagree, but I see most amateurs (and many pros) believe whatever they find on the internet. For example, your comment on the MMC diagram would be out there
for eternity unless you erase it but it is likely too late for that the way spiders work. Search information is not prioritized by veracity. Search engines are not very smart.

But, no worries. I would prefer having people checking my stuff whether their checking is right or wrong. At worst, they force me/us to review my posting. Always a good thing.

gmg

P.S. My compliments Chris! You owe me one free forgiveness. I am sure I will need it!
P.P.S. I see that Bunny took his post down.